Software to Help Surveying Engineers Deal with the Coordinate Changes Due to Crustal Motion in Alaska

Crustal motion in the western United States and Alaska, due primarily to tectonic forces, causes positional coordinates of points on the Earth’s surface to change over time. As a result, accurate surveying in these areas requires an equally accurate description of this deformation to allow survey measurements conducted at different times to be corrected for such movement. NOAA’s National Geodetic Survey (NGS) has developed the HTDP (horizontal time-dependent positioning) software that enables its users to make these corrections. HTDP may also be used to transform coordinates measured at one time to corresponding coordinates that would be measured at some other time. To accomplish these tasks, HTDP contains numerical models describing interseismic horizontal crustal velocities. HTDP also contains models describing the displacements associated with 30 major earthquakes, including two in Alaska. NGS updates the HTDP software periodically to incorporate newer models. Over the last few years, NGS has made substantial advances in developing models to allow coordinates and survey measurements in Alaska to be corrected for crustal motion. This includes models describing Alaska’s horizontal velocity field, and models for both the coseismic and postseismic displacements associated with the 2002 Denali Earthquake. The Denali postseismic model is the first such model included in a version of HTDP.