The latest version of Blue Marble Geographics’ coordinate conversion software continues the 25-year tradition of providing solutions for the most complex geodetic challenges. Geographic Calculator 2019 offers a number of requested improvements, such as a more user-friendly interface, a universal copy and paste function, a new angular unit conversion tool, as well as several enhancements to seismic file format support. However, a closer look also reveals several new features that provide some insight into more significant shifts in Blue Marble’s future development plans.
Geographic Calculator 2019 has added support for version 5.0 of the National Geodetic Survey’s (NGS) North American Datum Conversion tool (NADCON 5.0). This single line item in the release notes may be easily overlooked, but it represents years of work by NOAA-NGS. It also represents a fundamental change in the way United States coordinate reference system and reference frame transformations are performed. Before delving into the details, let’s answer this question…
What is NADCON 5.0?
The origin of this transformation methodology is rooted in the readjustment of the North American Datum of 1927 (NAD27) to the North American Datum of 1983 (NAD83). The differences between these reference frames were very regional and irregular, which resulted in shifts that could not easily be modeled with traditional mathematical transformations. The solution was the original North American Datum Conversion (NADCON) Utility, which was adopted in 1990 as the federal standard for modeling differences between the two systems.
The early versions of the NADCON transformations typically had an accuracy of 12 to 18 centimeters, which represented a significant improvement over most other large-scale models at the time. That said, the tool did have its shortcomings. According to NGS, it was “poorly documented, was applied inconsistently across regions, contained numerous errors, and was difficult to use”. Those kinds of defects were also shared by another NOAA-NGS tool called GEOCON, introduced during the realization of the NAD83(2007) reference frame. The significant difference between the original NADCON and GEOCON transformations was the latter’s ability to perform three-dimensional coordinate transformations among various newer NAD83 frames. This was also improved in GEOCON11 (version 2.0), but time constraints meant that only eleven states were able to provide data for these adjustments.
NADCON 5.0 was built to replace both of these imperfect NGS tools. Unlike its predecessors, it is well-documented, more “user-friendly”, it includes downloadable transformation grids that can be integrated into third-party software, and covers the entire United States (including overseas territories). Newer remote sensing technology and the ability to handle much larger datasets also allows for a finer level of detail during the transformation process. As with the GEOCON model, NADCON 5.0 also offers new transformations between many reference frames and three-dimensional coordinate systems, and it supports the US National Spatial Reference System (NSRS) as well as many other previously unrecognized local horizontal coordinate systems dating back to the late 1800s. It also provides local error estimates as a component of the transformation, which is likely to pique the interest of your friendly neighborhood geodesist.
How does NADCON 5.0 work?
Like the traditional NADCON version 4.0 transformation, version 5.0 is delivered as a set of grid files that can be used to move between individual reference frames. Unlike version 4.0 however, it is no longer limited to horizontal shifts. The new grid files contain fields for identifying 3D transformations between reference frames and an error metric where available. Furthermore, there is a much larger set of grids to choose from and it is no longer constrained to the traditional NAD27->NAD83 or NAD83(20xx)->NAD83(20xx) model. Instead, transformations can be performed between six separate realizations of NAD83, NAD27, and the US Standard Datum (USSD). NADCON 5.0 also provides access to precise transformations between other historic systems such as the Old Hawaiian Datum, Puerto Rico 1940, and local Alaskan systems, such as the St. Paul Island reference frame of 1897 and 1852.
Unlike the old version 4.0 transformation, NADCON 5.0 was designed to chain together various grid files to provide a more accurate result. This makes things a bit more complicated because of the need to keep track of individual transformations as components of a larger concatenated operation. The following diagram shows an example of a shift from the NOAA Technical Report NOS NGS 63.
The illustration shows a chain of transformations for moving a surveyed data point based on the USSD system to NAD83(2011). Any subset of the chain can be used independently as part of the NADCON 5.0 model. This chain process can support a new model once it is created (for example, the 2022 National Reference System) and with one grid file, associate it to all historical models.
So what is foreshadowing about the addition of NADCON 5.0 in Geographic Calculator?
NADCON 5.0 will be instrumental in the transition to yet another adjustment from NAD83 on the horizon — the introduction of the new National Reference Frame of 2022 (NATRF2022) and NSRS2022.
If you are a frequent visitor to Projections, the Blue Marble blog, you may have read an entry by Product Manager Sam Knight explaining why the new NSRS is being developed. If you haven’t seen Sam’s entry, the short explanation is that geoid and GPS-height accuracy have improved and that NAD83 did not account for the dynamic movement of our planet over time. Under the new system, all measurable gravity-related values (such as orthometric heights, geoid undulation, deflections of the vertical, etc…) will be time dependent for compatibility with the NATRF2022 coordinate systems — making for more accurate time-dependent transformations.
NSRS2022 will also replace all of the current vertical datums, which will require updates to VERTCON — another NGS transformation tool. Originally designed to transform between the North American Vertical Datum of 1988 (NAVD 88) and the North Geodetic Vertical Datum of 1929 (NGVD 29), the 2022 update to VERTCON will transform orthometric heights from the old datums into heights in the new North American-Pacific Geopotential Datum (NAPGD2022).
Small changes leading up to larger changes
With awkward acronyms, complex geodetic concepts, and NGS tool history, the 2022 update as it relates to NADCON 5.0 is a lot for a short blog entry. But hopefully you get the key message: NGS is making big changes that will lead to more accurate transformations and Geographic Calculator is an early adopter of these new geodetic parameters. Be on the lookout for more updates as tools like NADCON 5.0 develop and grow. Until then, the Geographic Calculator will continue to hold a finger on the NGS pulse.