5. Software

a. Conversion

Grid to and from Geodetic coordinates software is available from a number of sources and is incorporated in many mapping and GIS packages. They are based on published equations so results can be replicated regardless the platform. An example is the NAD83 Coordinate Conversion worksheet in the Software section.

b. Transformation

A large selection of transformation software is available based on different models and transformation conitions. TransPack is a simple package consisting of three main transformation types: conformal, affine, and projective. Becasue there are so many transformation conditions that can be applied, there is no single best mathematical model for all transformations. Experimentation, sometimes difficlut to replicate, may be necessary.

c. NGS

NGS which was responsible for the datum update as well as the national network's records mordernization, developed transformation and conversion tools to help users move to the new datum. All the individual software programs were part of their Geodetic Tool Kit.

(1) Conversion

Prior to NAD83 creation, NGS developed a range of programs to perform forward and inverse convesions for SPC and UTM. Their equivalents were created post-NAD83.

(2) Transformation

NGS' initial product was LEFTI (LEast Squares Fitting Tranfornation and Interpolation) developed primarily for the USGS to determine updated map corner locations. A series of 7.5 minute quadrangle maps were published with the shifted map corners printed in their correct spatial positions. LEFTI while accuate enough at the 7.5 minute topoquad scale was insufficient for large scale uses. Its life was relatively short.

The next product, and one of the best known, was NADCON (North American Datum CONversion) [it was unfortunate that NGS used "conversion" in the name of a transformation program]. Many GIS/Mapping software packages incorporated NADCON. NADCON was updated throught the NAD83(HARN) readjustment

LEFTI and NADCON were limited to (1) 2D positions, (2) Geodetic coordinates, and (3) NAD27 to NAD83(86) transformation.

Next chrolnologcally were GEOCON (GEOdetic CONversion) and GEOCON 11. These differed in that they were 3D (including ellipsoid height) and addressed specific readjustments: GEOCON transformed from NAD83(HARN) to NAD83(2007), GEOCON 11 from NAD83(2007) to NAD83(2011).

By this point, it was getting to be a bit of a mess. If you wanted to transform a NAD27 UTM position to NAD83(2011) using NGS products you had to follow this process:

• Convert NAD27 UTM grid coordinates to NAD27 geodetic coordinates
• Use NADCON to transform the NAD27 geodetic coordinates to NAD83(HARN)
• Use GEOCON to transform the NAD83(HARN) geodetic coordinates to NAD83(2007)
• Use GEOCON 11 to transform the NAD83(2007) geodetic coordinates to NAD83(2011)
• Convert the NAD83(2011) geodetic coordinates to NAD83(2011) SPC grid coordinates.

Not very efficient, regardless of which datum/adjustment you started at.

(3) Combined Operations

Sidebar

NGS developed VERTCON (VERTical CONversion) which was the vertical transformation equavalent of NADCON. Not long after it was released the US Army Corps of Engineers developed CORPSCON. CORPSCON combined NADCON with VERTCON and added coordinate conversion front- and back-ends. Input and output coordinates could be any combination of SPC/ UTM, or Geodetic. CORPSCON was the first readily available 3D datum-to-datum conversion/transformation software. Its last version was 6.0 and was officially retired in 2023.  

NGS replaced their conversion and NADCON/GEOCON products with NCAT (NGS Coordinate Conversion And Transformation Tool). This online tool can transform between any datum/adjustment (horizontal and vertical) up to the most current one and convert between coordinate types on any datum (NGS finally got "conversion" and "transformation" right). For the previous UTM NAD27 to SPC NAD83(2011) example, the user just has to provide:

• Source coordinates, type, and datum/adjusment
• Final coordinate type and datum/adjustment

NCAT automatically performs all the necessary conversions and transformations. In addition to SPC and UTM grid, NCAT can also convert to/from Earth Centered Earth Fixed (ECEF) X-Y-Z and US National Grid.

NGS is expected to further develop NCAT when the new datums and grid systems arrive in the c2026 time frame.