Coordinate Systems support topics

Datum-Adjustment Factor
A document that provides instructions for creating "modified" projection zones that match the scaled or "surface" state-plane coordinates used by some organizations. This scaling-only method is used by the Wyoming Department of Transportation to produce "surface" coordinates for highway-construction projects.

Modified State-Plane Coordinates
Instructions for creating "modified" state-plane projection zones that match the shifted-and-scaled state-plane coordinates used by some organizations. Such coordinates are often referred to as "ground" or "surface" coordinates. Includes an example from a Federal Highway Administration (FHWA) project.

NAD 83 Datum-Transformation Issue
Back in the 1990s the conventional wisdom of GPS was that NAD 83 was the same thing as WGS-84. The current realizations of those two datums differ by up to a meter, so that statement is obviously not true even though all the survey software we have seen still follows that earlier "wisdom." Trimble's Mapping & GIS Division wrote up a Support Note on this subject back in 2005 that started to address the issue but failed to address the time-derivative issue. If you are working in North America, at least until the publishers of survey software recognize this established fact, in almost all cases you will simply continue to enter NAD 83 values when the software asks for WGS-84.

Real-Time Correction Source Datums
Another Support Note from Trimble's Mapping & GIS Division. If you use real-time corrections, it is crucial that you understand the reference datum of your correction source(s).