Coordinate Systems and Terrain Reusability

Abstract

This paper is about a coordinate system problem for the “live” aspect of LVC training that impacts terrain reusability. Commonly used commercial software for producing correlated terrains has an outdated definition of datums that assumes that NAD83 and WGS84 are still equivalent, and does not take into account the newer realizations of these, or the fact that there have been significant plate tectonic and other changes of the earth since 1984. This results in a total overall difference of up to 1.7 meters between a synthetic 3D terrain made from NAD83 survey data and satellite imagery, and the live tracking data from a training exercise, which is in the latest realization of WGS84 at the epoch of the exercise. There is no available GIS software that includes an implementation of the latest realization of WGS84 (G1150) or time based transformations that can be used for imagery or elevation data. So even when pre-processing data, there is a remaining terrain error that varies depending on the plate tectonic velocity of the area of interest – currently about 65cm total error in some parts of California. The paper discusses two possible solutions to this problem, and their implications for terrain reusability. In particular, if a terrain is transformed to match the live tracking data for a particular event, then the terrain is specific to that epoch and not reusable for later events without loss of accuracy. A better solution would be to agree on a standard epoch for terrains, and use the Horizontal Time Dependent Positioning (HTDP) software to transform the tracking data to match that epoch. It is also important to ensure that all WGS84 source data includes the epoch as part of the metadata.

Keywords: coordinate systems, position accuracy, plate tectonics, terrain reusability, live training