Coordinate System Enhancements for CenterPoint RTX Corrections in Trimble Access 2020.20 and Trimble Business Center 5.40

The content of this blog was updated since its initial publication. Updates - Jul 2021, Nov 2021, Feb 2022, and Jun 2022: Additional reference frames and local displacement models have been added to the latest versions of Trimble Access and Trimble Business Center. The full lists of updates are available at the end of the blog.
 

Imagine taking just a rover into the field, turning it on and getting cm-level accuracy in your local coordinate system in less than a minute. No additional terrestrial infrastructure is required - no base station, no VRS network, no cellular networks, no radio or modem, and no need to set control.

The latest version of Trimble Access provides enhancements that allow you to use the same workflow for CenterPoint® RTX  that you would perform when using a single base RTK or network VRS for your GNSS corrections. You simply select the coordinate system that you’re working in, and you’ll be able to measure high-accuracy real-time CenterPoint RTX positions without requiring additional setup or transformation steps. The Access 2020.20 software will automatically transform CenterPoint RTX positions in real-time to match the coordinate system that you have selected. 

Surveyors have long been tasked to deal with site calibrations and offsets, the limited range of a VRS Network, and setting up base stations in the field to achieve their desired real-time accuracy. With new enhancements in Trimble Access 2020.20 and Trimble Business Center 5.40, working with CenterPoint RTX positioning eliminates all these hassles in the field. Utilizing these new software offerings enables surveyors to focus solely on the data collection with no interruptions, therefore maximizing efficiency in the field and providing more accurate positioning than ever before.

What is CenterPoint RTX?

CenterPoint RTX is a real-time high-accuracy GNSS correction service providing surveyors with cm-level positioning. Available globally and delivered via satellite or Internet, survey-grade accuracy is delivered to the GNSS rover without the need for a base station, radio, modem, or terrestrial connectivity. 

Trimble has a worldwide network of reference stations that are continuously tracking all GNSS constellations. Corrections are calculated for precise satellite clocks and orbits, along with global and regional atmospheric models. This information is sent to a satellite uplink and then to a set of geostationary satellites in orbit  around the globe; the geostationary satellites broadcast the Trimble RTX correction via the L-band frequency. Your Trimble GNSS rover is always tracking the standard GNSS satellites, and is also tracking the geostationary RTX satellite. Your rover is able to use the CenterPoint RTX correction directly from the geostationary satellite to calculate high-accuracy positions; there is no additional equipment required, however, you are able to receive the same correction stream via the Internet, if desired.

Coordinate Systems Revisited 

When performing any sort of point measurement, it’s necessary to have a frame of reference. In order to compare two or more point positions, you need to be using the same frame of reference for both. The entirety of this reference frame is referred to as your coordinate system, which includes the datum, ellipsoid, geoid, and projection that is being used. If positions are from different coordinate systems, then they must be accurately transformed before a suitable comparison is made.

Datums

Due to plate tectonics, the actual position of any point on earth is constantly changing, this is reflected in global datums such as the International Terrestrial Reference Frame (ITRF). The ITRF2014 coordinate of a position measured today, will have a different ITRF2014 coordinate in 5, 10, or 20 years time, because the tectonic plate will have drifted to a different location over time. All CenterPoint RTX positions are calculated in ITRF2014 at the epoch of measurement.

It’s challenging to deal with continuous coordinate change, so most national datums have coordinates that are static. By modeling the motion of the earth’s surface, these national datums project each coordinate to its position at a common date called the reference epoch, while still providing a link to the global systems.

Nearly all surveyors are using national datums in their day to day work, and the latest enhancements we outline in this blog provide a means of accurately transforming positions from a global ITRF datum to a local national datum. Time dependent transformations provide a way to transform positions measured in a global reference frame at the measurement epoch to a local reference frame at a fixed epoch. 

For more information on coordinate systems, refer to Trimble Geospatial’s Coordinate Systems 101 blog post.

What are the Trimble Access 2020.20 and Trimble Business Center 5.40 enhancements and where do they apply?

Simply stated: the enhancements to Trimble Access 2020.20 and Trimble Business Center 5.40 are an improvement in the accuracy of datum transformations in many regions around the world.

A datum transformation is a mathematical formula with parameters for converting coordinates from one datum to another. A simple example would be a 3 parameter transformation to indicate a shift in X, Y, and Z axes. A more precise computation involves 7 parameters —the 3 offsets plus 3 rotation parameters plus a scale parameter. Even more precise, and also necessary when dealing with dynamic datums or transforming between datums of different epochs, is a 14-parameter transformation—this is also referred to as a time-dependent transformation as the dynamic (or time-dependent) aspect of each parameter is now considered. In some countries (such as Australia) the tectonic plate motion is incorporated in 14-parameter datum transformation equations, and no further corrections are necessary to provide stable coordinates. However, for a country like the United States where part of the country lies across a plate boundary, a different strategy is adopted. Deformation in the stable part of the country may be done as described, but a deformation model is used for residual deformation, particularly in the plate boundary zone.

Previous versions of Trimble Access and Trimble Business Center were limited to static datum transformations. This meant that CenterPoint RTX positions could not be precisely transformed from ITRF2014 to a national reference frame through the use of transformations alone. In order to use CenterPoint RTX in a national reference frame, either a site calibration or an offset would have to be measured and applied.

Time-dependent transformations were first introduced in early 2020, and local deformation modeling has since been added in December 2020. With the additional support for local deformation modeling, the time-dependent transformations are now more accurate, allowing CenterPoint RTX positions to be directly used in a national reference frame. For additional details about local deformation modeling, please refer to the white paper.

Trimble Access 2020.20 and Trimble Business Center 5.40

CenterPoint RTX positions are now transformed using local displacement models. Improvements have been made to the time-dependent coordinate transformation feature, which is used to transform CenterPoint RTX positions between ITRF2014 at the epoch of measurement and the global reference frame:

• Local displacement models are used when available.

• Where no local displacement model is available, Trimble Access and Trimble Business Center use ITRF2014 tectonic plate velocities instead of the MORVEL56 tectonic plate models that were used in earlier versions.

• Country-specific realizations of the ETRS are used in Europe.

All these improvements result in a better representation of the position in the selected coordinate system, when using Trimble Access 2020.20 or Trimble Business Center 5.40, and CenterPoint RTX, ensuring that users obtain optimal accuracy in coordinate transformations and the best possible coordinates in the selected coordinate system.

Countries with coordinated system enhancements