April 25, 2023

NADCON 5 in Geographic Calculator 2023

Written by: Sam Knight


For a relatively long time, surveyors and mappers in the US have been familiar with working on the North American Datum. We have two major versions: the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83). NAD83 has had a number of realizations that have also become commonplace names: 1986, HARN/HPGN, FBN, CORS96, 2007, and 2011, leaving many possible combinations of realizations for folks trying to line up highly accurate and precise data over the years. For about 30 years, the official tool from the National Geodetic Survey to transform data between NAD27 and the original NAD83 (1986) was NADCON or (North American Datum CONversion). NADCON has been through version upgrades and expansions over the years but has largely done the same job, transforming data from the NAD27 frame to the NAD83 frame, specifically the 1986 epoch. Later updates added support for HARN/HPGN, and some FBN thrown in the mix as well. In 2012, NGS began work on a new tool to handle the 2007 and 2011 realizations, however that still leaves a patchwork of coverage at various epochs with no single-step solution to the growing transformation problems. 

Longitude differences of NAD27 and NAD83 (image courtesy of NGS)
Longitude differences of NAD27 and NAD83 (image courtesy of NGS)

In the early 2010s, an evolving ten-year plan got rolling to replace NAD83 after the 2011 realization. The new geometric reference frame would be called the North American Terrestrial Reference Frame of 2022 (NATRF22). With data all over the US in various realizations, and a lack of a comprehensive transformation set to move data between the realizations to prepare for the eventual new standard, a new toolkit was needed; enter NADCON5. This toolkit, which is supported by Geographic Calculator, is the current basis of the national transformations between any realization within the National Spatial Reference System. It completely replaces and supersedes the original NADCON kits. This toolkit allows definitive transformation of data in any epoch of the NSRS, all the way back to the US Standard Datum (pre-NAD27). Additionally, it will be the tool that supports the forthcoming NATRF2022 models.

Horizontal transformation vectors from NAD83(NRS 2007) to NAD83(2011)

With the advent of NADCON 5, more detailed information about transformations is also now easily accessible on an individual point basis, because of an impressive new capability that’s important in high-accuracy mapping applications as we move towards future surveying technologies and coordinate systems. In Geographic Calculator 2023, transformations involving the NADCON 5 methods report what NGS refers to as “uncertainty” of the transformation of the individual points, allowing users to assess the confidence in the output positions based on the transformation. These uncertainties are included for any Interactive or Point Database NADCON 5 transformation. For transformations involving multiple realizations, each step has an uncertainty. The NGS method for calculating a total uncertainty is cumulative; that is, each step’s uncertainty is summed as a total worst case. This can give a picture of the worst-case error that could be associated with a point. For most locations and combinations of epochs, the uncertainty will be in the order of a few centimeters. The methods used to sum errors in Geographic Calculator 2023 are the same as what you might encounter when running calculations on the NGS Coordinate Conversion and Transformation Tool (NCAT), so this provides another step for verifying the quality of a transformation output. For most datum transformations, we have a singular “accuracy” for the entire transformation area; this commonly reflects the accuracy of the transformation according to the IOGP’s EPSG dataset. This has been the state of the art for years; as transformations get much more tight and nuanced, more datums are likely to begin incorporating this more localized reporting.  For now, this reporting is limited to the NADCON 5 datum transformations, but represents an exciting step forward in high-accuracy geodetics.

Explore all the new features available in Geographic Calculator 2023 by downloading a 14-day free trial today! If you have any questions, please contact us.

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