For every geospatial expert, datums, geoids and coordinate systems are essential pieces of knowledge that drive position precision.
For Trimble customers, geodetic assurance is facilitated by the Trimble Geodetic Library (TGL). First established in the 1990s, TGL is a set of software libraries commonly used to transform geodetic coordinates (latitude, longitude, height) into plane coordinates and elevation (north, east, elevation).
Most importantly, the TGL facilitates consistency and accuracy through the same and the latest Coordinate Reference Systems (CRS) and transform coordinates using the same algorithms with the same results on all supported Trimble platforms.
The library is managed by an experienced and expert global team with contributors in many countries including the US, France, New Zealand, Germany and India, all working together to ensure geodetic accuracy and updated reference systems. TGL also maintains a strong relationship with geodetic national organizations around the globe and is a well respected leader amongst its industry peers. The work of this team behind the scenes has never been more important.
One of the biggest challenges in the geodetic space today is that positioning accuracy has so improved that geospatial professionals must consider tectonic motion—something that prior to GNSS was not possible. For example, it is known that points on earth are all moving with corresponding tectonic plates (up to 10 cm per year), with significant shift and drift following earthquakes.
Figure 1: Australian plate drift in meters.
That constant movement creates CRS challenges. Consider this—using NAD 83, the geocenter of the world is more than two meters off from the current best estimates of the International Terrestrial Reference System’s (ITRS) geocentric system of coordinates. That mismatch causes systematic errors in the -2 to +2-meter range in latitude, longitude and ellipsoid heights.
In the U.S., the National Geodetic Survey continues to push toward the National Spatial Reference System (NSRS) 2022 modernization that will replace the current national datums—the North American Vertical Datum of 1988 (NAVD 88) and the North American Datum of 1983 (NAD 83). It’s adjustments like this where TGL really shines.
The value of the TGL to Trimble customers is consistent results and centimeter accuracy across field, desktop and cloud applications, which are the same as those provided by national agencies. Accurately transforming coordinates from a global datum to a national datum is a non-trivial task.
Typically, these applications store measured coordinates in a CRS that is used while making measurements. For instance, Trimble RTX coordinates at the moment are stored in the International Terrestrial Reference System’s ITRF2020 at measurement epoch. The International Terrestrial Reference Frame (ITRF) is a set of instantaneous coordinates (and velocities) of reference points distributed on the topographic surface of the Earth. However, the ITRF2020 coordinates we get using precise point positioning (PPP) services like Trimble RTX are changing significantly over time.
Figure 2: ITRF coordinates versus time, US.
On the other hand, while national agencies worldwide are working hard to elaborate coordinate reference systems they fix on specific tectonic plates, cadastral coordinates are not changing too much with time.
Figure 3: NAD83 coordinates versus time, US.
The TGL team uses time-dependent transformations to convert coordinates between the ITRF that is used to position satellites and the National Reference Systems used in each country. This can be confusing for many users. As much as possible, the transformation should be handled automatically by software to reduce complexity on the user side and then coordinates should be presented in a clear and concise manner—and Trimble continues to make significant enhancements to key software packages, automating this task for its users.
For instance, through the TGL database, customers select a coordinate reference system that is needed for final publication of data to a customer (e.g., United States, NAD83, Colorado Central 0502 and others).
Currently, TGL supports the State Plane Coordinate System zones and the county coordinates for Iowa, Illinois, Indiana, Michigan, Wisconsin and Minnesota. However, the State Plane Coordinate System SPCS2022 will have nearly 1,000 projections due to the large number of low distortion zones. The TGL team has already begun to implement changes to Trimble software to make it easier to select the appropriate projection in this situation, filtering usable CRS from location.
TGL also continues to work on support for time-dependent transformations, an enhancement that allows TGL to transform coordinates more accurately than was possible in the past, particularly in tectonically active areas. It also allows us to support plate-fixed datums like NAD83 and NSRS2022. In practice both the velocity and earthquake shifts are stored as a series of grid files, which are used to estimate the appropriate values for an arbitrary point by linear interpolation. Time-dependent datum transformations were introduced with Trimble Access 2020.00 and TBC 5.30 and deformation models with Trimble Access 2020.20 and TBC 5.40.
Figure 4: Map showing countries with Dynamic Datums supported by TGL: Countries in light blue model crustal motion using an Euler Pole. Countries in dark blue have a velocity grid. Countries in green use a full displacement model, including a velocity model and earthquake grids, and countries in red provide an online calculator, which we implement as a distortion grid.
Further, the TGL team frequently receives requests to support new systems or new transformations. The team logs any request from our customers worldwide and launches studies to ensure we support the latest CRSs and provide expected coordinates with centimeter accuracy.
One recent study focused on a new deformation model and datum grid transformation for accurate transformation of RTX coordinates from ITRF2020 at measurement epoch to ITRF2014-TRIGNET & Hart94 datums used in South Africa. A second study is focused on an updated deformation model for accurate transformation of ITRF2020 coordinates into JGD2011 datum used in Japan.
Staying current with ever-changing worldwide geodetic needs is another key piece of the Trimble Geodetic library team. The TGL team regularly attends conferences and webinars to follow major changes in any National Reference System. If you have questions about coordinate reference systems, tectonic-affected datums, or other geodetic concerns, contact your local Trimble Geospatial Authorized Distribution Partner.