The Keys to the Datum Gates

A Users Guide to the Trimble TerraSync Workflow

Assumptions

• TerraSync and Pathfinder office workflow
• Real-time correction using NDGPS (beacon) or Omnistar
• Export to a NAD83 shapefile for ArcGIS
• Case 8 on Datum worksheet ^{Ref 1a}

Purpose

Users must pass through many datum gates to successfully collect and process GPS data.  The increasing availability of sub-meter accurate GPS, imagery and other spatial datasets is keeping users even more honest every step as once ‘minor datum shifts’ can no longer be overlooked.  Although a challenging topic for users of all levels, ignoring datums will introduce significant errors in the data you have worked so hard to collect.^{Ref 2}

When moving GPS data to a GIS, at each step, ask yourself: 1) to which datum is the GPS data currently referenced, 2) how might this step change that, and 3) to which datum is the resulting data file referenced.   For steps where datums demand your consideration, this paper provides -

• Background and terms
• Table outlining the datum for each file
• Software screenshots and explanations
• Workflow posters, and
• References

May this guide offer you the keys to successfully pass through each Datum Gate.

BACKGROUND & TERMS

A. Datums

A datum is a point of reference from which measurements are made.  It is where the coordinate grid is tied to the earth’s surface.  Datums are recalculated and updated as technology allows for more precise measurements.  These updates are referred to as realizations, flavors or many similar terms.  See references 3a and 4a for more on the evolution of datums and the implications for GPS.  Encountered in this paper are:

World Geodetic System 1984 (WGS-84)

GPS satellites broadcast signals in WGS-84 and most GPS receivers, including Trimble mobile devices, calculate data in WGS-84.  The latest realization of WGS-84 is WGS84(G1150).

International Terrestrial Reference Frame (ITRF00)

The latest realization is ITRF2000 (aka ITRF00).  WGS84(G1150) and ITRF00 are within millimeters of each other and are considered equivalent.  This is referred to as WGS-84/ITRF00 in this series of papers.

North American Datum 1983 (NAD83)

Many maps in the United States are based on this datum.  The realization most currently used is NAD83(CORS96) with specific updates for Alaska and the Pacific Islands.  The first realizations of WGS-84, WGS-84(1986) and NAD83, NAD83(1986) were equivalent. Through multiple updates of each datum, the difference between the NAD83(CORS96) and WGS-84(G1150) is now many meters in some areas.

B. Local epochs of NAD83(CORS96)

An epoch is a temporal term that refers to the state of that datum fixed to a certain date.

Continental US

The most commonly used epoch of NAD83(CORS96) is NAD83(2002.00).  This epoch of NAD83 CORS96 is referenced to January 1, 2002.

Alaska

The most commonly used epoch of NAD83(CORS96) is NAD83(2002).  This epoch of NAD83 CORS96 was adjusted after the 2002 Denali quake.

Pacific Islands

The most commonly used epochs of NAD83(CORS96) CORS in the Pacific are NAD83(PACP00) for the Pacific plate and NAD83(MARP00) for the Mariana plate.  These contain parameters to adjust for the tectonic movement of these plates.

C. Datum transformations

Transformations move data between two datums.  GPS coordinates are in WGS-84 but most GIS data is referenced to another datum such as NAD83, thus the need for a transformation.  As datums are updated, the transforms are updated as needed.

To the view the parameters Pathfinder Office (PFO) uses for any datum transformation, from the main menu choose Utilities / Other / Coordinate System Manager. Click on the Datum transformations tab and choose the transform of interest.  Encountered in this paper are:

NAD83 (CONUS), NAD83 (Alaska)

These are both null transforms in Pathfinder Office.  All parameters are held at 0 and data does not move at all.  Despite different names, they are functionally equivalent in PFO.  NDGPS users can choose these transformations to keep from applying the same transformation twice to their GPS data.

NAD83 (CONUS) CORS96

In the Continental US and Alaska, choose this transformation in PFO to move data between WGS-84 (G1150) and NAD83.

NAD83(PacP00), NAD83(MarP00)

NAD83(PacP00) is for users on the Pacific plate, such as the Hawaiian and  Marshall Islands and American Samoa.  This transformation was updated in the last days of 2008 and the latest parameters have not yet been incorporated in PFO. ^{Ref 1b} They must be added manually to all versions of PFO.  Instructions for this are included at the end of this paper.  For users on Guam and others on the Mariana plate, use NAD83(MarP00).

NAD83 (Hawaii), NAD83(Aleutian)

These transformations are only gross approximations and should not be used at all in PFO.

D. Differential correction

A base station continually calculates its position as reported by the satellites. Since the base station remains stationary, it can calculate the difference between the location GPS-reported position and its true, surveyed location. ^{Ref 4b} Differential correction subtracts this same known amount of error from your GPS data. Data can be corrected immediately in the field against a real-time correction source such as WAAS or corrected later in the office against downloaded base files.  While real-time corrections are ideal for immediate needs such as navigation in the field, it is usually less accurate than post-processing.  When the best available accuracy is desired, upon return to the office, post-process your real-time data against stored base station files. For post-processing, CORS (continually operating reference stations) are held to the highest standards and are the recommended base station.  Anytime corrections are applied via real-time or post-processing, the GPS data may take on the spatial reference of the correction source.  Therefore, proceed carefully when collecting and processing all GPS data.

SUMMARY TABLE

CASE 8a: Without post-processing

• TerraSync 3.2 or greater with NAD83 (CONUS) CORS96 correction datum

• NDGPS real-time correction

• No post-processing

• Export to NAD83 ArcGIS shapefile

CASE 8b: Post-processing

• TerraSync 3.2 or greater with NAD83 (CONUS) CORS96 correction datum

• NDGPS real-time correction

• CORS base station for post-processing

• Export to NAD83 ArcGIS shapefile

CASES 8c: Without post-processing and older TS

• TerraSync versions earlier than v3.2, which lacks a correction datum

• NDGPS real-time correction

• No post-processing

• Export to NAD83 ArcGIS shapefile

Follow the datum choices made and the results at each step

SOFTWARE SCREENSHOTS

GLOBAL POSITIONING SYSTEM

STEP ONE: GPS signal

Satellites broadcast GPS signals in WGS-84/Lat-Long.

ON THE MOBILE DEVICE

STEP TWO: GPS receiver

Most GPS receivers (including Trimble devices) only receive GPS signals in WGS-84/Lat-Long. Be assured knowing that TerraSync data can NEVER be collected in the ‘wrong’ coordinate system and datum.

STEP THREE: Data storage in TerraSync ^{Footnote 1}

When the real-time correction is applied, the .ssf file not only stores the raw GPS-satellite data, but it also stores in parallel, the real-time corrected positions which are transformed to the reference frame of the correction source.  NDGPS and Omnistar signals are broadcast in the local epoch of NAD83(CORS96). ^{Ref 4c}

A. TerraSync v3.2 and later

In the Set-up/Real-time Settings menu, choose Integrated Beacon.

Click the Wrench icon to open the Integrated Beacon settings menu and set the Correction Datum.

The correction datum should match that of the incoming correction signal.  For NDGPS or Omnistar, choose NAD83 (CONUS) CORS96.

The correction datum keeps your GPS data from taking on the NDGPS reference frame.  Your real-time corrected positions are transformed back to WGS-84.

As a result, stored in a Trimble .ssf file are two parallel sets of data.

a) your real-time corrected positions are referenced to the

NAD83 (CONUS)  CORS96 datum of the beacon,

b) The correction datum, transforms a parallel set your real-time corrected

positions back to WGS-84. This is the same spatial reference in which the

GPS signal was received from the satellites.

B. TerraSync versions prior to v3.2

TerraSync versions prior to v3.2 do not set a correction datum.

In the Set-up/Real-time Settings menu, choose Integrated Beacon.

As a result, your NDGPS-corrected positions are stored in a Trimble .ssf file in the local epoch of NAD83(CORS96). You will not be able to post-process your data.  Skip ahead to STEP FIVE.

STEP FOUR: Post-processing GPS data in Pathfinder Office

This step is for those wishing to post-process their GPS to take advantage of the increased accuracy post-processing offers or a chance to re-correct data if not all positions were real-time corrected in the field.

If you do not wish to post-process your data, skip ahead to STEP FIVE.

For all base stations, ALWAYS compare the ITRF00 coordinates for the L1 Phase Center of the GPS antennae reported on the base station’s website……

The numbers should match out past the 4th decimal point.  If they do not match, choose another site as any mismatch between the two shifts your differentially corrected data by this same difference.  Alert the base station administrator of any discrepancies.  Their contact information can be found on the Base Provider Properties / General Tab in PFO.

After verifying the base station coordinates, you are ready to proceed with the differential correction.

The .ssf file stores the raw GPS-satellite data referenced to WGS-84.  The .ssf file also contains the parallel real-time corrected positions referenced to the local NAD83(CORS96) epoch of the correction source.   The differential correction wizard affects only the raw WGS-84 positions in the .ssf file. Therefore, the following instructions apply to all users wishing to post-process data regardless of the use of real-time correction in the field.

As when any correction source is applied, post-processed GPS data takes on the datum of the reference position of the correction source.  For CORS, the recommended base station, base files are referenced to a local epoch of NAD83(CORS96).  The differential correction wizard offers the choice to use the reference position from the base files themselves or from the ITRF00-based provider list in PFO.  Choose the latter option as shown here.

In the Differential Correction Wizard, choose to ‘Use the base station antennae position from the base provider.’ This 2^nd option uses the Trimble ITRF00 base provider list.

Since we chose the Trimble ITRF00 base provider list (the 2^nd option), the raw, WGS-84 positions are transformed to ITRF00. As a result, your post-processed data is now stored in a newly created Trimble .cor file referenced to ITRF00. ITRF00 and WGS-84 are within millimeters of each other.  Your post-processed GPS data remains in essentially the same reference frame in which it was received from the satellites.

STEP FIVE: The ArcGIS Projection file (.prj)

ArcGIS uses a projection file (.prj) to determine the reference frame of each exported shapefile^{Footnote 2}.  Unless you create this .prj file, ESRI ArcGIS does not know to which reference frame Trimble Pathfinder Office will export the data.

At the bottom of the PFO Export tab click Browse and navigate to the spatial reference of your GIS data.  Shown here is NAD83 UTM ZONE 11N. Choose the zone and/or coordinate system of your GIS. ^{Footnote 3}

STEP SIX:  PFO Export to a GIS Shapefile

Your GPS data is currently stored in a Trimble file and is now ready to export to an ESRI shapefile for ArcGIS.  The datum transformation you choose will depend on whether or not you post-processed your GPS data.

A. Data was not post-processed: Export from a Trimble .ssf file (Case 8a or 8c)

Your .ssf file contains the raw GPS-satellite data referenced to WGS-84 and the parallel real-time corrected positions referenced to the local epoch of NAD83(CORS96).   To specify which set of positions to export:

In the Export Setup Properties / Position Filter tab, check the box next to ‘Real-time Code’ to export your real-time corrected positions.

Your NDGPS data is currently in a local epoch of NAD83(CORS96). This already matches the datum of your NAD83-based GIS.  You do not want your data to be transformed or shifted again.

In the PFO Export utility, select the NAD83(Conus) datum transformation.  This null transform has all parameters held at zero and does not move data at all.

Your GPS data retains the local epoch of NAD83(CORS96). from the real-time beacon.  Data in this screenshot will be projected to the UTM Zone 11N coordinate system.  Choose the zone and/or coordinate system of your GIS. ^{Footnote 3}

Click OK to export your data.

Your GPS data is exported from a Trimble file to an ESRI shapefile in NAD83(CORS96) and is ready for ArcGIS.

B. Data was post-processed: Export from a Trimble .cor file (Case 8b)

Your post-processed GPS data is currently stored in a Trimble .cor file in ITRF00.

In the Export Setup Properties / Position Filter tab, be sure the box next to ‘Postprocessed Code’ is checked as we wish to only export post-processed positions.

During Export, the datum transformation depends on where the data was collected.

i. Continental US

In the PFO Export utility, select NAD1983 (Conus) CORS96 from the datum drop-down menu.  This transformation moves data from WGS84 or ITRF00 to NAD83(CORS96). Data in this screenshot will be projected to the UTM Zone 11N coordinate system.  Choose the zone and/or coordinate system appropriate to your GIS. ^{Footnote 3}

Click OK to export your data.

Your GPS data is exported from a Trimble file to an ESRI shapefile in NAD83(CORS96) and is ready for ArcGIS.

ii. Alaska and Aleutian Islands

For the best results, follow the instructions for the Continental US to select the NAD83 (Conus) CORS96 transformation ^{Footnote 4}. Do not use the NAD83(Alaska) transform which is a null transform with all parameters held at zero. Your data will not moved at all by NAD83(Alaska) in PFO.   Do not use the NAD83(Aleut) transform which is a gross approximation. Your data will not be moved accurately by NAD83(Aleut) in PFO.

iii. Pacific Islands

NAD83(PacP00) is transformation to choose for the Hawaiian Islands, Marshall Islands,  American Samoa  and other users on the Pacific plate.  The PacP00 transformation was updated in the last days of 2008 and the latest parameters have not yet been incorporated in PFO.  Instructions to manually add the updated PacP00 transform are included at the end of this paper.

In the PFO Export utility, select  NAD1983 NEW (PacP00) from the datum drop-down menu.  Data in this screenshot will be projected to the UTM Zone 4N coordinate system.  Choose the zone and/or coordinate system appropriate to your GIS^.. Click OK to export your data .

Existing versions of NAD83(Hawaii) or NAD83 (PACP00) in PFO should not be used since they no longer contain the correct transformation parameters.  Users on the Mariana plate, which includes Guam, should use NAD83(MarP00).

Your Pacific GPS data is exported from a Trimble file to an ESRI shapefile in NAD83(CORS96) and is ready for ArcGIS.

CONGRATULATIONS !!!

Your GPS data is now ready to add to your GIS as a NAD83(CORS96) shapefile complete with a projection (.prj) file.   See reference 1e for more on projection-on-the-fly in ArcGIS.

SUMMARY POSTERS

The following posters summarize your route through each datum gate.  As a self-test, provide the datum for each file in the provided spaces for your workflow.  The answers are in the summary table and the text of this paper.  You can also download the completed one-page posters as guides.^{Ref 1f}

Posters with areas for you to enter answers

• Case 8a
• Case 8b
• Case 8c

Completed Posters

• Case 8a
• Case 8b
• Case 8c

FOOTNOTES

1) Step 3 discusses data storage settings only. To specify a coordinate system and datum for field purposes such as the correct display of a background image, current coordinates or the entry of a navigation target, see Data Display in TerraSync.

2) Versions of PFO earlier than v3.1 did not create a projection file. The .prj file can be create in ArcGIS using either the Define Project tool in ArcToolbox which assigns a projection file to one shapefile at a time1c or the Batch Project Define tool in ArcCatalog1d which defines the projections of all shapefiles in the same folder at once. Regardless of either tool you choose, remember that the exported GPS data has to be in the projection of the .prj file you assign. Neither tool re-projects data; they only define the existing spatial reference of the data.

3) The USFS (US Forest Service) R6 Albers projection is a NAD83 projection. Select the NAD1983 (Conus) CORS96 datum transformation as shown. Consult with your local GIS Specialist to install the R6 coordinate system to PFO.

REFERENCES

1. Spatial-ed.com

A. Datum Worksheet, Joel Cusick, National Park Service

Quick guide to datum settings for ArcPad and TerraSync data collection scenarios.

B. PacP00 transformation, Joel Cusick, National Park Service

Summary of a Dec 2008 e-mail thread following a proposed revision of Pacific transform

C. Project define instructions

D. Batch define zip files and instructions

E. Projections in ArcGIS

2. The_Right_Place, Michael Dennis

www.ak-aug.org/presents/2006%20GIS%20jam/mdennis_The_Right_Place.pdf

3. Electronic Data Solutions, Jerome, ID

A. Understanding the Evolution of WGS84 and NAD83

www.elecdata.com/gps/technote/Understanding_the_Evolution_of_WGS_1984

_and_NAD_1983_REV3.pdf

B. Sending Background Images to terrasync

www.elecdata.com/gps/technote/supportnote_terrasyncbackgroundfiles.pdf

4. Trimble

A. Datum transformations in GPS

Trl.trimble.com/docushare/dsweb/Get/Document-170369/sprtnote_PFO-GPSA_NAD83Datum.pdf

B. Why post process GPS data?

Trl.trimble.com/docushare/dsweb/Get/Document 210840/

MGIS%20Postprocessing%20White%20Paper_0205_lr.pdf

C. Common Real-Time Correction Source Datums

Trl.trimble.com/docushare/dsweb/Get/Document-407823/

MGIS_sprtnote_terrasync_correctiondatums.pdf