> ![](media/image1.png){width="0.6041666666666666in"
> height="0.7916666666666666in"} 11/13/2023 SLS
> g:/docs/HOWTOs/GPS_INFO/GPS_Differential_Correction.doc

# Spatial Analysis Center

## Center For Resources

> Yellowstone National Park
>
> GPS Differential Correction with PathFinder Office:
>
> This HOW-TO describes how to differentially correct a GPS rover file
> using PathFinder Office software (version 2.1 and later).
>
> **Introduction:**
>
> Differential correction of GPS files is the process of combining
> information in a *base station file* with that in *rover file* to
> obtain high accuracy files. Rover files are those collected in the
> field while surveying features. Base station files are those collected
> by fixed GPS receivers at various locations. Currently, Yellowstone
> does not have its own base station, so these files must be procured
> from other locations. The final step in differential correction is
> exporting the file to another software format, such as ArcView (GIS)
> shapefiles or MicroStation (CAD) design files, and performing checks
> and manual edits on the data.
>
> **The Steps:**

1)  Create or open a Project in PathFinder Office

2)  Transfer rover file from DataLogger to the computer

3)  Download Base Station files

4)  Perform the differential correction

5)  Export file

6)  Edit and check data.

> **Create PathFinder Office Project:**
>
> \*\*\* Temporarily, in order to run Pathfinder, you must be logged on
> as **guestarc**. We hope to have this problem solved soon! \*\*\*

PathFinder Office is currently on *Populus* and *Mollisol*. Hopefully we
will have it installed on a couple more machines soon. Use the shortcut
to Pathfinder already on the desktop to start the program. You'll be
asked to select or create a "project." Projects are directories on the
hard drive that will hold your rover, base station, and export files.
Try to create your project in your *personal/project* directory.

> **Upload Rover files from DataLogger:**

Connect the DataLogger to the computer via the cable that's connected
the Office Support Module (OSM) or the Trimble Support Module (TSM)
(*the battery chargers*!) (if it's not there, find someone to yell at).
The TSC1 DataLoggers use the cable with the round plug on the end. The
GeoExplorers use a black cable with a round plug on the end that\'s NOT
connected to either an OSM or TSM. Then, from the DataLogger's main
menu, select \"**File Manager**", then "**File transfer**."

In PathFinder Office under the \"**Utilities**\" menu, select "**Data
transfer**."

**\*\*\* If you are downloading from a ProXR or ProXRS unit, you must be
on COM2. If you are downloading from a GeoExplorer, you must be on COM1.
If you are unable to connect to your datalogger after selecting \"Data
transfer,\" this probably means you\'re on the wrong COM port. The COM
port can be changed in the \"Data transfer\" window. \*\*\***

After attempting to connect to the DataLogger, a menu will appear that
allows you to select which files you'd like to upload to the PC. Select
the files you need and hit the \"**add**\" button. Make sure your
Destination Directory is correct, then press the "**transfer**" button
and the selected files will be transferred from the datalogger to your
directory. Once all files are transferred, press the "**disconnect**"
button, and you can unplug the datalogger and turn it off. Do not delete
your rover files from the datalogger yet!

> **Downloading Base Station Files:**

You'll need to download all of the files for the hours you surveyed.
Open your rover file(s) to be corrected in Pathfinder Office. Select
***View/Timeline*** to display the visual record of when (local time)
data were collected; this will help you decide which base files are
needed. If you were recording locations in the field from 10:25 am to
2:15 pm, you'll need the files for that day for hours 16, 17, 18, 19,
and 20 (assuming the survey was in the summer).

Base station files can be downloaded over the Internet or from and FTP
site. We use the following sites:

**FTP Sites:**

(easier to download from these sites)

*[Idaho Falls, ID GPS Community Base Station]{.underline}:*

ftp.inel.gov/pub/gps

*[Helena, MT GPS Community Base Station]{.underline}:*

ftp.mdt.state.mt.us/GPSFILES/SSF_FILES

**Internet Sites:**

*[Idaho Falls, ID GPS Community Base Station]{.underline}:*

ftp://id.inel.gov/pub/gps

*[Helena, MT GPS Community Base Station]{.underline}:*

ftp://ftp.mdt.state.mt.us/GPSFILES/SSF_FILES

*[Missoula, MT GPS Community Base Station]{.underline}:*

http://www.fs.fed.us/database/gps/missoula.htm

*[Casper, WY GPS Community Base Station]{.underline}:*

http://www.sdvc.uwyo.edu/cme/cbs.html

> Not all of these stations are operational at any given time, so if you
> can't find a file at one site, try another, *but remember that the
> closer the station is to where you collected data, the more accurate
> your correction will be!* ***NOTE:*** Base station files are not
> posted immediately. It can take up to 24 hours before files are
> available. Also, some "surfing" of each site is required to get to a
> listing of base station files.

Base station files use a standard naming convention. The first letter is
base station site code, the second digit is the last number of the year,
the third and forth digits are the month, the fifth and sixth are the
day, and the seventh and eighth are the hour. The date and time are
usually Mean Greenwich, or ZULU, time (6 hours *later* than in
Yellowstone in the summer -- no daylight savings -- and 7 hours in the
winter -- with daylight savings). For example, the file "I8041422.EXE"
contains the base station information for the INEEL site that was
recorded at 10 pm to 11 pm on April 14^th^, 1998, Mean Greenwich time.
The file extension is usually ".EXE" which is a self-extracting
executable, or "\*.SSF" which is the standard base station file
extension.

Once you've decided on the files you need, right-click files in the web
browser and select "Save Target As..." to download the file to a spot on
the hard drive. Or select the files from the ftp site and download all
at once.

**Performing the Differential Correction:**

Gladly, this step is one of the easiest. Under the "**Utilities**" menu
in PathFinder Office, select "**Differential Correction**." You'll need
to select the file or files that you'd like to differentially correct.
You'll also need to select the base station files to use for the
correction. In most cases, hitting the "**Autolocate**" button will
search the available base station files and automatically select the
needed files. After selecting the base files, a window will open
entitled, "**Confirm Selected Base Files**." The "**Coverage**" column
should read "Full;" if it reads "Partial," you will need to designate
and/or download additional base files to correct your entire survey
period. Sometimes, even when you know you have all the base files you
need, the coverage will read "partial." In these cases, just go ahead
and do the correction -- it should work. When the "**Reference
Position**" window opens, select "**Set From Base File**" and then OK..

Once you've specified the input rover files and base station files,
verify that the output directory (where the program will store the
corrected files) is correct, and hit the "**OK**" button. A progress bar
should appear while the files are being corrected. Pay attention to the
results-you need to check if all features in the rover file were
corrected. If your base station files didn't cover the full time span of
your rover file or if there was a problem with one of the base station
files (we've seen both happen), they may not have been. If you do think
you have a "problem" base station file, you'll need to download one from
another base station.

The corrected file will have the same name as the rover file with a
"\*.cor" extension.

**Exporting**

Under the "**Utilities**" menu in PatherFinder Office, select
"**Export**." The dialog that appears has options for the type of file
you'd like to export along with specific projections. For each format,
there's a predefined set of format parameters. We normally use the
"**Sample ArcView Shapefile Setup**" format, with the projection set up
as follows:

System: Universal Transverse Mercator

Zone: 12 North

Datum: NAD 1983 HPGN (Wyoming)

Coordinate Units: Meters

In most cases, these defaults are fine. Just look them over to verify
that each looks reasonable.

*NOTE:* There will be three files created for every feature type for
ArcView formats (with \*.shp, \*.shx, and \*.dbf extensions). For
example, if you collected points and lines in your rover file, the
export routine will create three files for the point features and three
files for the line features.

> **Editing and Checking:**
>
> Editing and checking the data is beyond the scope of this document.
> This section is just a strong reminder that the work is not complete
> just because you've created the differentially corrected files. The
> exported files need to be edited, perhaps attributed, and definitely
> documented (remember *metadata?*). Good luck!
>
> **\*\*\*\* Once you have completed the differential correction and are
> sure your data are correct, it is then safe to delete your rover
> file(s) from the datalogger.**