NGS (the National Geodetic Survey) recently released a revised version of its Blueprint for the Modernized NSRS and has hosted several webinars on this topic, which are available to the public in recorded form. These materials are part of the NGS’s educational efforts around its plans to modernize the NSRS (National Spatial Reference System) originally scheduled for rollout in 2022 (but since delayed).
The NSRS is the coordinate system, developed and managed by NGS, that provides authoritative latitude, longitude, elevation and other locational values across the US. It is made up of a network of continuously operating reference stations (CORS), a set of thousands of monumented passive control points, and a suite of models describing dynamic, geophysical processes that affect spatial measurements.
The modernization effort includes replacing NAD 83 and NAVD 88 with a new spatial reference frame (NATRF2022 for the coterminous US) and vertical datum (NAPGD2022) that explicitly account for crustal motion over time.
In an earlier article I discussed the five different types of coordinates defined in the Blueprint document. This article looks at one of the use cases in the document: transitioning data to the modernized NSRS.
Resurveying, Readjusting, or Transforming
As with the last major datum modernization, when NAD 83 and NAVD 88 supplanted NAD 27 and NGVD 29, NGS’s policy is that the most accurate way to establish coordinates in the new reference frame/datum is by resurveying. Readjustment of original observations to new control is seen as less accurate than resurveying, but more accurate than mathematical transformation, which is viewed as the least accurate method to generate new coordinates (p. 90).
Unfortunately, for most GIS users, it is this last option – transformation – that will be used to convert existing data to the new system. This includes both existing vector and raster data such as orthophotos and Lidar data.
Resurveying means returning to the field to collect new observations, relying on geodetic control that has coordinates in the new reference frame and datum. To facilitate resurveying, NGS will provide geodetic control in the modernized NSRS and will expand its OPUS (Online Positioning User Service) software to support a wider variety of surveying methods. OPUS will be able to provide recommendations to users so that their surveys – if correctly processed – will be tied to the NSRS (p. 92).
Readjusting means using existing observations to compute new coordinates based on geodetic control defined in the new reference frame/datum. The OPUS tool will be available for users to upload existing observations and adjust those data to geodetic control in the modernized NSRS (pp. 92-93).
Transforming means taking a finished product like a map or geospatial database based on coordinates in NAD 83/NAVD 88 and applying transformation software to estimate coordinates in the new reference frame/datum. To facilitate such transformations, NGS will offer two overlapping transformation tools: NCAT (NGS Coordinate Conversion and Transformation Tool) and VDatum. With ongoing work to align these two tools, they will both soon offer the same official transformations between the current NSRS (NAD 83 and NAVD 88) and the modernized NSRS (NATRF2022 and NAPGD2022). Transformations will be the prevalent method used to update legacy geospatial data. (pp 92-93)
To further support transformations of data, NGS plans to support users in the following ways (pp. 93-95):
- NCAT and VDatum source code will be made available so that these tools can be incorporated into non-NGS software. The datasets and spatial grids that define how NCAT and VDatum operate will also be available in standard formats.
- Documentation and instructions for performing transformations will be provided, including transformation equations and proper sequencing of multiple transformations.
- Sample data sets will be provided so that users can test other transformation software against NCAT and VDatum. Uncertainty estimates for transformations will be included.
- Older transformation software (such as NADCON and VERTCON) will continue to be available on the NGS website. Older transformations, such as those that exist in NADCON up through and including the transformation to NAD 83(2011) epoch 2010.00 will stand unchanged. Although the US Survey Foot will be deprecated on December 31, 2022, NGS will also continue to support the Survey Foot in software for historic applications.
Handling Large Datasets
Users have asked NGS for transformation tools capable of handling large datasets such as Lidar point clouds. Since NGS servers will be unable to handle a large volume of such data transformations, several options are available to users (p. 94):
- NCAT and VDatum can be downloaded to run locally.
- Software developers can incorporate the NGS-provided tools into their own code.
To ensure third-party software contains the correct transformations and methods, users will be able to check with NGS test datasets online and NGS will assist communities with technical advice necessary for their efforts (pp. 94-95).
Note that WSRS2022 – the Wisconsin Spatial Reference System 2022 Task Force – is also considering a plan to develop an online software tool to accommodate current and new reference frames/datums for Wisconsin-specific projections not found in NCAT, such as WISCRS. See the WSRS2022 web page for more information on Task Force activities and plans.