Instructions on taking GPS data into a GIS
v.1.1, 3june98

Note, in the following documentation:

*	GPS field receiver = rover (contrasted with fixed receiver or base station)
*	menu selections on the receiver are noted below as, for example, GPS Status > Sat Hlth & URA, meaning select GPS Status on the Main Menu and select Sat Hlth & URA from the GPS Status menu
*	press the ESC key to move up one menu level, press the H and P keys to scroll up or down through the choices in a menu or display, and press the F key to make a selection
*	to turn the rover off, you must depress the power key for a 5­second countdown

Collecting GPS Data

The GPS field receivers owned by the department can be used easily to collect position information for both points and features. A point would be useful to map things such as sampling or well locations. Examples of features to be mapped would be the outline of a lake, or a line corresponding to a transect.

In order to gather data as quickly and accurately as possible, a few steps should be taken before the GPS receiver is taken out into the field.

The day before you take the GPS receiver out into the field, you should make sure the batteries are charged and you should collect an almanac file. Collecting an almanac file involves turning the receiver on and letting it sit in one place, with a clear view of the sky, for about fifteen minutes. This will allow the GPS receiver to locate itself faster in the field the next day.

Once in the field, it is necessary to allow the rover to locate itself. You will know when this happens when the rover displays GPS POSITION under the Position menu, rather than OLD POSITION. The GPS POSITION display will be updated every second, and the display will show 3D in the lower left corner, indicating a position fix in 3 dimensions.

You can review the satellites tracked by the receiver under the GPS Status > Sat Tracking menu. The arrows to the left of The double digit numbers correspond to the satellites, and the satellites designated by arrows are the ones the receiver is currently tracking. Four satellites are necessary for accurate locating. The bar chart in the lower left of the Sat Tracking display is an indication of signal:noise ratio (SNR); the receiver will not collect data if SNR < 5. The PDOP (position dilution of precision) value in the lower right of the Sat Tracking display reflects the geometry of the satellites; lower values are better, and the receiver will not collect data if PDOP > 6 (you can change this threshold if needed), although it will try to switch to other satellites in the constellation to improve the PDOP.

Whether you are collecting positions for a single point or for a feature, a roving file is appropriate. Under the Data Capture menu, select Open Rov. File. If you are collecting positions for a point, stand still until about 100 to 200 positions are calculated. The number of positions calculated is reported in the upper right corner of the display. When the positions are calculated, select Close File, and then YES. Your file is automatically saved.

To collect positions for a feature, open a roving file as in the above paragraph. Then, once positions are being collected, walk the feature to be mapped. Walk at a comfortable pace, covering about 1 meter every 1 or 2 seconds. You may continue to walk as long as positions are being collected. If collection ceases, pause until they begin to be collected again. When you have walked your feature, close the file and move onto your next collection point.

Differential Correction of GPS Data

In order to correct the files you collected in the field, you must have access to the Pathfinder Office software. The computer will need to have the software key (the green 'dongle') plugged into its parallel port and the data transfer cable connected to its serial port (COM1). Once at the computer that has this software:

1.  Download rover files to the project directory

a. Rover filenames have the format Rmmddhhi, where

	R=rover (you can change this prefix on the rover)
	mm = 2 digits for the month
	dd = 2 digits for the date
	hh = hour of data collection (time zone is usually UTC, but can be set otherwise on the rover; be sure you know!)
	i = letter in sequence denoting order of file creation during the hour (a, b, ...etc.)

	b. connect the transfer cable between the PC and the rover, then turn on the rover
	c. Start Pathfinder Office, and select the desired project (a dialog may display allowing this choice when you start the program or select Projects from the File menu; if necessary, create a new project).
	d. select Data Transfer from the Utilities menu; the dialog box will display the files available on the rover, select the ones you want to download, click Add, and then Transfer. If you intend to download all of the files on the rover, click Add All. If you don't see any files displayed, make sure the Data Type box just above the file list is set to Data
	e. the data will be downloaded into the c:\Pfdata\<project> directory and copied to the c:\Pfdata\<project>\Backup subdirectory . The files will be copied and the .ssf extension added to each filename.
	f. click Close to leave the Data Transfer dialog, turn off the rover, and disconnect the rover from the PC.

2. You can now display your data with the understanding that there will be drift introduced by anti-spoofing in the satellite signals and there may be large discontinuities that result from the rover switching among different constellations of satellites. Select the files you want to display with the Open dialog on the File menu, and set the View menu to the Map option.

3. In order to differentially correct your data, you need base-station data, which are available for many areas from the National Geodetic Survey (NGS), which is a division of the National Oceanic and Atmospheric Administration (NOAA), through the CORS (Continuously Operating Reference Station) network at www.ngs.noaa.gov/CORS/cors-data.html.

	a. to get base station data, select Differential Correction from Pathfinder's Utilities menu.
	b. select the files you wish to correct using the Browse option in the Rover Files window.
	c. select Internet Search in the Base Files window to locate the files. Select the appropriate Base Data Provider ( in the Lehigh Valley - Wilkes-Barre), click OK in the Internet Search window (if necessary, click Yes to confirm Internet Setup). The program will go to the above site, find the files, download them to Pathfinder Office, and unzip them. In the Confirm Selected Base Files window which opens, check to see if there is base-file coverage for each rover file you want to correct. When you click OK in this window, a Reference Position Window will appear; here you may select a coordinate system, or default to Lat/Long. Once you click OK, your files will be corrected. The corrected files will all have the same name as the corresponding .ssf file, but with a .cor extension. The .ssf file is saved (and you still have the backup files in your <project>backup directory).

4. In order to get the files into GIS, they must be exported into the proper format, which depends on whether you want to use ARC/Info or ArcView.

	a. from the Utilities menu, select Export.
	b. using the Browse option, select the set of corrected files you wish to export as one file for the GIS.
	c. Select the appropriate setup for the destination GIS, select Change Setup, and click Positions Only. You can make other changes to the setup as needed.
	d. if each .cor file corresponds to a point, select the option that allows you to export the entire file as one point [one point per input file (mean position)]; it will average them all and export the file according to your setup instructions.
	e. if the .cor files are features, select the option which allows you to export the file as one line per input file, or one area per input file. The features will be exported as a set of files according to the setup instructions.
	f. Pathfinder Office always exports files with the name posnpnt.xxx, posnlin.xxx, or posnarea.xxx, depending on the file type (where xxx is an extension that depends on the setup). You must manually change the file name (using the Windows95 Explorer) if you plan on exporting more than one file at a time.
	g. The .pts, .gen, .dbf, and .shp files are necessary to build ARC/Info coverages. If you are exporting many files at a time, it is only these whose names must be changed, the rest of the files may be overwritten.

5. At this point, the exported files can be transferred via ftp to a directory where ARC/Info can be accessed, or they can be moved to the appropriate directory on the PC for ArcView.

Changing Exported Files into GIS coverages

Once in ARC, use the generate command to begin creating a coverage. At the ARC prompt type:

	Arc: generate <coverage name>
	Generate: input <.pts file, or .gen file, or .shp and .dbf files >
	Generate: line, or points, or polygons depending on the file type
	Generate: quit

The program will create external tic and boundary files and a coverage will be created.

If there is more than one input file, they can be either inputted in the generate command, or added into a text file and then imported with generate.

The describe command will allow you to look at the format of your coverage.

From here, you may use the necessary arc commands to build and edit your coverage.

created by Erika Hammar-Klose May 1998, modified by Carl Moses June 1998