Solving Common Coordinate System Configuration Problems in GIS Workflows
Part 3: What is Trimble doing to solve these problems?
We’ve spent the last several blog posts covering the basics of coordinate systems and looking at key challenges in field data collection workflows that relate to them. It goes without saying that these topics are relevant to users of any GIS field data collection workflow.
Now we turn our attention to work that Trimble is doing to significantly improve our core Mapping & GIS data collection workflows in this area.
Prior to November 2020, Trimble® TerraFlex™ software stored data in the global WGS 1984 coordinate system (geographic coordinates or latitude/longitude) in both the mobile and cloud environments. Real-time corrections from local sources (VRS/RTK) were transformed from their regional or national coordinate system to WGS 1984 using the standard, static datum transformation methods that Trimble has always provided. On the other hand, real-time corrections from global sources (e.g., free SBAS corrections or paid services like Trimble RTX™) were assumed to be already in a WGS 1984 (or the equivalent ITRF) coordinate system and were not transformed. Feature heights were always stored as height-above-ellipsoid (HAE) values. This served to keep the storage of GNSS data, and hence feature geometries, consistently in one coordinate system from which we could transform out to any other as required.
If you were only interested in getting WGS 1984 data out of TerraFlex through export or downloading through one of the ArcMap® add-ins, then no additional transformations would have been required. If, however, you wanted to get the collected data into your GIS system-of-record in a local coordinate system (e.g., a regional or national one), then an additional datum transformation would be required. Your data may have even taken a round-trip from the coordinate system used for real-time corrections, to WGS 1984 for storage and transit through TerraFlex, and back to the original coordinate system for storage in your GIS.
TerraFlex With Coordinate System Support
In the November 2020 release, TerraFlex allows you to configure your data collection project using a regional or national coordinate system. Additionally, you can choose either geographic (latitude/longitude) or projected (northing/easting) coordinate storage and display. Finally, you also can select a geoid model for storing feature heights as orthometric (or mean-sea-level, MSL) instead of being limited to HAE. The selection of coordinate system and geoid is set at the TerraFlex project level and affects not only the storage of feature geometries but also coordinate display and navigation in the field.
In the Trimble Connect Map Viewer, coordinate system selection is done as part of a new workspace creation wizard. The wizard guides you through the selection of the most appropriate coordinate system by first asking for the area-of-interest where you will be collecting data. The list of available coordinate systems is then filtered to only present the relevant choices (includes both geographic and projected coordinate systems). In addition, the wizard will display information to help you along the way.
In the desktop context of Trimble Terra™ Office add-in for ArcGIS® Desktop, the existing project creation wizard has been augmented to present a complete picture of project geodetic information, the mapping of Esri-to-Trimble coordinate system information, and the selection of the desired coordinate system in the TerraFlex project. This pertains to both the cloud (Connect) and offline workflows.
Note: The list of supported local coordinate systems and geoids is curated and will expand over time. In the November 2020 release, the following coordinate systems are supported:
Improvements to Positioning Settings and Behaviors
As discussed earlier, incorrect configuration of datum transformations can be a significant source of error. With the inclusion of a richer set of coordinate system information in the TerraFlex workflow, we have added more intelligence into the configuration and use of datum transformations used with real-time correction sources.
In the Positioning settings area of the Map Viewer, you are able to create real-time correction settings for use with Trimble GNSS receivers. Now that the Map Viewer workspaces can utilize coordinate systems other than WGS 1984, additional validation is applied when associating real-time correction settings. If the workspace is using a local coordinate system based on a local datum, only VRS/RTK real-time correction settings that use the same local datum can be used with it. This ensures that no transformation will be applied when using those local correction sources. If, on the other hand, you intend to use global correction sources like Trimble RTX or SBAS for workspaces that utilize a local coordinate system, then TerraFlex will actually use the most accurate, kinematic (time-dependent) datum transformation in the field. For certain regions, this will provide significantly improved transformation accuracy compared to earlier versions.
If you are initiating your TerraFlex workflows from the Terra Office add-in for ArcGIS Desktop, a similar set of configuration options and validation behavior exists to ensure the optimal configuration of datum transformations for real-time correction sources.
In the previous blog post in this series, we identified several key challenges in coordinate system configuration as part of GIS data collection workflows. One of the overarching goals of this TerraFlex release is to address these very challenges.
Challenge: My field-collected data looks correct by itself, but is nowhere near to my source of truth. They barely show up on the same map!
As the first step of the workspace creation wizard, the Map Viewer asks for the location where you intend to collect data and presents a list of coordinate systems that are valid for that area. This includes the system, zone, datum, geoid, and units. This prevents the selection of an incorrect coordinate system (or any of the components) from the very beginning. This also ensures predictability when the data is taken out of the Map Viewer and into your GIS system-of-record.
In the workflows that start with the Terra Office add-in, the coordinate system of the TerraFlex project is connected directly to the coordinate system of your ArcGIS infrastructure where the data is stored. The add-ins know how to map between equivalent (or compatible) Trimble and Esri® coordinate systems.
All coordinate system setup and configuration is done by the office user and not by the field user.
Challenge: My field-collected data looks close to my source of truth but is shifted by a constant amount (e.g., an offset).
With this update to TerraFlex, the data collection project now has a well-defined coordinate system as do any positioning settings that you have associated with the project. The coordinate systems we supported in TerraFlex have been specifically curated to ensure that appropriate datum transformations are always available. When working with a WGS 1984 project, you can associate a positioning setting that uses any coordinate system and those positions will be transformed using the most current datum transformation from Trimble’s coordinate system library. When working with a project in a local coordinate system, you will only be able to associate positioning settings that use the same local coordinate system, or, are global correction sources like Trimble RTX or SBAS.
The field user never has to select a coordinate system, a datum, or a datum transformation in TerraFlex—they need only to select the positioning configuration they want to use. The office user has already set the project and positioning configurations to use the appropriate coordinate system and even then, they never had to select a specific datum transformation.
Challenge: My field-collected data is very close to my source of truth, but not within the accuracy estimates of what the GNSS receiver is telling me.
The coordinate system components in TerraFlex support not only static datum transformations but also kinematic, or time-dependent datum transformations. Among other use-cases, this allows for the most accurate transformations between global (e.g., ITRF2014) and local (e.g., NAD 1983 (2011)) coordinate systems. If your project is set to use a local coordinate system and you are using a positioning configuration for say Trimble RTX, positions will be transformed using the most accurate time-dependent methods automatically. As with before, there is no additional action necessary by either the field user or the office user to take advantage of this.
Support for local coordinate systems has long been recognized as a critical part of enabling a first class data collection workflow in TerraFlex, and the team behind it is excited to get it in the hands of our customers. We will continue to improve the workflows in this area by adding support for additional local coordinate systems and geoids, as well as simplifying the world of geodetics for GIS data collection users. Look out for future blog posts in this series as we continue to evolve coordinate system support in TerraFlex.
Check out our webinar on "Understanding Coordinate Systems in GIS Data Collection" to learn more!
Continue to Part 4: GNSS Correction Services and Geodetics