This presentation covers issues relating to datum transformations between the several horizontal/geometric and vertical geodetic datums most commonly used in the United States. These include: The North American Datum of 1927 (NAD 27), the North American Datum of 1983 (NAD 83), the International Terrestrial Reference Frame (ITRF), the World Geodetic System of 1984 (WGS 84), the National Geodetic Vertical Datum of 1929 (NGVD 29) and the North American Vertical Datum of 1988 (NAVD 88). The presentation demonstrates and examines several commonly used transformation utilities from NGS and the National Geospatial-Intelligence Agency (NGA) specifically, the NGS Coordinate Conversion and Transformation tool (NCAT), VDatum Transformations, 3-parameter abridged Molodensky transformations originally computed by the U.S. Defense Mapping Agency (DMA) and the more rigorous 14-parameter Helmert transformations supported by the NGS HTDP (Horizontal Time Dependent Positioning) utility.
Objectives
- Be aware that all vendors of geospatial software use transformations from various sources
- Understand the accuracy of any transformation utility
- Being aware of the primary source of any transformation tool
- Importance of having complete metadata for positions and heights
