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Trimble Positions Desktop add-in: About Base Station Reference Position Handling 

12-21-2020 17:08

Introduction

As part of the support for local coordinate system workflows in TerraFlex v5, the functionality and user experience for managing base station reference positions has been improved in Trimble Positions Desktop add-in v10.8.1.1. Instead of splitting up the information between two tabs in Base Station Properties, we have consolidated the reference position information into one tab, and provided explicit control over reference position handling in post-processing workflows: use the reference position from the database (list), use the reference position from the base station RINEX files, or let Trimble Positions decide based on project geodetic settings (currently applies only to TerraFlex workflows; see this article for more information).

The text below describes the options available on this form, with the same content being available from within the app by clicking on the More info link on the form itself.

Base Station Properties: Reference Position

General

Base station reference position information is displayed here. Base stations in the Trimble database (CBS list) will have at least one reference position, typically for a global datum or frame (e.g., WGS 1984) but can also have additional reference positions in other datums or even epochs. Base station data files will also typically have a reference position, the datum and epoch of which varies by provider. In general, this information cannot be changed by the user except for custom or user base station entries. All coordinate system information in this area uses a Trimble definition.

Reference position(s) from database

The reference position(s) contained in the Trimble database for this base station, expressed in latitude, longitude, and altitude, along with the datum or frame.

In the classic CBS list distributed by Trimble, there is a single reference position per base station - this is indicated here as ‘(classic / legacy)’. Future distribution of CBS data will include multiple reference positions (one per datum + epoch) per base station but they can be added manually now. Base station providers typically provide authoritative reference positions in datums other than the global WGS/ITRF. Use the ‘Edit…’, ‘Add…’, and ‘Delete’ buttons to manage the reference positions for user base stations.

Reference position from base data files (in ECEF coordinates)

The datum or frame of the reference position seeded in the base station data files (e.g., the RINEX file header). This is generally expressed as an Earth-centered, Earth-fixed coordinate.

This critical piece of information is typically included for each base station in the CBS list and is obtained from the base station provider. Use the ‘Change…’ button to manage the datum of the base data file reference position for user base stations.

Handling for differential correction

The setting for how base station reference positions should be used in post-processing. There are 3 choices:

  • Use best match for current project geodetic settings (default)
    The reference position will be selected from EITHER the database OR the base data files depending on what is a better match based on the geodetics of the project. For the ‘Minimize Transformations’ and 'Best Available' project geodetic workflows, it will use the reference position in the same datum from either source (base files first). For the ‘Classic’ project geodetic workflow, it will use the reference position in the WGS 1984 global datum from either source (base files first) and if one does not exist, the first one from the database will be used and transformed from the specified datum to WGS 1984 at the start of post-processing. In the database, the ‘(classic / legacy)’ position is assumed to be WGS 1984.
  • Use only reference position from base station data files
    Only select the reference position from the base station data files. For the ‘Minimize Transformations’ and 'Best Available' project geodetic workflows, the reference position datum must match the project GNSS storage datum for this base station to be available for use in post-processing. For the ‘Classic’ project geodetic workflow, the reference position will be transformed from the specified datum to WGS 1984 at the start of post-processing.
  • Use only reference positions from database
    Only select a reference position from the database. For the ‘Minimize Transformations’ and 'Best Available' project geodetic workflows, a reference position using the datum of the project GNSS storage must be available in the database in order for the base station to be available for use in post-processing. For the ‘Classic’ project geodetic workflow, a global WGS 1984 reference position from the database will be used and if one does not exist, the first one from the database will be used and transformed from the specified datum to WGS 1984 at the start of post-processing. In the database, the ‘(classic / legacy)’ position is assumed to be WGS 1984.

For More Information

For more information on geodetics related to high-accuracy GIS data collection workflows, consult this site.

#terraflex
#positions
#trimble_positions
#esri
#arcgis
#arcmap

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Comments

Matthew Morris
02-17-2021 15:30

@Alison Walker Great question. This will depend on what the setting is for base station reference position handling in the user stations. If you create TerraFlex projects in NAD 1983 (2011) using either the "Minimize Transformations..." or "Use a standard reference frame..." workflow, then we will only let you use base stations for which we have a NAD 1983 (2011) reference position. The goal of these workflows is to minimize the transformations.

For CORS stations, we know that the reference positions in the base station (RINEX) files are in NAD 1983 (2011). So for those base stations, you will need the reference position handling set to either 'Use ​best match for current project geodetic settings' or 'Use only reference position from base station data files'​.

If the customer does not have a NAD 1983 (2011) reference position for these user base stations, then either the Classic/WGS84 workflow can be used or we can discuss further how this particular customer has historically handled the difference between NAD83 realizations in their data. Mainly, are they regarding NAD 1983 (2011) as the same as older realizations or are they modeling the difference using Esri composite transformations...or are they using NGS tooling to actually calculate a difference.

Alison Walker
02-16-2021 11:28

Does this change what base stations can be used to post-process?  I just had a case where a user had a bunch of user-defined base stations set up in Positions.  These still work in their ArcPad projects, but in their TerraFlex project, they were not able to see any profiles available that were user-defined.  When we added a profile from a station off the list, then everything worked as expected.  Must have something to do with those old base stations not being compatible with this project?

Matthew Morris
01-29-2021 20:19

Thanks, Joel. I'll follow-up over email.

Joel Cusick
12-27-2020 13:20

Hi Matt,

This enhancement is long awaited and will meet what I think is our most pressing need - to use in CONUS and Pacific Plate region of the United States the reference / datum position of all CORS adopted stations and in the case of National Park Service, our growing number of NTRIP stations.  Let me clarify, or let you clarify for me our situation.

First some background:  We are about to send to Trimble upwards of 20 new base stations reference positions via CBS list submissions. These are NETR9 and Alloy base stations that have been computed with 12-24 hour solutions sent to OPUS and averaged to obtain a NAD83 (2011 / PA11) Epoch 2010.0 solution.  These coordinates along with Ellipsoidal height are entered into the reference position of the Trimble web interface that manages the base station reference position and these are the solutions streamed out to Rovers as well as logged in 1 hour files stored on FTP for consumption in Post Processing software (PFO, Positions, TBC).  In other words, the base files RINEX header contains the local coordinate system we want our users to adopt.  Okay, thats the background.

In our case therefore, using the traditional CBS submission form we can only provide one and only one reference position.  See https://www.trimble.com/trs/trsform.asp  .

Thhe form, above the coordinate portion of the page says, and i quote

"Reference Station Position:

The reference position coordinates must be in Latitude/Longitude format, in terms of the WGS-84 datum or ITRF00 reference frame. If you are using a different datum or reference frame, please specify it in the Comments field below.

Reference positions that are not supplied in terms of WGS-84/ITRF00 will be transformed to WGS-84 coordinates in the base station list."

In every comments section for each station I will have text that explicitly states the coordinates reference frame (either NAD83 (2011) or PA11) with additional info on how we derived said coordinates.

On the submission page, I will also therefore select Local and "an averaged GPS location" in the selection box.  Is that TRUE in the case of NPS submissions?

Next, following the information on this page, since this local position is "in" a local CS either NAD83 (2011) or NAD3(PA11), the latter for Pacific plate regions, the end user will select in 10.8.1.1 wizard

Use only reference position from base station data files

I could also use 

Use best match for current project geodetic settings (default) since this application sees, in our cases a Map dataframe in ArcGIS set to NAD83 (2011/PA11) and does nothing.

Do I have this right?

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