52.2 - Global Positioning System (GPS). GPS is a system utilizing signals transmitted from satellites to determine the coordinate location (x,y,z) of points on the ground. When these points are located on the perimeter of a closed traverse they form a polygon, and it's area can be calculated. 52.21 - GPS Traverse Standards. The following standards apply to GPS traverses used for area dependant cruises, appraisals, timber sale contracts, and various post-sale purposes.

1. All crew members using the GPS equipment for timber sale preparation shall be trained in its use. It is important that users have a basic understanding of how GPS works before collecting field data.

2. All positional fixes must be differentially corrected to a reference/base station that is no farther than 500 kilometers (300 miles) away. Position fixes collected by the remote GPS receiver shall be collected under the following conditions:

- a Position Dilution of Precision (PDOP) no greater than 6.
- a minimum satellite elevation angle of 15 degrees.
- a minimum signal level of 6, or the manufacturer's recommended level for good signal quality.
- in the 3-D manual mode.

3. The reference/base station shall be of a third order (or better), NAD 83 datum position.

4. Position fixes shall be collected while moving around the perimeter and shall be collected at a time interval that will accurately define the perimeter of the traverse. A 5 second interval is suggested for a walk file, however if moving more rapidly a 1 second interval may be appropriate. Loss of position fixes (signal) for a short duration while moving in a straight line is acceptable, however position fixes must be received while moving through all turning points inorder to portray the true representation of the traverse.

5. Only 3-D positions that are differentially corrected will be used for area determination. A computer display or plot of the file should show positional fixes lying one after another in a relative sequential/pattern, defining the perimeter of the traverse. There should be only slight irregularities (jumping from side to side) of positional fixes, if some of these are outliers (do not represent the traverse) they should be edited from the file. A computer display or large scale plot of the data should be reviewed by the person who collected the data and it should be reviewed for the following:

- consistency of data collection (look for a smooth string of fixes).
- large spacing between points. Large spacing would indicate loss of signal and the possibility of missing positional fixes at critical turning points that would define the traverse.
- the polygon should resembling the traverse walked in the field.

At this time if the reviewer is confident that the data collected meets the above standards, an area calculation can be performed. This calculation shall be made using NAD 83 datum.

6. If the established standards are not met, it will be necessary to recollect the data. Data for a portion of a traverse or in the case of a large traverse, may be collected at different dates and times as long as it is collected while moving in the same direction as the origional traverse.

7. The following information will be kept as part of the final record:

- the corrected data file containing the differentially corrected positional fixes in NAD 83 datum.
- a plot of the traverse at 1:24,000 scale in the current (NAD 27) datum. This is because all of our primary base series maps are displayed in the NAD 27 datum.
- the date and name of the person who collected the data.
- the name of the reference/base station used for the differential corrections.
- the method used for area calculation (software).

Please send Comments to: Tony Jasumback, Missoula Technology Development Center, USDA Forest Service or DG: A.Jasumback:r01a