Spatial - Ed.com

The Keys to the Datum Gates

A Users Guide to the Trimble TerraSync Workflow

Assumptions

• TerraSync and Pathfinder office
• No real-time correction or with  WAAS
• Export to a NAD83 shapefile for ArcGIS
• Case 7 on Datum worksheet ^{Ref 1a}

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. 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 accordingly.

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 select the transform of interest.  Datum transformations encountered in this paper are:

NAD83 (CONUS) CORS96

In the Continental US and Alaska, choose this transformation in PFO to move data between WGS-84 (G1150) and NAD83 (CORS96).

NAD83 (CONUS), NAD83 (Alaska)

These are both null transforms in PFO.  All parameters are held at 0 and data does not move at all.  Despite different names, they are functionally equivalent in PFO.

NAD83(PacP00), NAD83(MarP00)

NAD83(PacP00) is for users on the Pacific plate, which includes 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.  They must be added manually to all versions of Pathfinder Office.  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 in PFO and should not be used at all.

C. 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, 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 7a Assumptions

• TerraSync 3.2 or greater

• No real-time correction

• Post-processing against CORS base station

• Export to NAD83 ArcGIS shapefile

CASE 7b Assumptions

• TerraSync 3.2 or greater with WGS-84 correction datum

• WAAS real-time correction

• Post-processing against CORS base station

• Export to NAD83 ArcGIS shapefile

Case 7b only differs from Case 7a in that it is recommended that a correction datum be set to keep collected data from taking on the ITRF00 reference from frame from the WAAS real-time correction.   The files created and datums they are referenced to are identical in the two cases.

CASE 7c Assumptions

• TS earlier than v3.2, which lacks a correction datum setting

• WAAS real-time correction

• Post-processing against CORS base station

• Export to NAD83 ArcGIS shapefile

Case 7c only differs from Case 7a in that older version of TerraSync lack a correction datum setting.  Collected data takes on the ITRF00 reference from frame from the WAAS real-time correction, however WGS-84 and ITR00 can be considered equivalent.  Therefore, this point is made more for demonstration than to imply shift in your data more than a few millimeters.  The files created and datums they are referenced to are identical in the two cases.

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

Data storage varies depending on the presence and type of real-time correction ^{Footnote 1}.

A. No real-time correction

GPS data is stored in a Trimble .ssf file in WGS-84/ Lat-Long.

Your GPS data is not transformed or shifted from the spatial reference in  which it was received from the satellites.

B. WAAS real-time correction

When WAAS 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 WAAS reference frame.  WAAS signals are broadcast in ITRF00. ^{Refs 4c,5}

In the Set-up/Real-time Settings menu, select Integrated SBAS since WAAS is a type of satellite-based augmentation system.  Click the Wrench icon to open the Integrated SBAS Settings menu and view the Correction Datum ^{Footnote 2}.

The correction datum should match that of the incoming correction signal.  For WAAS this is ITRF00.  WGS-84 is already chosen for you.  Note, Trimble uses ITRF00 and WGS-84 interchangeably.  As a result, your WAAS-corrected positions are stored in a Trimble .ssf file in WGS-84.  This is the same spatial reference in which the GPS signal was received from the satellites.

ON THE DESKTOP COMPUTER

STEP FOUR: Post-processing GPS data in Pathfinder Office

This step is for those wishing to post-process GPS data because:

a) a real-time correction source was not used in the field,

b) a real-time positions are to be re-corrected to take advantage of the

increased accuracy of post-processing, or

c) not all positions were real-time corrected in the field.

If  you choose to not 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 the 4th digit.  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.

Your GPS data is currently stored in a Trimble .ssf file in WGS-84 (or WGS-84 /ITRF00 for WAAS users with earlier versions of TerraSync).  When differentially corrected, the GPS data takes on the spatial reference of the base station position.   PFO offers the choice to use an ITRF00 reference position from either the PFO base provider list or from the base files themselves.  Continually Operating Reference Stations(CORS),  CORS are held to the highest standards and are the recommended base station.  CORS are referenced to NAD83(CORS96). The reference datum of stations other than CORS may vary and provide mixed results with the PFO differential correction utility.

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

The wizard corrects the raw GPS-satellite data in the .ssf file.  Since we chose the Trimble ITRF00 base provider list (the 2^nd option), the 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 GPS data remains in essentially the same reference frame in which it was received from the satellites.

STEP FIVE: The ArcGIS Projection file

ArcGIS uses a projection file (.prj) to determine the reference frame of a shapefile ^{Footnote 3}.  Unless you create .prj files, ESRI ArcGIS does not know to which reference frame Trimble Pathfinder Office will export your GPS data.

At the bottom of the PFO Export/Coordinate System 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 appropriate to your GIS.  ^{Footnote 4}

Click OK to export your data to a shapefile.

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 use in ArcGIS.

A. Users who did not post-process their GPS data

You will be exporting real-time corrected positions, which are referenced to ITRF00 and currently stored in a Trimble .ssf file.

B. Users who did post-process their GPS data

You will be exporting post-processed positions, which are referenced to ITRF00 and currently stored in a Trimble .cor file.

The correct datum transform during export 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.  Data in this screenshot will be projected to the UTM Zone 11N c

oordinate system.  Choose the zone and/or coordinate system appropriate to your GIS. ^{Footnote 4}

Click OK to perform to export your data to a shapefile.

Your Continental US 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 above instructions for the Continental US to select the NAD83 (Conus) CORS96 transformation. Despite the “Conus” name, this is the choice for Alaska and the Aleutians.  Do not use the NAD83(Alaska) transform in PFO.  This is a null transform with all parameters held at zero. Your data will not moved at all by NAD83(Alaska).   Do not use the NAD83(Aleut) transform in PFO.  This is a gross approximation. Your data will not be moved accurately by NAD83(Aleut).

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

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 on this site in the Datum CheatSheets section.

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 perform to export your data to a shapefile.

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 a GIS-ready NAD83(CORS96) shapefile complete with a projection (.prj) file.  For more on ArcGIS projection-on-the-fly, see Reference 1e.

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 7a
• Case 7b
• Case 7c

Completed Posters

• Case 7a
• Case 7b
• Case 7c

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) TerraSync versions prior to v3.2 do not set a correction datum. In this case, the GPS data is transformed to the ITRF reference frame of WAAS. As a result, your WAAS-corrected positions are stored in the Trimble .ssf file in WGS-84/ITRF00. ITRF00 and WGS-84 are within millimeters of each other. Your GPS data remains in essentially the same reference frame in which it was received from the satellites.

3) 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 time Ref 1c or the Batch Project Define tool in ArcCatalog Ref 1d 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.

4) 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 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

5. Differential correction - WAAS

www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/