ASPRS is preparing the geospatial industry for the modernized National Spatial Reference System

By Christopher Parrish, Qassim Abdullah, Linda Foster, Stephen White, and Jenna Borberg

The National Geodetic Survey (NGS) is modernizing the National Spatial Reference System (NSRS) in the United States. The modernization involves significant updates to the official reference frames and vertical datum used across the country, affecting the entire geospatial industry. The ASPRS NSRS Modernization Working Group prepared this article to help prepare the geospatial industry for the upcoming changes.

The geospatial industry is on the brink of a major advance that will affect all facets of our work. For the first time in over four decades, the official reference frames and geopotential (vertical) datum of the U.S., including territories, are scheduled to be updated.

The primary reasons for the updates include the non-geocentricity of the current North American Datum of 1983 (NAD 83) frames, bias and tilt of the North American Vertical Datum of 1988 (NAVD 88), multiple vertical datums, sea level change, the dynamic movements of geodetic control marks, and vast improvements in survey technologies and accuracies since the 1980s. As large volumes of existing maps and geospatial data are reference to NAD 83 and NAVD 88, these updates are a significant undertaking with broad-reaching implications.

The agency leading these updates is the National Geodetic Survey (NGS), a program office within the National Oceanic and Atmospheric Administration (NOAA), National Ocean Service (NOS). NGS is mandated to define, maintain, and provide access to the National Spatial Reference System (NSRS), the official system that defines latitude, longitude, gravity, scale, orientation, and height throughout the nation.

Most geospatial professionals understand the impending changes. They are highlighted in Table 1, and you can view them online in the ASPRS PE&RS publication, November 2024 edition.

This article looks at the benefits of NSRS modernization for the geospatial industry, including those working in photogrammetry, lidar, sonar, remote sensing, mobile mapping, surveying, and GIS, among others. It presents recommendations for geospatial firms in preparing for NSRS modernization. These recommendations are separated into those for geospatial service providers, software manufacturers, and the entire industry.

The article concludes with a look ahead at the anticipated NSRS modernization schedule and opportunities for getting involved in ongoing efforts to assist with the integration of the modernized NSRS into geospatial infrastructure and workflows.

BENEFITS OF A MODERNIZED NSRS FOR THE GEOSPATIAL INDUSTRY

The improved accuracies and data interoperability that will be enabled through NSRS modernization will provide tremendous benefits across all segments of the geospatial landscape. The modernized NSRS will better support data sustainability, meaning that geospatial data will remain useful over longer time periods and across multiple applications.

Just a few examples of specific applications that stand to benefit tremendously from the Modernized NSRS include:

• Floodplain modeling
• Coastal storm inundation modeling
• Improved hydrodynamic modeling (e.g., in support of salmon migration protection on the Columbia River)
• Precision navigation (including autonomous vehicles)
• Marine navigation safety, including computation of real-time under-keel clearance
• Infrastructure positioning and monitoring
• Transportation and engineering projects construction and maintenance

Also of importance, NGS is building in mechanisms to support time-dependent coordinates through the use of reference epoch coordinates (RECs), which will be computed by NGS every five or 10 years, and survey epoch coordinates (SECs), which will provide the position at the time of survey.

PREPARING FOR NSRS MODERNIZATION IN THE GEOSPATIAL INDUSTRY

To prepare to take full advantage of the benefits enabled by NSRS modernization, it is imperative that geospatial service firms and software providers take certain steps now. The following are ASPRS Working Group recommendations for geospatial firms, separated into those that apply mainly to geospatial service providers, those that apply mainly to geospatial software manufacturers, and those that apply to the entire profession.

A critical aspect of these recommendations is ensuring forward and backward compatibility of coordinates.

WORKING GROUP RECOMMENDATIONS FOR GEOSPATIAL SERVICE PROVIDERS

Geospatial Service Providers, including those who collect, process, and provide aerial and satellite imagery, lidar, sonar, hyperspectral imagery, and other forms of geospatial data, are advised to take the following steps:

• Ensure that all metadata for all archived data (not just final deliverables) is complete and correct, paying particular attention to reference frames, coordinate epochs, units (if feet, be sure to document whether international feet or U.S. survey feet), geoid models applied (e.g., GEOID12b, GEOID18), and acquisition dates and times.
• For all control points and checkpoints, archive the survey report and store the observation data files (for example, RINEX raw observation files, processed GNSS vector solutions, or total station observation files), so that they can be reprocessed later relative to the modernized NSRS. To the extent possible, store data using the NGS standard file formats. Reprocessing or readjusting the raw data or processed observations (such as GNSS vectors) are the most accurate forms of relating legacy data to the new datums. Users can also transform data, but the transformed coordinates will not be as accurate as if the raw data are reprocessed in the new datums.
• For all data deliverables (and possibly important intermediate products), store versions with geodetic coordinates (latitudes, longitudes, and ellipsoid heights) relative to the current NSRS (e.g., NAD 83(2011) epoch 2010.00), even if the project deliverables call for, say, SPCS 83 northings, eastings, and NAVD 88 heights.
• Document the full project workflows with particular attention to any coordinate transformations or conversions.
• Work with software manufacturers for all steps in your end-to-end project workflow to ensure they are aware of and preparing for NSRS Modernization.
• Assess and document the uncertainty of spatial coordinates in all geospatial data products. This will enable additional uncertainties associated with transformations to be accounted for and used in assessing whether transformed products still meet requirements.

WORKING GROUP RECOMMENDATIONS FOR SOFTWARE MANUFACTURERS

Geospatial software manufacturers are advised to take the steps listed below. As a note on terminology, many of these recommendations refer to handling of what are widely (if somewhat loosely) referred to as “Coordinate Reference Systems” or “CRSs” in geospatial software. Ideally, a CRS provides a complete definition of the reference frame (e.g., NAD 83, ITRF2020, or, in the future, NATRF2022), the realization (e.g., 2011), and epoch (date for which coordinates are valid), and, if applicable, the map projection system (e.g., Universal Transverse Mercator (UTM) or SPCS 83), zone, units (e.g., international feet, or meters), and vertical datum. (Unfortunately, current methods of storing CRS do not allow specifying the epoch, except in the remarks, but this is anticipated to be addressed in future standards revisions.)

• If your software uses European Petroleum Survey Group (EPSG) codes or International Organization for Standardization (ISO) Geodetic Registry (ISOGR) to define CRSs internally and/or in exported data products, ensure that the EPSG codes or ISOGR entries for new terrestrial reference frames and NAPGD2022 and SPCS2022 are supported. (Side note: the intent is for EPSG to be replaced with ISOGR, although the timeline is yet to be determined.)
• Ensure SPCS2022 coordinates can be computed in units of both meters and international feet (1 international foot = 0.3048 meter, exactly)
• Ensure coordinate conversions and transformations (if provided in your software) are consistent with those of NGS
• Ensure proper and consistent use of geoid models. Importantly, any geoid model is designed for and valid for only a specific reference frame (and often also a specific realization of the frame) and region. For example, in the current NSRS, NGS’s GEOID18 is designed only for coordinates in the North American Datum of 1983 (2011) epoch 2010.00 and will convert ellipsoid heights to orthometric heights in the following datums: NAVD 88 (in the conterminous U.S. only, not Alaska), the Puerto Rico Vertical Datum of 2002 (PRVD02), or the Virgin Islands Vertical Datum of 2009 (VIVD09). Applying GEOID18 geoid heights to WGS84 ellipsoid heights is invalid and does not provide heights in any recognized system. Similarly, applying Earth Gravitational Model 2008 (EGM08) geoid heights to NAD 83(2011) ellipsoid heights is invalid and does not provide heights in any recognized system. Other examples of geoid models designed for specific reference frames include GEOID09 associated with NAD 83 (NSRS 2007) and GEOID99 or GEOID96 with NAD 83 (HARN). When a geoid model is used to compute heights relative to a particular datum, it is important to document the specific geoid model (e.g., GEOID12b, GEOID18, etc.). In some software and metadata, the geoid model is included in parentheses after the datum, such as NAVD 88 (GEOID18).
• Provide uncertainties in output geospatial data products, accounting for uncertainties associated with coordinate transformations. Note that NGS is planning to provide uncertainties for transformations between current and modernized reference frames conducted using NGS’s software utilities. This is already done in the existing NGS Coordinate Conversion and Transformation (NCAT) software for transformations between all frames and datums, and that will continue in the Modernized NSRS.

WORKING GROUP RECOMMENDATIONS FOR THE ENTIRE GEOSPATIAL INDUSTRY

A recommendation on terminology is to avoid using the term “height above mean sea level” or “MSL height” when referring to NAPGD2022 orthometric heights. The correct term for height above the geoid, measured along a plumbline, is “orthometric height.” To explain, local mean sea level (MSL) is a tidal datum that varies along the coast, not only in response to changes in geopotential, but also to currents, local hydrodynamics and other variables.

For example, if one were to set a series of benchmarks along the coast, each adjacent to a tide gauge and each set at MSL = 0.000 m, differential levels run between these marks would show them to be at different NAVD 88 (or, in the future, different NAPGD2022) orthometric heights. Future versions of NOAA’s vertical datum transformation tool, VDatum, will enable transformation between NAPGD2022 and tidal datums, such as MSL, mean lower low water (MLLW), and mean high water (MHW).

A final, and most important, recommendation for everyone in the geospatial industry is to take advantage of NSRS modernization educational materials and opportunities. NGS, as well as university partners, have developed training modules, workshops, and short courses related to coordinate transformations, geoid models, map projections and distortion (including overviews of SPCS2022), and geodesy. A list (although not intended to be comprehensive) of recommended training modules and continuing education resources can be found on geodesy.noaa.gov. 

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Christopher Parrish, Oregon State University; Qassim Abdullah, Woolpert, Inc.; Linda Foster, ESRI and NSPS; Stephen White, NOAA National Geodetic Survey; Jenna Borberg, Oregon State University.

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