The NGS GEOCON Page v1.1

The National Geodetic Survey has released the GEOCON set of tools for performing three-dimensional coordinate transformations.

GEOCON performs the transformation between NAD 83 ("HARN") coordinates and NAD 83(NSRS2007) coordinates. GEOCON also issues information about the quality of the transformation at each point, and notifications regarding poor quality results.

NGS policy had been to allow a 5 cm tolerance before changing an official NAD 83 coordinate derived from the HARN surveys to a coordinate derived from the FBN surveys, causing some significant overlap between the two datasets. As such, for GEOCON specifically, the term "NAD 83(HARN)" does not refer only to coordinates from HARN surveys, but rather, is a shorthand notation to mean "the most recent, pre-NSRS2007" NAD 83 coordinate on a point. Such coordinates may have their origins in either the HARN, the FBN, or a mix of the two surveys.

GEOCON requires, as input from the user, a file of three dimensional coordinates in one of two allowable formats. Either as pairs of FGCS Blue Book standard *80* and *86* records (one pair per point), or in a Free Format style, one record per point. The coordinate transformation and the associated quality values are obtained through biquadratic interpolation within a series of grids. Third party applications may obtain identical results if they use the same grids and algorithms.

GEOCON employs high resolution grids (1 arcminute by 1 arcminute) to obtain unprecedented fidelity in modeling coordinate differences. Frequently, one may see that the reported quality is extremely good (e.g. 1 cm or better), but this is not comparable to a geodetic readjustment of survey measurements. The National Geodetic Survey considers actual readjustment of survey measurements, and not coordinate transformations, as best practice.