8. TimberCruisingHandbook.pdf

2409.12_50

FOREST SERVICE HANDBOOK

NATIONAL HEADQUARTERS (WO)

WASHINGTON, DC

FSH 2409.12 – TIMBER CRUISING HANDBOOK

CHAPTER 50 – AREA DETERMINATION

Amendment No.: 2409.12-2012-2

Effective Date: July 19, 2012

Duration: This amendment is effective until superseded or removed.

Approved: Leanne M. Marten Acting Associate Deputy Chief, NFS

Date Approved: 07/13/2012

Posting Instructions: Amendments are numbered consecutively by handbook number and calendar year. Post by document; remove the entire document and replace it with this amendment. Retain this transmittal as the first page(s) of this document. The last amendment to this handbook was 2409.12-2012-1 to 2409.12_40.

New Document

2409.12_50 63 Pages

Superseded Document(s) by Issuance Number and Effective Date

2409.12,50_Contents (Amendment 2409.12-2000-3, 04/20/2000) 2409.12_50 (Amendment 2409.12-2000-4, 04/20/2000)

2 Pages

9 Pages

Digest:

50 - Revises and updates entire chapter. More specific changes are set forth in the following sections.

50.2 - Establishes code, caption, and sets forth “Objective” statements.

50.3 - Establishes code, caption, and sets forth “Policy” statements.

50.4 - Establishes code, caption, and sets forth direction for “Responsibilities”.

50.5 - Establishes code, caption, and sets forth new “Definitions” terminology.

WO AMENDMENT 2409.12-2012-2

EFFECTIVE DATE: 07/19/2012

DURATION: This amendment is effective until superseded or removed.

FSH 2409.12 - TIMBER CRUISING HANDBOOK

CHAPTER 50 - AREA DETERMINATION

Digest--Continued:

50.6 - Establishes code, caption “Practicality,” and sets forth direction for the use of ‘reasonable judgment’ when applying applications.

52 - Removes captions and obsolete direction for “Traverse” and establishes new captions and sets forth direction for “Training” in this section.

53 - Removes captions and obsolete direction for “Aerial Photography and Mapping Methods” and establishes new captions and sets forth direction for “Survey Methods” in this section.

54 - Removes captions and obsolete direction for “Additional Equipment” and establishes new captions and sets forth direction for “Point Accuracy Estimations” in this section.

55 - Establishes codes, captions, and sets forth direction for “Area Error Estimations and Limits” in this section.

56 - Establishes code, caption “Equipment”, and sets forth direction for the use of approved and appropriate equipment.

DURATION: This amendment is effective until superseded or removed.

Table of Contents

50.2 - Objectives

50.3 - Policy

50.4 - Responsibility

50.5 - Definitions

50.6 - Practicality

51 - REQUIREMENTS

51.1 - General Requirement for All Survey Methods

51.2 - Direction-distance Survey Method

51.3 - GPS Walk and GPS Angle-Point Survey Methods

51.4 - Orthophotography Survey Method

52 - TRAINING

52.1 - All Survey Methods Training

52.2 - Direction-Distance Survey Method Training

52.3 - GPS (GNSS) Survey Methods Training

52.4 - Orthophotography Survey Method Training

53 - SURVEY METHODS

53.1 - Direction-Distance Survey Method

53.2 - GPS (GNSS) Survey Methods

53.21 - GPS (GNSS) Walk Method

53.22 - GPS Angle-Point Method

53.23 - GPS Receiver Always Active–Logging Only at Features (Points)

53.24 - GPS Augmentation

53.3 - Orthophotography Survey Method

53.31 - Points at Ground Level

53.32 - Points Not at Ground Level

53.4 - Combined Survey Methods

54 - POINT ACCURACY ESTIMATIONS

54.1 - Direction-distance Survey Accuracy Estimation

54.2 - GPS Accuracy Estimation

54.3 - Orthophotography Survey Accuracy Estimation

54.31 - Points at Ground Level

54.32 - Points Not at Ground Level

54.4 - Combined Survey Accuracy Estimation

54.5 - Alternative Values for Accuracy

54.6 - Precision of Measurement

55 - AREA-ERROR ESTIMATIONS AND LIMITS

55.1 - Direction-distance Area-Error Estimation

55.2 - GPS Walk and GPS Angle-Point Area-Error Estimation

55.3 - Orthophotography Survey Area-Error Estimation

55.4 - Combined Survey Area-Error Estimation

56 - EQUIPMENT

DURATION: This amendment is effective until superseded or removed.

50.2 - Objectives

An accurate area determination is required in area-dependent sampling to calculate sale volume.

Accurate acreage measurements are required for non-volume operations such as timber sale appraisal, contract preparation, reporting and monitoring, various post-sale activities, and resource measurement activities, such as archeology, wetlands assessment, or similar activities.

Defined limits for GPS and orthophotography surveys are given in section 55 of this handbook.

Defined limits for direction-distance methods are given in section 55.1 of this handbook. Other references to limits are explained throughout the handbook and are repeated in some sections for clarity.

50.3 - Policy

Determine areas by approved methods, such as: direction-distance traverse, GPS surveying, orthophotography, or similar methods derived from evolving technologies.

50.4 - Responsibility

Regional foresters may approve exceptions for:

1. Area determination using modified or other techniques,

2. Area accuracy exceptions in specific situations, and

3. Specific equipment or other equipment as appropriate for a forest or project.

The regional forester may delegate authority to a regional staff director responsible for forest management, who may further delegate the authority to a regional measurement specialist.

It is the responsibility of the regional forester to approve the use of orthophotography (aerial volume estimates) for area-expanded volume estimates. This authority may not be delegated.

However, orthophotography may be used for inspection or for missing data processes as explained in section 51.4 without such approval.

50.5 - Definitions

The following definitions apply to timber cruising operations. Other definitions may exist outside this discipline.

Accuracy. Closeness of an estimated value to a standard or accepted value (truth).

Accuracy (absolute). Closeness of an estimated value to a global or universal standard or accepted value; for example, the absolute accuracy of a point relative to a location on the Earth.

DURATION: This amendment is effective until superseded or removed.

Accuracy (expected). Long-run average value of a variable over many independent repetitions of an experiment.

Accuracy (relative). Closeness of an estimated value to a standard or accepted value relative to a local region or concern. For example, the relative accuracy of the points to each other in one unit (polygon) though the location on the Earth may be unknown.

Angle point. The points along a boundary or navigation survey where there is a distinct change of direction and which are observed by steadying the GPS receiver in one position for a period of time in order to average a number of observations at that common location.

Area-error. The difference in the measured area and the accepted “true” area. For example, if the measured area is 11 acres and the known area is 10 acres, the area-error is 1 acre.

Area-error (percent). Area-error is often expressed as a percentage. For example, 1 acre of area-error in a 10-acre unit gives an area-error of 10 percent (1/10).

Azimuth. A horizontal angle reckoned clockwise from North. Measurements are typically given in degrees: 0 degrees is North, 90 degrees is East, 180 degrees is South, 270 degrees is west, and 360 degrees is North again. North references may be true, magnetic, grid, or another method.

Beginning point. Point of Beginning (POB). The starting point of a direction-distance traverse; it is often the same point upon which the traverse ends.

Blunder. A mistake caused by mental confusion, carelessness, or ignorance. This is different from an error. See Error.

Boundary Point. Location point defining the perimeter of a land parcel.

Burst (GPS NMEA). A series of digital strings [values] sent from a GPS receiver to a recorder. The series constitutes one location measurement. Bursts are usually averaged to obtain a more reliable location measurement. Also called fix, hit, or ping.

Canopy closure. The proportion of the sky hemisphere obscured by vegetation when viewed from one point on the ground.

Closing error. The amount by which a series of survey measurements fails to yield a theoretical or previously determined value at a desired survey quality. The distance between the coordinates of a terminal point of a direction-distance traverse compared to

DURATION: This amendment is effective until superseded or removed.

the accepted (true) coordinates of the point being measured to or closed upon. Common terms for closing error are “closing distance,” “traverse error,” “traverse closure,” “traverse closing error,” “error of closure,” and “misclosure”. Closing error is not a blunder but a geometric value from measurement.

Common point accuracy. The accuracy value assigned to all points of a polygon.

Continuous Operating Reference Station (CORS). The system of base reference stations with known locations used in GPS location augmentation operations and post-processing operations.

Datum. A set of constants specifying the system used for calculating coordinates of points on the Earth. Such definition are NAD27 (North American Datum of 1927), NAD83, WGS84 (World Geodetic System 1983), and ITRF (International Terrestrial Reference Frame).

Digital Elevation Model (DEM). A raster surface where each pixel represents an elevation.

Differential Global Positioning System (DGPS). An enhancement to GPS that uses a network of fixed, ground-based reference stations (for example, CORS) to determine the difference between positions indicated by the satellite systems and known fixed positions.

See Nationwide Differential Global Positioning System (NDGPS).

Digitize. Selecting points on a map or drawing and recording the coordinate values of the locations. This can be performed manually or by automated means and results put into digital or hardcopy form.

Differential correction. The process of enhancing measured GPS positions with corrections to the Global Navigation Satellite System (GNSS) (such as the Global Positioning System) that use a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.

Direction. The angle between a measured line and a reference line. Azimuth is the direction of a line relative to North measured clockwise. Azimuth directions are commonly used in timber cruising (see Azimuth). (Some geodetic activities measure azimuth from the south; that is not done in timber cruising. Bearings are directions measured east or west of North or South with angles of 0 to 90 degrees. The bearing quadrants are: 1=NE, 2=SE, 3=SW, 4=NW. Bearings are seldom used in cruising.)

Direction-distance traverse. The process of surveying or measuring vectors around a unit or between control points (that is, GPS points). Vectors are defined by direction and distance.

DURATION: This amendment is effective until superseded or removed.

Distance. The measured or calculated interval between two points. Horizontal distances are used in timber cruising area determination.

Estimated maximum area-error (EMAE). (See Area-error). Estimates are approximations of measurements. This estimation is expected to be at the 95 percent confidence level for timber cruising.

End point. The terminal point of a survey. End point is often used as the terminal point of a direction-distance traverse. Point of beginning (POB) is the start of a direction-distance traverse.

Error. The difference between the observed value of a quantity and the theoretical or defined value of the quantity. Error is not a mistake or blunder; it has statistical meaning.

See Blunder.

Error of closure. See closing error.

Ephemeris. A table of the locations and related data of a celestial body (satellite) for given dates at uniform intervals of time. The precise ephemeris is the ephemeris of a satellite computed by adjustment of observations obtained from a worldwide tracking network in order to obtain maximum accuracy.

Filtered information. Processed data from satellite communications that has been reduced to meet selected criteria, such as using only 3D GPS calculations or limiting values of Horizontal Dilution of Precision (HDOP).

Fix, Fixes. Coordinate values determined by the GPS receiver from satellite signals.

Usually, a number of fixed coordinates are averaged to determine a location or position.

Other terms include: hits, bursts, and pings. See Burst.

Forest Management Service Center (FMSC). Located in Fort Collins, CO, FMSC is a sub-staff of the U.S. Forest Service, National Forest Management Staff, Washington Office. For more information, go to www.fs.fed.us/fmsc.

Geographic Coordinate System (GCS). A reference system that uses latitude and longitude to define the locations of points on the surface of a sphere or spheroid. A geographic coordinate system includes a datum, prime meridian, and angular unit.

Geo-referenced data. Aligning geographic information to a known coordinate system so that it can be viewed, queried, and compared to other geographic data. Geo-referencing may involve shifting, rotating, scaling, skewing, and in some cases, warping, rubber sheeting, or orthorectifying the data.

DURATION: This amendment is effective until superseded or removed.

Geographic Information System (GIS). An integrated collection of computer software, processes, and data used to view and manage information about geographic places, analyze spatial relationships, and model spatial processes. A GIS provides a framework for gathering and organizing spatial data and related information so that it can be displayed and analyzed.

Global Navigation Satellite System (GNSS). "Global navigation satellite system" is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. A GNSS allows electronic receivers to determine longitude, latitude, and elevation of the receiver on the Earth's surface.

Global Positioning System (GPS). Radio-emitting and -receiving satellites used to determine the location of points on Earth. The orbiting satellites transmit signals that allow a GPS receiver anywhere on Earth to calculate its own location through trilateration. Developed and operated by the U.S. Department of Defense, the system is used in navigation, mapping, surveying, and other applications in which precise positioning is necessary. GPS is a part of GNSS.

GPS angle-point. GPS points (vertices) at distinct changes of direction between lines defining the unit, generally with considerable spacing between each point.

GPS Walk. A unit’s boundary determined by GPS points that are closely spaced, such as an interval of a few steps.

Ground control points. Positions of known value on which the location and accuracy of other measurements depend; for example:

1. Surveyed points on the ground serve to locate aerial imagery, and

2. GPS points can control direction-distance traverses.

Horizontal Dilution of Precision (HDOP). Measurements of the geometric quality (strength of figure) of a GPS satellite configuration in the sky. HDOP is concerned only with horizontal measurement precision. The smaller the dilution of precision number, the better the geometry.

Hit. See Bursts.

Inertial Navigation System Survey. An inertial navigation system survey uses an instrument which is ordinarily composed of motion sensors (accelerometers) and/or rotation sensors (gyroscopes) to measure direction and distance between survey points.

This process is sometimes called dead-reckoning.

DURATION: This amendment is effective until superseded or removed.

Least Count. The least count is the smallest subdivision markings on a measuring tool.

For example, a tape marked with tenths of an inch can be used to measure to the tenth of an inch by reading the markings directly. A tenth of an inch is the least count on the tape.

Estimation is required to measure to the nearest hundredth of an inch. Another example is a compass rose marked in degrees that can be read directly to a degree, making one degree the least count of the compass. Parts of a degree must be estimated.

Leg. A term for a line or vector used to measure a boundary or to navigate. A leg can be a line defined by two end points or a line defined by direction and distance.

Magnetic declination. The angle between magnetic North and true North observed from a point on Earth. Magnetic declination varies from place to place, changes over time, and is subject to local anomalies. Magnetic variation is sometimes used for this term in timber cruising operations.

Metadata. Data (information) about the data. Metadata describes the overall history of specific data, including content, quality, condition, and other characteristics. For more information go to: http://www.fs.fed.us/gac/metadata/

Misclosure. See closing error.

Mosaic. Several maps of adjacent areas with the same spatial reference and scale whose boundaries have been matched and dissolved.

MTDC Accuracy Matrix. Missoula Technology and Development Center (MTDC) spreadsheet of GPS accuracy measurements for given GPS receiver and antenna configurations, measurement procedures, and forest conditions, such as canopy closure.

MTDC follows National Standards for Spatial Data Accuracy (NSSDA) for stated accuracies.

Multipath errors. Errors caused when a satellite signal reaches the receiver from two or more paths, one directly from the satellite and the others reflected from the boles of trees, nearby structures, or other surfaces. This error is particularly troublesome for forest surveyors who unknowingly measure the bounced signal while the direct signal is blocked.

NAD83. See Datum.

National Agriculture Imagery Program (NAIP). NAIP acquires aerial imagery. A primary goal of the NAIP program is to make digital orthophotography available to governmental agencies and the public.

DURATION: This amendment is effective until superseded or removed.

Nationwide Differential Global Positioning System (NDGPS). NDGPS service is organized by the U.S. Department of Transportation. This service uses ground reference stations to determine the differential between the GPS solution and the true solution.

This differential is then broadcast on a low frequency carrier for users to access, free of direct user charge. See Differential Global Positioning System.

National Marine and Electronics Association (NMEA). A nonprofit association composed of manufacturers, distributors, dealers, educational institutions, and others interested in peripheral marine electronics occupations. The NMEA has created a standard that defines an electrical interface and data protocol for communications between marine instrumentation that has been adopted as an industry standard by the GPS industry. GPS receivers receive and send NMEA strings to recorders with appropriate software to perform location and mapping functions.

Offset. A line running parallel to the line being measured. Sometimes this term is erroneously substituted for sideshot. See Sideshot.

Orthophotograph. A vertical aerial photograph from which distortions due to camera tilt and ground relief have been removed. An orthophotograph has the same scale throughout and can be used as a map. The scale is only true at ground level.

Orthophotography surveying requires digitizing orthophotographs for coordinates.

Common short names are orthos and orthophoto.

Positional Dilution of Precision (PDOP). A measure of the geometric quality (strength of figure) of a GPS satellite configuration in the sky. PDOP is a measurement using horizontal and vertical components. The smaller the DOP number, the better the geometry. See Horizontal Dilution of Precision.

Photo-identifiable point (photo ident point). A spot on an orthophotograph, aerial photograph, or similar remotely sensed imagery from which a feature on the natural ground can be inferred, such as road intersections, ant hills, or utility covers. See Ground control point.

Ping. See Bursts.

Post-Processing. Correcting GPS observations after the surveying is complete. Post-processing uses Differential GPS to obtain precise positions of unknown points by relating them to known points such as Continuous Operating Reference Stations.

Precision. The tendency of a set of random numbers to cluster around a number determined by the set, such as the mean of the set. Precision measures the quality of the method used, not the quality of the results (accuracy). Standard deviation is the statistical measurement usually used to report precision. A set of measurements whose range is small is relatively more precise than a comparable set whose range is large.

DURATION: This amendment is effective until superseded or removed.

Projection. A method by which the curved surface of the earth is portrayed on a flat surface. This generally requires a systematic mathematical transformation of the earth's arcs of longitude and latitude onto a plane. Some projections can be visualized as a transparent globe with a light bulb at its center (though not all projections use the globe's center), casting lines of latitude and longitude onto a sheet of paper. The common methods have the paper flat and placed tangent to or cutting through a portion of the globe (a planar or azimuthal projection) or formed into a cone or cylinder and placed over the globe (cylindrical and conical projections). Every map projection distorts distance, area, shape, direction, or some combination thereof.

Root mean square error (RMSE/RMS error). A measure of the difference between locations that are known or considered to be true compared with locations that have been interpolated or digitized. RMS error is derived by squaring the differences between known and unknown points, adding those together, dividing that by the number of test points, and then taking the square root of that result. The FMSC (and timber cruisers) use this process to determine the accuracy of orthophotography for surveying and for determining the quality of GPS observations against known values on GPS test courses.

Real-Time Kinematic/Real-Time Navigation (RTK/RTN). A technique used in survey based on the use of carrier phase measurements of the GPS (GNSS) signals in which a single reference station provides the real-time corrections, providing up to centimeter-level accuracy.

Satellite-Based Augmentation System (SBAS). Nationwide Differential Global Positioning System (NDGPS) and Wide Area Augmentation System (WAAS) are examples of satellite-based augmentation systems for GPS. See NDGPS and WAAS.

Sideshot. A sighting or measurement from a survey point to locate a point which is not intended to extend the traverse line. A sideshot is often made to determine a location of an inaccessible object or a point obscured to GPS observations.

Static angle-points. See Angle point.

Traverse. Historically the term traverse means a direction-distance survey (chain and compass). Colloquialism use has extended the term traverse to sometimes mean any type of survey, including GPS surveys.

Traverse closing error (TCE). See closing error.

Unit. An area defined by a closed boundary: for example, a forest unit, polygon area, or project site.

Vertex. See Angle point.

DURATION: This amendment is effective until superseded or removed.

Vertical Displacement. In aerial photographs, the outward dislocation from the center (nadir) of the top of a tree relative to the base of the same tree. The same condition can occur for elevated ground positions but is not noticeable because there is no base for reference.

Wide Area Augmentation System (WAAS). WAAS corrects GPS signal errors caused by ionosphere disturbances, timing, and satellite orbit errors. It provides vital integrity information regarding the health of each GPS satellite. A WAAS-capable receiver can yield a position accuracy of better than 3 meters, 95 percent of the time, in open sky conditions. There is no fee to use WAAS. For more information, go to:

http://www8.garmin.com/aboutGPS/waas.html.

For additional definitions refer to the GIS Dictionary at:

http://resources.arcgis.com/glossary.

50.6 - Practicality

Natural resource area measurements do not typically follow straight lines or occur on flat ground. Strict adherence to the following rules may not be practical; reasonable judgment should be applied. Operations following the intent of this section are allowable.

For example, boundary lines marked by painted trees usually do not fall in a straight line but are frequently represented by a straight line. It is practical to represent these trees in a straight line as long as the trees fall equally on either side of the line. Lines representing tight curves need to reference each marked tree to represent the curve well.

In another situation, a large unit may need to be broken into sub-units for payment purposes. It is appropriate to measure the area of the total unit to meet the area-error standard. Then, the unit may be broken into sections. Area differences in the sub-units are compensated for when all sub-units are completed, of course, this requires that all the sub-units are completed; if there is a possibility this might not happen or if the prescriptions for the sub-units are quite different, then each sub-unit must be addressed independently.

Still another example would involve a unit intruded by a riparian corridor. It is appropriate to measure the area of the unit first without concern for the corridor. A poly-line for the thread of the stream can be determined by any survey method explained herein. Then, the width of the corridor along the stream-thread may be used to determine the area of the riparian area and subtracted from the total area of the unit.

These examples may be extrapolated to other applications demonstrating what practically may be done. The intent of this section is not to give an open license to ignore procedures, but allows common sense to be applied

DURATION: This amendment is effective until superseded or removed.

51 - REQUIREMENTS

The following metadata are required for area-based sampling methods in timber cruising.

51.1 - General Requirement for All Survey Methods

1. Document acreage determinations, calculations, and methods used; also include a map of the area.

2. Include information about the data and process (metadata). Metadata should include:

a. Thoroughly document the collection procedure or reference the standard procedure/process that was used.

b. Document the data sufficiently to allow users to determine the accuracy, value, purpose, restrictions, and sufficiency of the information.

c. Include equipment details, such as: type, measurement units, least count, and other similar information associated with the data or data collection.

d. Include information about: personnel directing or authorizing the work and procedure, crews performing the survey, field situations, personnel checking and verifying the field work, personnel processing the data, and so forth.

e. Forest, region, district, time, date, sale, and so forth.

f. Mapping projection, coordinate system, and datum used.

g. Adjustments made, such as: GIS modification of GPS features, office corrections to the original data, and transformations or conversions of coordinates.

h. Special conditions that might influence the outcome of the survey.

i. Differential GPS augmentation methods or similar operations (if used).

j. Software programs used for data collection, processing, and so forth. Store appropriate data files used for processing and keep common readable files showing the same general information with the project. These files need to be in readable form, such as text, ASCII, or *.doc files.

3. Involve field personnel in the review and validation process.

DURATION: This amendment is effective until superseded or removed.

Coordinates for determining area may be calculated in any appropriate coordinate system, datum, and projection. The mapping projection, coordinate system, and datum used must always be associated with the coordinates. The process used must be clearly identified for each and every operation, such as data collection, calculations, mapping values, and any conversions or translations performed. Methods of conversion or translations between different systems must be documented. The USDA Forest Service presently requires data be reported in the NAD 83 datum for official mapping operations. Additional information can be found on the Forest Management Service Center (FMSC) Web site at http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml

All measurements include error; therefore, area measurements include error. Estimates of the maximum amount of difference between the true area and the measured area are determined by the estimated accuracy of the points defining a boundary. All methods determine area-error by using point accuracy values for GPS surveys (including orthophotography or similar methods), the closing-distance value of a direction-distance traverse or a similar accuracy metric. The GPS methods estimate the accuracy by using the Missoula Technology and Development Center (MTDC) Accuracy Matrix monitored by FMSC, or using procedures that are similar to or produced the Accuracy Matrix. Metadata must document which procedures are used for each project. For additional information go to http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml.

51.2 - Direction-distance Survey Method

In addition to the general requirements, document the following:

1. Type of equipment used to measure direction.

2. Magnetic declination determination and application or similar direction rotation procedures.

3. Type of equipment used to measure distance, such as laser, chain, or other tool.

4. Type of vertical or inclination angle tool and other significant tools.

5. The means used to change slope to horizontal distance.

51.3 - GPS Walk and GPS Angle-Point Survey Methods

In addition to the general requirements, document the following:

1. GPS receiver type, model, and serial number.

2. Native datum, native projection, or geodetic system.

DURATION: This amendment is effective until superseded or removed.

3. Precision indicators, geometric strength indicators (such as Horizontal Dilution of Precision (HDOP)), filters or masks, and so forth.

4. Possible error sources that may cause interference, such as signal attenuation, multipath, or blockage.

51.4 - Orthophotography Survey Method

This method of determining area for area-expanded cruises can only be used with approval from the regional forester. However, there are no restrictions for the use of orthophotography for inspection of other survey methods as stated in sections 53.2 paragraph 8d, or for incidental determination of corners due to missing/lost data in GPS or direction-distance traverses, or corridor lengths, or similar operations were the area determination is only supplemented by orthophotograhy positions.

Orthophotography used to gain boundary coordinates must meet or exceed the following conditions to be acceptable for timber cruising work. The source and use of such information must be documented:

1. For USDA Forest Service timber cruising purposes, the pixel size should be one meter or less, unless otherwise authorized. Be aware that the size of the pixel is the precision of resolution of the image and not the accuracy of the point the pixel represents.

2. Orthophotography imagery meeting National Mapping Standards does not necessarily guarantee that digitized (or measured) coordinates are suitable for the project. Be sure the imagery is from an approved source, and even then, perform checks to verify the accuracy of the image’s geo-referencing.

3. Check the accuracy of the orthophotography if there is any question about the quality of the source materials. Verify orthophotography imagery accuracy by comparing selected control points across the imagery with superior control, such as GPS points or previously validated imagery. Additional information available at:

http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml.

4. Obtain or define the metadata documenting the orthophotography source, resolution, accuracy, projection, datum, photography date, and other relevant details. When possible, use Forest Service-approved orthophotography.

5. Two different measurement accuracies concern the timber cruiser. For cruising area-determination purposes, the relative accuracy should be used. This is the accuracy of the pixel points with regard to each other, but not how well all the related points are geographically referenced. At this time, the range of relative accuracy from approved sources is from one to about fifteen meters. (This is appropriate for most area and area-error determination.)

DURATION: This amendment is effective until superseded or removed.

6. The geographic coordinate absolute accuracy ranges from about 3 meters to 17 meters.

(This is applicable for absolute geographic location in most timber activities.) There is frequently more distortion on the orthophotography where the images are joined (mosaic lines). Skilled and trained personnel are needed to use imagery when mosaic lines are inside the unit being measured.

7. Generally, for one-meter resolution orthophotography, the relative accuracy is less than 8 meters and quite possibly as accurate as 3 meters.

52 - TRAINING

52.1 - All Survey Methods Training

Train all crew members in the correct use and care of appropriate equipment for the appointed task. Training should go beyond the proper use of equipment by including proper application and calibration of the equipment. Furthermore, since determining area involves measurement, mapping, and navigation, the cruiser should be trained in the knowledge of: equipment precision, accuracy, mapping, and proper recording of relevant information in the metadata.

52.2 - Direction-Distance Survey Method Training

In addition to the general training requirements, conducting a direction-distance traverse requires training in:

1. Use of the sighting compass. See http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml.

2. Azimuth and magnetic declination.

3. Significance of traverse closing errors and adjustments (sec. 54.1)

4. Slope chaining and inclination measurements.

5. Use of a distance meter (such as a laser).

6. Other devices or systems that produce line vectors, if used.

7. Observer alertness to avoid measurement blunders.

52.3 - GPS (GNSS) Survey Methods Training

In addition to the general training requirements, conducting a GPS or satellite-navigation-system survey requires training in:

DURATION: This amendment is effective until superseded or removed.

1. Awareness of datum and projections.

2. Units of measurement and conversion.

3. Unit formats.

4. Proper setting to collect three-dimensional positions.

5. Satellite geometry and associated satellite observation errors.

6. Knowledge of GPS receiver peripherals, such as antennas and recorders.

7. Location accuracy augmentation methods (such as WAAS or any other SBAS, satellite based augmentation system, or NDGPS—National Differential Global Positioning System).

8. Knowledge of differential correction procedures and data export (if used).

9. Observer alertness to avoid obvious measurement blunders.

52.4 - Orthophotography Survey Method Training

In addition to the general training requirements, train all crew members in appropriate methodology for determining survey points from orthophotography. The forester shall have photogrammetric training, be able to understand aerial photography distortions and errors, and be able to use proper photographic interpretation procedures. The person digitizing the location shall understand and recognize image error. Train crewmembers in using only appropriate imagery and verifying the accuracy of an image.

Individuals should have orienteering map skills to help analyze objects in the photography. They should have knowledge of how the terrain at the project site is uniquely portrayed. Different areas of the country may demand new terrain familiarization to perform accurate photo-identification.

Train observers to be alert to see possible problems and avoid measurement blunders.

53 - SURVEY METHODS

53.1 - Direction-Distance Survey Method

The distance-direction traverse employs traditional survey techniques to measure area using directions (angles, bearings, or azimuths) and distance. Originally, a directions-distance traverse encompassed an entire unit and had to meet the 20-acre rule. Expanding the original process allows a direction-distance traverse between control points using the 5,000-foot rule (see sec. 55.1).

DURATION: This amendment is effective until superseded or removed.

Use instruments capable of meeting accuracy standards. Record all raw measurements.

Document all techniques used and the results of calculations. Inertial navigation system surveys may use procedures explained by this method.

53.2 - GPS (GNSS) Survey Methods

The Global Positioning System (GPS) utilizes signals transmitted from satellites to determine the coordinate location (X, Y, and Z) of points on the ground. Only the X and Y coordinate values are relevant to area determination. When these points are located on the perimeter of a closed traverse, they form a polygon whose area can be calculated. The Global Navigation Satellite System (GNSS) refers to international point-navigation-time satellites systems of which GPS is the United States of American part.

1. Boundary angle-points are determined by the GPS position (or sideshot or traverse points relative to the GPS position). The boundary of the unit is determined by the lines (legs) that join these boundary angle-points.

2. Before taking measurements, allow the GPS receiver to “warm up” for a sufficient amount of time to download a current GPS ephemeris. This may take as long as ten minutes and may be done while walking to the site if the antenna is kept in view of the satellites (avoid heavy canopy closure and obstacles).

3. During measurements, position the antenna of the GPS receiver above local obstacles (including the body) as much as possible. Take care to avoid situations that could cause multipath errors (reflected, redirected, or bounced signals) such as proximity to cliffs, large rocks, buildings, trucks, signs, power lines, and nearby large tree boles. If these situations cannot be avoided, consider alternative methods of measuring the boundary.

4. GPS receivers calculate a position when they receive signals from satellites. Any interference with these signals can cause errors in positioning. The optimal observing environment is a clear, open sky with little or no canopy closure.

5. Use only approved GPS receivers (see sec. 56) that meet the accuracy requirement necessary for determining the desired area accuracy. See:

http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml. (The required accuracy standards needed for each application are explained throughout this handbook.)

6. Re-collect the data if the established standards are not met. Data may be collected for different portions of the boundary on separate dates or times.

7. Report area in the accepted Forest Service-required datum. Unless otherwise required, use the NAD 83 datum. The metadata must show in which datum and projection the survey was collected and in which datum and projection the report is made.

DURATION: This amendment is effective until superseded or removed.

8. Include with the project:

a. Data files containing the position fixes or results from those fixes (such as, *.ssf , *.cor, *.inf, *.txt, *.tt, *.gpx, or similar file types).

b. Data files that are readable, such as text files and spatial files like Shapefiles (*.shp, *.dbf, *.shx, or *.prj). These files should be populated with all relevant attributes.

c. Method used for area calculations and any software used.

d. Plot of the survey in a digital format. Overlaying the plot on an orthophotography image is strongly suggested for checking purposes.

e. Date and name of the persons who collected the data.

f. Only filtered information from a GPS receiver is adequate for area-determination measurements. Filtered information means satellite information that has been inspected, ordinarily by software, before acceptance into a position calculation. The filtering process checks whether criteria such as HDOP limits and 3D mode are being met. Signals not meeting defined criteria are rejected for measurement. Some GPS receivers can filter satellite information as it is collected: this type of information does not need further filtering. For GPS receivers that do not do this, software to filter GPS information data or similar procedures can be used. This second situation usually requires that the GPS receiver have an internal operating system or be connected to a data recorder with an operating system that can run filtering software.

Programs like TwoTrails© and ArcPad© can be configured to meet this requirement.

See: http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml.

9. Use only three-dimensional GPS measurements. This requires a minimum of four satellites. (Note: GPS satellite measurements are made in the 3D mode but area measurements are made in horizontal 2D space. These are essentially two different definitions for different applications.)

10. HDOP may be set to maximum productivity. Setting the HDOP to 20 or less is simply good practice; setting it to 6 or less may improve accuracy if that is needed to meet area-error limitations. Future setting may change as technology develops. See:

http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml.

11. SBAS (Satellite Based Augmentation Systems) should be used where available, such as WAAS (Wide Area Augmentation System for GPS).

12. See GPS Augmentation (sec. 53.24) for possible additional observation restrictions.

DURATION: This amendment is effective until superseded or removed.

53.21 - GPS (GNSS) Walk Method

1. To measure the boundary line, the field person must walk directly on the boundary

(ex. 01 of this section) or use and note appropriate sideshots or offsets. Use another survey method if the desired accuracy is not possible by this method, such as working in canyons, heavy tree canopy, steep northern slopes, or cliffs.

53.21 - Exhibit 01

Walk Method Example.

Dots Depict Frequent Position Measurements While Walking the Unit

2. Any distance the field person walks off the boundary line that is not noted is improper and adds unknown inaccuracy to the boundary and, consequently, to the area. Both the inaccuracy of the GPS measurement plus the distance the observer is off-line contribute to error in measuring the area. This handbook describes methods to estimate the GPS accuracy; walking off the boundary without referencing it with an offset or sideshot is a blunder to be avoided.

3. Collect position fixes (coordinate values determined by the GPS receiver from satellite signals) while moving around the perimeter at a time interval that accurately defines the boundary. In general, a 5-second interval can be used for a walk file. However, if moving more rapidly, a more frequent interval may be suitable. Loss of position fixes (satellite signal) is acceptable for a short duration while moving in straight lines;

however, position fixes must be acquired while moving through turning points (corners or curves) to portray the polygon correctly.

DURATION: This amendment is effective until superseded or removed.

4. The receiver must be receiving and recording positions at major direction changes in the unit’s boundary. Ensure this by walking slower around bends and monitoring the receiver. See that all fixes continually move forward and that they do not bounce backward relative to a previous fix. This can happen when the GPS inaccuracy is bigger than the actual distance covered between fixes.

5. Substantial spacing between position fixes indicates a loss of signal and the possibility of missing positions at critical turning points. The polygon shape must represent the boundary walked in the field. If not, the survey must be redone.

6. The field personnel should review the resulting map. The collected data must consist of a smooth string of fixes. The position fixes must lie in order, one after another, in a sequential pattern. GPS walk method boundaries of units are often jagged. Frequently, adjacent units have coincident sides that do not match. Allow only slight irregularities (such as jumping from side to side) in taking fixes.

7. When all measuring conditions are appropriately observed and the roughness of the boundary appears to be within positional tolerance defined for the project, it is allowable to smooth the lines. If some irregularities are obvious outliers, they may be deleted upon the advice of the field personnel. It is inappropriate to eliminate small irregularities without proper reason. Eliminated points should be noted.

8. If coincident boundaries of adjoining polygons are modified to match each other, comments must be placed in the metadata as to which boundary segments were maintained and which were modified.

9. Boundaries may be constructed from separately measured polylines created by the GPS walk method taken at different times and in different directions.

DURATION: This amendment is effective until superseded or removed.

53.22 - GPS Angle-Point Method

In this method, the observer measures the unit’s boundary angle-points (vertices) by occupying only the points on or near a boundary direction change. The observer measures while on the boundary point or takes sideshots (or similar measurement) to the angle-points when they are near the boundary change (ex. 01 of this section).

53.22 - Exhibit 01

Triangles are GPS or Ortho Control -- Double Lines are Boundary -- Dashed Lines are Sideshots. The Southwest Points are Shown as Sideshots but They Could be Traversed

Points if the Sideshots are Not Possible

1. It is best to support the receiver’s antenna in a stable manner, such as a tripod or staff, which is held reasonably steady while collecting satellite information. (This is not mandatory, but has proven to yield better results.) Natural resources surveying allows the receiver to be “wiggled” slightly, such as rotating the antenna around the survey pole or wrist.

2. Boundary angle-points located in poor environmental conditions suffer poor GPS signals. It is better to obtain a position in an open site and then take sideshots (or traverse) into the covered or blocked location (ex. 02 of this section).

DURATION: This amendment is effective until superseded or removed.

53.22 - Exhibit 02

A Sideshot Diagram

3. Sideshot positions are created by measuring the direction and distance from a good GPS location to a nearby boundary point. The sideshot should be short, rarely more than 30 meters, because longer distances can produce greater error (for example: a 1-degree azimuth error in a 30-meter sideshot causes about a half meter position error at the sideshot point). Longer sideshots can be taken when the boundary point being determined by the sideshot is perpendicular to the boundary, as when measuring across a narrow strip unit. Observations should be taken very carefully when the sideshots involve sharp direction changes along the boundary.

4. Direction-distance traversing from an open-site GPS point into heavily canopied boundary points and back out to another good GPS control point is an excellent method to locate points where GPS signals are obscure. See section 53.4, exhibit 01.

5. Observe and average a sufficient number of good fixes. Good fixes meet filtering criteria such as 3D, appropriate HDOP, and so forth. Averaging 60 good GPS fixes has proven to be sufficient for positions that require more solid determinations. Lesser fixes may be averaged for more general measurement needs as explained in other sections of this chapter. See http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml for changes in averaging requirements as new technology is developed.

6. Choose another survey method if the satellite signals are suffering error such as multi-path or being blocked. It is important that the observer continually monitor the quality of the fixes and only average appropriate values.

DURATION: This amendment is effective until superseded or removed.

53.23 - GPS Receiver Always Active–Logging Only at Features (Points)

This method modifies and uses parts of the two previous methods. This method is quite efficient. Accuracy is quite good relative to the other methods and the time to observe a point is reduced greatly. The problem of the walk-method being off the line being measured is lessened in this method. Further, this method takes much less time than the angle-point-method.

Have the GPS receiver on but not logging as you walk to the point. Upon arriving at the point or feature, stabilize the receiver in an appropriate manner as explained in the previous sections (usually at head height over the point to be measured). Then immediately take 5-10 measurements and average them to determine the location coordinates.

When sideshots are used, make them after determining the GPS location. Be sure to leave the receiver active and orientate the antenna reasonably well, but do not log positions when moving between points or features. For more details and field reports, see http://www.fs.fed.us/fmsc/measure/geospatial/index.shtml.

53.24 - GPS Augmentation

Using real-time differential methods to improve the GPS positional accuracy is encouraged.

Examples of such methods are: WAAS, National Differential GPS, and real-time kinematics.

Post-processing GPS surveys can be beneficial, but is not necessary or mandatory for timber cruising work. Studies by MTDC have shown that data collected with WAAS or NDGPS is rarely improved significantly with post-processing.

Real-time measurements are often used in field work, but later post-processing may be desired.

Special requirements are necessary if post-processing is expected to be done at a later time to improve the measurements. If post-processing is expected, follow the manufactures guidelines to be sure the data collection is appropriately set for post-processing.

The following special requirements may be needed to properly post-process GPS information relative to…