Trimble XRS Pro GPS Instructions for the LTER Laboratory


In use from 2008-07-15 to 2012-03-30


The LTER GPS units are used for recording the locations of microplots, experiments,instrumentation, plot corners, sampling sites, events and many other LTER features that need to be mapped in the LTER GIS.
They are also used to navigate back to these locations.

These instructions detail how to use the GPS in normal conditions and more challenging situations and also how to use the software in the lab to download, upload and convert data.

When using the Trimble XRS Pro GPS with the Omnistar real-time correction service,
we can expect submeter accuracy (+ 70 cm or better).





The KBS LTER has two identical Trimble GPS units. “GPS I” is to be used exclusively by the KBS LTER staff. “GPS II” can be loaned to others with permission by the LTER Project Manager.

Two components make up each GPS unit: the Trimble Data logger type TSCe with touch screen, and the Trimble Receiver Type Pro XRS model 33302-51.

The GPS can be used in temperatures from -4 F to 140 F. The logger is designed to withstand accidental immersion so it should be fine to use it in the rain.

The data logger is set to power off after 10 minutes of inactivity to conserve the batteries.

Start, Settings, Control Panel, Power Off tab, “On Batteries suspend after: 10 minutes”


  1. TerraSync – Used for collecting and updating GPS data on the Trimble data logger.
  2. Trimble GPS Pathfinder Office (Version 2.9)- Used to manage and process GPS data on the PC.
  3. Microsoft Active Sync – Used to connect your PC to the Trimble Data Logger in order to transfer data.
  4. Omnistar is a GPS service that gives real-time differential correction. The Omnistar service will be maintained on “LTER GPS I”, if funding allows. Omnistar service will not be purchased for LTER GPS II unless another user requests to borrow the unit and pays for the Omnistar service. See the Appendix for service details.

Set Up

GPS Mission Planning

GPS mission planning allows you to determine what time of day the satellites are in suboptimum position so that you can avoid collecting GPS data during these times. Mission planning is critical when collecting GPS data in forested areas or other challenging sites (see “Collecting Data in Trees or near Obstructions” in the Appendix).

  1. The Planning Software should be installed on the two computer’s in the Robertson Lab that have Trimble’s GPS Pathfinder Office software, however, if it is not installed then do the following.
    1. Go to Trimble website.
    2. Click on: “Support and Training”
    3. Scroll down and click on: “Planning Software”
    4. Click on: “Downloads”
    5. Download: Trimble Planning Software
  2. Update the ephemeris file if it has been more than one week since it was last updated.
    1. Go to Trimble website:
    2. Click on: “Support and Training”
    3. Scroll to and click on: “Planning Software”
    4. Click on: “Downloads”
    5. Download: “Ephemeris File” Save it to D:\Program Files\Common Files\Trimble\Almanacs\current.ssf.
  3. Open GPS Pathfinder Office.
  4. If the “Select Project” window appears, deselect “Display this dialog at start-up”.
  5. Click “Utilities” on the Tool Bar.
  6. Scroll down to “Quick Plan”.
  7. “Select Date” window appears. Choose the date of interest, and click OK.
  8. “Edit Point” window appears.
  9. Choose “City” and select Battle Creek, MI
  10. The Status window opens.
    1. Choose “Options” on the Tool Bar and scroll to “Elevation Mask”, enter 10 degrees.
  11. To view the PDOP status
    1. Click “Graphs” on the Tool Bar, and scroll down to PDOP. A graph of PDOP over time will be displayed. You should plan to do your GPS data collection when the PDOP is equal or less than 4 if possible. See the “Understanding PDOP” section in the Appendix for an explanation of PDOP.

Data Dictionary

A data dictionary is used in the field to control the collection of features and attributes. It prompts you to enter information or select from a menu of choices to ensure that the data collected is complete, accurate, and compatible with the LTER database and GIS.

  1. A data dictionary called ’KBS LTER already exists and should be loaded on both data loggers.
  2. If you are collecting GPS locations for a feature that is not already defined in the data dictionary, you will need to add your feature to a data dictionary so that the appropriate attributes are collected for the LTER database.
    1. If it is a main feature on the LTER then add it to the KBS LTER data dictionary, otherwise create your own data dictionary. Do not edit the “KBS LTER” data dictionary without permission from the LTER Project Manager or GIS Manager.
    2. See “Making and Editing a Data Dictionary” in the Appendix if you need to edit the data dictionary or create a new one.

Transferring Files to GPS Data Logger

You may need to transfer files from your computer to the data logger, such as files to be edited, files to be used for navigation, or a new or edited data dictionary.

  1. Log yourself onto the computer that has Pathfinder Office installed on it. If you do the following step before logging in then you will get an error message.
  2. Connect the data logger to your computer using the nine-pin connector cable.
  3. Microsoft ActiveSync software will open automatically and will establish a link between your PC and the GPS data logger.
  4. (You may get the message – Microsoft Active Sync does not recognize the device you are trying to connect. Retry.) and/or Would you like to set up a partnership?
    1. Select “No” (you do not want to set up a partnership).
  5. The green Microsoft ActiveSync icon will state “connecting” and then “connected”.
  6. Open GPS Pathfinder Office. If the “Select Project” window appears, deselect “Display this dialog at start-up”.
    1. “Utilities”/“data transfer” /select the “send” tab.
    2. Select the “Add” dropdown menu and choose “Data File”.
    3. Highlight the desired file(s) and click " Open".
    4. “Transfer All”
    5. You should get a message on the PC – “Transfer Completed”. It will tell you how many files were transferred.

Preparing the GPS

  1. Connect the batteries to the receiver.
    1. A green light on the battery charger indicates that the batteries are charged.
    2. Connect the batteries to the GPS receiver inside the backpack by hooking them to the cable coming from the data receiver; the cable is split in two to connect to two batteries. Use the batteries in the front of the charger first.
    3. Place batteries in the pockets next to the receiver.
    4. Take an extra set if you plan to be out for more than four hours.
  2. Connect the data logger to the receiver using the cable with a nine-pin connector on each side. One side of the cable should already be connected to the receiver.
  3. Open the TerraSync application by double-clicking on the icon on the data logger touch screen. It may be slow to open if there are lots of files stored on the data logger.
  4. Connect the antenna to the receiver using the TerraSync software. This connection needs to be completed in order to verify the battery power of both the receiver.
    1. Select the “Set Up” section button drop down menu in the upper left.
    2. “Connect”
  5. When you connect, you should get a small message at the top of the screen- “almanac received”. Then the following message will appear to show that your Omnistar service is working and when your subscription needs to be renewed: “Integrated Satellite real-time service Information, User Access- Enabled, End Date-”.
  6. The battery icon at the top of the screen gives the battery status: the right hand side shows the logger battery charge and the left side shows the charge of the receiver batteries. You can touch the icon for the percent of battery charge available.
  7. Non-LTER folks need to sign out GPS II before leaving the lab.

Data Logger Settings

Verify that all your settings are appropriate in the TerraSync Setup. Never change the settings on LTER GPS I. If you change the settings on LTER GPS II, be sure to set them back to the following settings when you return the GPS.

The Section drop-down menu is located in the upper left hand of the logger screen. Select the “Setup” section.

  1. “Real Time Settings”
    1. Choice 1: Integrated Satellite
    2. Choice 2: Wait for Real Time
    3. RTCM Age Limit: 10s
    4. Integrated Satellite:
      1. Service: Omnistar
      2. Name: Custom
      3. Frequency: 1,530.359MHz (this is preset by Omnistar)
      4. Data Rate: 1200 (preset by Omnistar)
    5. “Coordinate System”
      1. System: UTM
      2. Zone: 16 North
      3. Datum: NAD 83 (Conus)
      4. Altitude Reference: MSL (Mean Sea Level)
      5. Geoid Model: Other
      6. Geoid: DMA 10 × 10 (Global)
      7. Coordinate Units: Meters
      8. Altitude Units: Meters
    6. “Logging Settings”
      1. Log Velocity Data: no
      2. Log Super Correct Data: yes
      3. Log QA/QC Data: no
      4. Antenna Height: 1.5m Use the standard of 1.5m, a typical height of the antennae when the backpack is worn, unless the antennae will be positioned lower or higher.
      5. Allow Position Update: Confirm
      6. Confirm End Feature: no
      7. File name Prefix : g – This is irrelevant since we will assign our own filename.
      8. Between Feature Logging:
        1. Style: time
        2. Interval: none
    7. GPS Settings”
      1. GPS Receiver Port: COM1
      2. Velocity Filter: off
      3. Configuration Style: Custom
      4. DOP Type: PDOP
      5. PDOP Mask: 4 (See “Understanding PDOP” in the Appendix for more information)
      6. SNR Mask: 6 (Signal to Noise Ratio of the satellite signal. The higher the number the better.)
      7. Elevation Mask: 10

Collecting Data

  1. Open TerraSync on the data logger by double-clicking the TerraSync icon (if not already open).
  2. If not connected, connect antenna to receiver (see step 5 of “Preparing the GPS”).
  3. From the “Data” Section menu, a screen appears that prompts you to create a new data file.
    1. Assign the file a name.
      1. The default name is the month-day-hour that you began the file; don’t use this name.
      2. Instead, name the file by feature and date, such as lysimeters_20041230, TSF_corners_20051230, or grad_student_name_sites_20031012.
    2. Choose the desired data dictionary. LTER personnel use the “KBS LTER” dictionary except for special projects.
    3. Press “Create”
    4. Select “Options”
      1. To ensure you don’t begin to log data right away, select “Log Later”.
      2. Verify that the " Logging Interval" is set to use “time” as the Logging Style and a 1 second interval.
  4. When using the KBS LTER data dictionary, it will list feature types (such as lysimeter). Select a feature and press “Create”. If using a generic data dictionary, that does not have predefined features, select point, line or area and then “Create”.
  5. Place the antenna directly over the feature and press “Log” (on the right side of the screen) to start collecting GPS data for that feature.
    1. A pencil icon on the top of the screen will wiggle back and forth to indicate that data are being logged or will flash to indicate that data cannot be logged. Each time a point is logged, you will hear a click and the number next to the pencil icon will increase.
    2. Note that data will not be logged if the PDOP is too high or if you do not have a real-time connection since we have set these data quality restrictions on the data logger during setup. For some applications, such as collecting data under trees, you may find that the PDOP setting is too restrictive, see “Understanding PDOP” and “Collecting Data in Trees or near Obstructions” in the Appendix.
    3. Fill out the attributes that are requested on the screen.
    4. Press “Pause” to pause the logging of points. Press OK when you want to stop logging the feature; this will save it.
      1. In order to assure good quality data, it is best to collect 30 points. When using the KBS LTER data dictionary, you must collect 30 positions for each feature or the following message will appear: “Insufficient positions- Do you want to store the feature now?”.
  6. Repeat step 4 and 5 for each feature.
  7. To view the data, go to the “Map” Section menu.
  8. To save the file, press “Close” in the upper right hand part of the screen. The data are not saved until you close the file, therefore you should close it occasionally.

Updating a File

Sometimes you may need to work with an existing file and either update the data or collect new features in the file. Before you update a file that has already been sent to the Data Manager, you will to need to get permission from the Project Manager and also discuss this with the Data Manager to let him/her know that you plan to edit data in the database. If the file is not stored on the data logger, you first need to transfer the file from your computer to the GPS data logger (see the “Transferring Files to GPS Data Logger” section).

  1. Open TerraSync on the data logger.
  2. Select the “Data” section button (in the upper left).
  3. “Existing File”
  4. Select the file to be updated, press open.
  5. To update a feature
    1. “Data”
    2. “Update Features”
    3. “Begin”
  6. To collect a new feature
    1. “Data”
    2. “Collect Features”
    3. “Begin”


You can use the GPS to navigate to a feature that was GPS’ed at an earlier time.

If the GPS data have already been entered in the KBS LTER database, you must query the database to get the desired data (see “Getting GPS Data out of the Database”), and then create an import file in GPS Pathfinder Office (see “Importing Data”) (Do not use the original Pathfinder files since they may be outdated.

If the file is not stored on the data logger, then see “Transfer Files to GPS Data Logger”.

  1. In TerraSync, open the file containing the points to navigate to.
  2. Select the “Data” Section button (in the upper left).
  3. “Existing File”
  4. Select the desired file.
  5. “Open”
  6. Highlight (touch once) the feature to which you want to navigate to (touching twice will open the update feature dialogue box).
  7. “Options” This options menu will only appear after you have opened the file.
  8. “Set Nav Target”
  9. Select the “Navigation” Section button.
  10. You will now see a large circle with an arrow inside. The arrow indicates the direction to which you want to walk to get to the target. When you get to within 5 meters of your target, the screen will change. You will now see a small circle, which is the target, and an “x”, which is you. Continue walking in the direction that will move the “x” (your position) into the circle. When the “x” is in the circle, you have reached your target. To the right of this target there are numbers that tell you the actual distance you are from your target.

Definition of words to the right of navigation screen:

  1. Bearing – the angle to the navigational target relative to north. This angle will change as one moves.
  2. Heading – the angle of the direction you are traveling.
  3. Turn – the angle you should turn to be on the designated bearing (always clockwise).

To navigate to a station flag along the plot paths:

Walk in a straight line until the bearing states either 0, 90, 180, 270 or 360 (depending on which station you are navigating to and your direction); you are now parallel to a station flag and can now turn right or left.

Returning the GPS Unit

Putting the GPS unit away

  1. It is important to disconnect the data logger from the receiver via TerraSync software before you disconnect the batteries. If you disconnect the batteries first then you may get an error message when you reconnect them (see Trouble Shooting if this happens).
    1. Select the"Setup" Section button (in the upper left).
    2. “Disconnect”
  2. Connect the data logger to its recharging cable. Leave the data logger turned on when connected to its recharging cable; this allows Microsoft Windows to monitor the charging process.
  3. Remove the batteries from the receiver and put them in the charger to recharge. The two batteries that were used for the most time should be placed in the back of the charger.
  4. Complete the sign out sheet if using GPS II for non-LTER projects.

Complete Log of GPS Usage on the LTER Intranet

  1. Enter KBS LTER intranet web site:
  2. LTER Intranet”
  3. Click “here” for access to the KBS LTER Intranet. You will need a password that is available from the LTER Data Manager.
  4. Centralized Data Editing
  5. Enter your KBS LTER intranet user name and password. If you do not have a KBS LTER intranet password then email your report to at, put KBS039-001 in the subject line.
  6. “Log GPS” If you don’t see this option maybe you should not be here
  7. Scroll to the bottom of the web page, select “New”.
  8. The “ObsNumber”, should be one unless there is more than one entry on a given date.
  9. Your “Observation” entry should be written in complete sentences, and should tell what you did and why you did it. You should also include any comments about conditions or results, data quality, etc. If you set the maximum allowable PDOP to a higher value, you should mention that here. Your entry will appear on the web at: (it may not appear here immediately).
  10. Click “Submit New”.

Downloading the data files

When you finish collecting GPS data, download the Trimble GPS files (.ssf files) from the data logger to your computer.

  1. Connect the data logger to your computer via the cable with 9-pin connectors and turn on the data logger.
  2. Open Microsoft ActiveSync on your desktop if it does not open automatically.
  3. (You may get the message – Microsoft Active Sync does not recognize the device you are trying to connect. Retry.) and/or Would you like to set up a partnership?
    1. Select ""No"" (you do not want to set up a partnership).
  4. You should see the Microsoft ActiveSync icon turn green and state “connecting” and then “connected”.
  5. Open GPS Pathfinder Office.
  6. If the “Select Project” window appears, deselect “Display this dialog at start-up”.
  7. “Utilities”
  8. “Data Transfer”
  9. Select the “Receive” tab
  10. Select the “Add” dropdown menu to pick the desired files.
  11. Select “Data File”
  12. “Destination”, browse to enter where you want the files saved.
    1. Save on the Gpr network: \LTERnt2\LTER\GPS\Pfdata\
    2. Create a new subdirectory within this directory that describes the feature name such as lysimeters or plot_corners. Example: \LTERNT2\LTER\GPS\Pfdata\lysimeters.
    3. Highlight the desired files.
    4. “Open”
  13. “Transfer All”
  14. Verify that files were saved properly.

Checking the Trimble data file

After returning from the field, you need to map your data in GPS Pathfinder Office so that you can verify that your data were collected correctly.

  1. If desired, select a background map, such as the LTER plot map, to overlay your data onto: “File”, “Background”, click on the desired map from the list.
  2. Open your data file: “File”, “Open”, select your file.
  3. “View”, “Map”
  4. When you double-click on a point you will get the attribute information associated with that point. Select the arrow on the toolbar and point to a GPS point. Check the information to verify that it is correct.

Deleting files from the data logger

After downloading your files, delete them from the data logger unless you plan to update them or use them before another user may need the unit.

  1. Open TerraSync on the data logger.
  2. Select the “Data” Section button (in the upper left).
  3. “File manager”
  4. Select the files to delete.
  5. “Options”
  6. “Delete”
  7. If you were using a data dictionary other than the “KBS LTER” data dictionary, you will need to delete it when you are finished using it.

Exporting Files

When the Trimble data files (.ssf files) are complete and you have checked them over, export files as a shapefile (.shp). A folder (with the original file name) is created and discrete shapefiles are created for each feature type; shapefiles are used in the GIS and they also have a dBase file associated with them that will be used for the database. Also, a text (.txt) file is created every time exporting is performed and an information (.inf) file is also generated with general information.

  1. Open GPS Pathfinder Office.
  2. “Utilities”
  3. “Export”
  4. “Selected File”": browse to choose the files to export.
  5. “Output Folder”: browse to choose the desired output folder.
    1. Save shapefiles on Gpr network: \LTERnt2\LTER\GPS\Export\
    2. Create a subdirectory within this directory to organize the files. Examples: \LTERnt2\LTER\GPS\Export\T01R01 or \LTERnt2\LTER\GPS\Export\lysimeters.
  6. “Choose an Export Setup”: “LTER Shapefile Export UTM”. If you want to use settings that are different from this setup (for a non-LTER project for example), please do not change this setup but instead create your own setup or use another one of the existing setups.
  7. If setup “LTER Shapefile Export UTM” is available, you should verify that the settings are correct.
    1. “Properties”, check the desired settings listed below.
  8. If “LTER Shapefile Export UTM” is not available, then create it by selecting “New” and entering the following settings.
    1. “Setup Name”: type “LTER Shapefile Export UTM
    2. “Create; New Setup”: shapefile
    3. Select the “Data” tab.
      1. “Type of Data to Export: Features- Positions and Attributes”: Export All Features
    4. Select the “Output” tab.
      1. “For each input file create output subfolder(s) by the same name”
      2. System File Format- DOS Files.
  9. Select the “Attributes Tab”.
    1. “Export Menu Attribute As”: “Attribute Value”
    2. “Generated Attributes”
    3. For “All Feature Types”, select: PDOP, HDOP, Correction Status, Receiver Type, Date Recorded, Update Status, Feature Name, Data File Name, Total Positions and Data Dictionary Name (i.e., don’t select Time Recorded, Filtered Positions, GPS Week or GPS second).
    4. For “Point Features”, select: Height, Vertical Precision, Horizontal Precision, Standard Deviation and Position (i.e., don’t select Point ID).
      1. For “Line Features” and “Area Features”, you can select all attributes although presently we only collecting point data for the database.
    5. Select the “Units” tab.
      1. “Use Exportunits”
      2. “Change”
      3. “Select Units”: Meters, Square Meters and Meter per Second
      4. Decimal Places- choose 5 for North/East, choose 9 for Lat/Long
    6. Select the “Coordinate System” tab.
      1. “Use Export Coordinate system”
      2. “Change’
        ###. Select “Coordinate System and Zone”
      3. “System” UTM
      4. “Zone” 16 North
      5. “Datum” NAD 83 (Conus)
      6. “Altitude Measured From”: “Mean Sea Level (MSL)”
      7. “Geoid Model”: “Other”
      8. DMA 10 × 10 Global Geiod
      9. “Coordinate Units”: meters
      10. “Altitude Units”: meters
    7. Export Coordinates As: XYZ.
    8. After entering all settings listed above on the “Export Setup Properties” form, select “OK”.
  10. When you are ready to export, click “OK” on the “Export” form.

LTER Database

When the Trimble data files and export files have been checked, they should be moved to the LTER database. You can query the database to get GPS data needed for navigation or other needs,
Moving GPS Data into the Database

  1. You created a dBase (dbf) when you exported your data as a shapefile.
  2. Open this dbf file in Excel to verify that the columns are in the following order: Treatment, Replicate, FeatDes, Microplot, Comments, Max_PDOP, Max_HDOP, Corr_Type, Rcvr_Type, GPS_Date, Update_Sta, Feat_Name, Datafile, Unfilt_Pos, Data_Dicti, GPS_Height, Vert_Prec, Horz_Prec, Std_Dev, Northing and then Easting.
  3. Save as file type: CSV (Comma delimited).
  4. Move the comma delimited files to: LTERnt2\LTER\GPS\ready for database\comma_delimited_files.
  5. Move the Trimble files (.ssf) to: \LTERnt2\LTER\GPS\ready for database\ssf_files.
  6. Inform the LTER Data Manager that the files are ready for the database.
  7. The Data Manager will move the comma-delimited files to the appropriate table in the database. These data will be accessible by querying the database.
  8. The Data Manager will move all the original Trimble files (.ssf files) to a storage location; these files will not be stored in the database and can only be accessed by making a special request to the Data Manager.
  9. Once the data are in the database, the LTER Data Manager will delete these files from their original location.

h4.Getting GPS Data Out of the Database

Query the KBS LTER database on the web for the desired GPS data.

  1. Select “KBS039 (GPS data)”. If you plan to use the data for navigation then select “KBS039-002 (Navigational GPS Points)” which has only the attributes needed for navigation. If you need more information about the data then select “KBS039-003 (Navigational GPS Points Expanded)”.
  2. “Missing Value Indicator:” leave blank
  3. “Reformat As:” don’t select any reformat.
  4. Select and copy the desired data
  5. Open a blank file in Microsoft Excel.
    1. Paste in the data from the query.
    2. If the data will be used for navigation, Save In: \LTERnt2\LTER\GPS\navigation\.
    3. Give it an appropriate name with the letters NAV_ in front, the feature, and the date the data was exported from the database at the end. Example: NAV_T5_corners_20040530.
    4. If you plan to import these data back into GPS Pathfinder Office for navigation, you must save the file as a dBase file. “Save As Type” DBF 4 (dbase IV) (*.dbf).

Importing Data

You may want to put a data file (from the LTER database or another source) on the GPS logger for navigation. To do this you must import a dBase file back into GPS Pathfinder Office. The file will be saved as an import file (imp).

  1. Open GPS Pathfinder Office.
  2. Select “Utilities” and then “Import”
  3. “Selected Files”: browse to choose your desired dBase files.
  4. “Output Folder”: browse to choose the desired output location. LTER files to be used for navigation should be saved in \LTERnt2\LTER\GPS\navigation\
    1. Note: The files in the navigation directory should be in the database first and should not be edited; they are for navigation only.
  5. “Choose an Import Setup”: “LTER Navigation Import”.
  6. If the “LTER Navigaton Import” setup is available, then verify that the settings are correct.
    1. Select “Properties”, and see the settings listed below.
  7. If the “LTER Navigation Import” setup is not available, then create it.
    1. “New”.
    2. “Setup Name”: type LTER Navigation
    3. “Create”
      1. “NewSetup”: dBase
    4. Select the “Data” tab
      1. “Type of Data to Import”: “Features with Data Dictionary”
    5. Select the “dBase” tab
      1. Select “Import Coordinates of Point Features”,
      2. “Specify Column Names”: “Northing/Latitude”: NORTHING (use capital letters), “Easting/Longitude”: EASTING (use capital letters)
      3. “Altitude Values”, “Use Fixed Value”, 0
    6. Select the “Coordinate System” tab
      1. Note: If the coordinate system of the input data file does not match the coordinate system displayed here, then the imported data file will contain incorrect positions. Select the “Change” button
      2. Select by: “Coordinate System and Zone”
      3. “System” : UTM
      4. “Zone”: 16 North
      5. “Datum”: NAD 83 (Conus)
      6. “Altitude Measured From”: “Mean Sea Level (MSL)”
      7. “Geoid Model”: “Other”
      8. DMA 10 × 10 Global Geoid
      9. “Coordinate Units”: meters
      10. “Altitude Units”: meters
    7. Select the “Output” tab.
      1. Create one output file for each input file.
    8. After entering all settings on the “Import Setup Properties” form, select “OK”.
  8. When you are ready to import, click “OK” on the “Import” form.
  9. The file will be saved as a Trimble imported data (.imp file).
  10. To use the file for navigation, you need to transfer the file to the data logger. See " Transfer Files to GPS Data Logger".


Understanding PDOP

DOP, dilution of precision, is an indication of the current satellite geometry. A low DOP indicates a higher probability of accuracy.
The PDOP (position dilution of precision) is perhaps the most important factor in data accuracy. A PDOP of " 4.5 is good. A PDOP of 4.6- 6.0 is poor but usable. A PDOP of "6 is not usable.

We set the PDOP limit on the receiver so that positions are not recorded if the PDOP is above 4; this limit is called the PDOP mask (to set the PDOP mask see Data Logger Settings, GPS settings). Our PDOP mask of 4 is fairly restrictive, to allow us to collect the best data possible. For some applications, such as collecting data under trees, you may find that setting the PDOP mask higher is necessary (see “Collecting Data in Trees or near Obstructions”). In such situations, the PDOP mask could be set to 6, but be aware that the quality of the data that you collect could be inferior. If you do set the PDOP mask higher; record this in your GPS usage log (see “Complete Log of GPS Usage on the LTER Intranet”) and be sure to change it back to 4 when finished.

Collecting Data in Trees or near Obstructions

  1. Collecting data in forested area presents special challenges because the trees can block the satellite reception.
  2. If possible, it is very helpful to collect data during the leaf-off period in Fall, Winter or Spring.
  3. Mission planning is critical when working in forested areas. Set your criteria such that you have plenty of satellites available (such as ="7) and a high quality PDOP (such as " 3).
  4. Be patient! When collecting data in the forest, you may find that you can’t collect any data for a few minutes and then the satellites realign and data start coming in.
  5. This protocol requires collecting 30 positions per point. However, depending on the accuracy needed for your application, you may need to settle for " 30 positions per point in the forest.
  6. This protocol requires setting the PDOP mask to 4 but you may find this too restrictive when collecting data under tree cover. If needed, you can set the PDOP mask to 6, see “Understanding PDOP” above.
  7. If you do the above and cannot collect data then you can use an offset point (see “Using an Offset Point to Collect Data” below).

h4 . Making and Editing a Data Dictionary

Before creating a data dictionary, think of the features (physical object or an event) that you will want to collect and the attributes (descriptive information) that you want to record about the feature. Enter only the 5 attributes listed below if the GPS data will be stored in the LTER database.

To create a new data dictionary:

  1. Open GPS Pathfinder Office
  2. “Utilities”
  3. “Data Dictionary Editor”
  4. “File”
  5. “New”
  6. “Name”: Select a name for the data dictionary.
  7. “New Feature”
    1. Select the “Properties” tab
      1. Give the feature a name. You can use word(s) with more than 8 characters but only the first 8 characters will be used in the file name when it is exported.
      2. Select a Feature Classification: point, line, or area. Data to be stored in the database should be collected as a point feature.
    2. Select the “Default Settings” tab
      1. “Logging Interval”: time, 1 second for a point
      2. “Minimum positions”: 30
      3. “Accuracy”: Code
    3. Select the “Symbol” tab
      1. Select the type of symbol and its details.
  8. “New attribute”
    1. “New Attribute Type”
    2. Select “Menu”, to enter a menu of choices that will allow data to be entered in a consistent manner or
    3. Select the appropriate type of data, such as “numeric” or “text”, if you want your GPS operator to enter the attribute information from the keypad.
    4. Select a “name” for the attribute
    5. Field entry: select required on creation, if appropriate, to ensure that the GPS operator does not forget to enter critical information.
    6. GPS data to be stored in the LTER database must have only the following 5 attributes:
      1. Treatment; use “menu” to enter 1,2,3,4,5,6,7,8,CF,DF,SF.
      2. Replicate; use “menu” to enter 1,2,3,4,5,6.
      3. FeatureDescription; use “menu” to enter the relevant descriptions for the feature. For example: For lysimeters, use “menu” to enter 1,2,3,4,5; For leaf litter traps, use “menu” to enter 1,2,3,N-S, E-W; For microplot corner, use “menu” to enter NW, NE, SE and SW and center.
      4. Microplot; use “text”, length 50 characters
      5. Comment; use “text”, length 50 characters
  9. Save file in LTERnt2\LTER\GPS\data dictionaries.

To edit an existing data dictionary (do not edit the LTER data dictionary without permission):

  1. Open GPS Pathfinder Office.
  2. “Utilities”
  3. “Data Dictionary Editor”
  4. “File”
  5. “Open”
    1. Select an existing data dictionary. KBS LTER is stored in: LTERnt2\LTER\GPS\data dictionaries
    2. Double-click on a feature or attribute to edit it.

To transfer the edited or new dictionary into the data logger, see “Transferring Files to GPS Data Logger”.

Using an Offset Point to Collect Data

Sometimes you may not be able to collect good GPS data for a point because the satellites are blocked by trees, buildings, etc. In these situations, you can use the “offset” option in TerraSync and collect the GPS location for a nearby (offset) point, and enter the bearing and distance to the desired point from the offset point that you GPS’ed. The GPS will then give you the coordinates of the desired point.

  1. Select an offset point near the desired point that allows you to get a good PDOP. Mark the offset point with a temporary stake if needed. You will get better quality data if the offset is close to the desired point because the bearing measurement error is exaggerated with greater distance.
  2. Set up your compass on a tripod over the offset point. Level the compass using the bubble.
  3. Measure the distance (in meters) from the offset point to the desired point and record it.
  4. Measure the bearing (azimuth), from north as 0-360 degrees, from the offset point to the desired point and record it.
    1. If your compass does not account for declination then you need to add the declination for the KBS area: -5.1 (note the negative sign) to your bearing. Example: If the bearing on a compass without declination correction is 321 degrees + -5.1 degrees for declination correction = 315.9 degrees. If it does account for declination then verify that it is set correctly to 5.1 degrees west (for KBS area).
  5. There are 2 compasses available in the KBS storeroom that are okay for the job.
    1. The Keuffel and Esser surveying compass fits onto a tripod and therefore works nicely. Unfortunately the Keuffel and Esser compass does not use 360 degrees but rather is split into four quarters of 90 degrees each. In order to record the bearing from north, locate the arm of the needle that has an arrow inscribed on it (this points to north) and read your angle from that needle as 0-360 degrees not the 0-90 degrees that the compass reads. The first few times that you record bearings with the Keuffel and Esser compass also measure the bearing with another compass to verify that you are reading the Keuffel and Esser compass correctly.
    2. A Suunto, code KB-14/360 R, is also available from the KBS storeroom. This compass allows you to sight through to your point or pole but can be a bit tricky to get a stable reading. There are 2 bearings given on the compass, read the larger number off of the compass since this gives the bearing from your location to the sighted location.
  6. In TerraSync
  7. “Data”
  8. “New file” You could use an existing file if needed.
  9. “Create”
  10. “Option”
  11. “Offset” The offset option will not appear until the file is created.
  12. Choose offset type: “Distance-Bearing”
    1. This is where you enter the distance and bearing that you measured above.
    2. Distance-Bearing offset for 1 point:
      1. Bearing (T):
      2. Horizontal distance:
      3. Vertical distance:
  13. Collect data as you would normally; the GPS coordinates given will be for the desired point, not the point that you GPS’ed.

Omnistar Differential GPS Service

The Omnistar service must be renewed annually. In 2004 the cost was $800/year. When you connect the datalogger, the Omnistar message will tell you when your present service contract will expire. Take note of this date so that you can renew the subscription prior to the expiration date if needed. To renew the subscription or for technical help, call Omnistar : 1-888-883-8476.

Additional Resources

  1. General GPS information
    1. Dave Lusch’ Powerpoint presentation, saved on LTERnt2\LTER\GPS\protocol & documents.
    2. Mapping Systems, General Reference by Trimble
  2. Trimble GPS Receiver Information
    1. GPS Pathfinder Systems, Receiver Manual by Trimble
  3. Trimble Data Logger Information
    1. Trimble Data Collector Product Guide, 4 pages
  4. GPS Pathfinder Office Information
    1. GPS Pathfinder Office, Getting Started Guide by Trimble
    2. Help within the software
  5. TerraSync software Information
    1. TerraSync Operation Guide, found on the TerraSync software CD
    2. Dave Lusch’ Powerpoint presentation on LTERnt2\LTER\GPS\protocol & documents.
    3. Help within the software


  1. As mentioned earlier, it is important to disconnect the data logger from the receiver before disconnecting the batteries. However, if the batteries do get disconnected before the data logger, then when you try to reconnect you may get an error box telling you that the com port is already connected and it will not allow you to reconnect. The solution to this problem is a soft reset.
  2. A soft reset of the data logger allows you to keep the RAM intact; all data and software will remain intact. To do a soft reset: [Shift] (yellow button with up arrowhead) + {Ctrl} + {On/Off].

Date modified: Tuesday, Oct 24 2023