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About geographic transformations and how to choose the right one
I just stumbled on this most helpful post About geographic transformations and how to choose the right one by Aileen Buckley of ESRI's Mapping Center. I even learned a few things from the comments...
I'd highly recommend the read, but if you only have a moment, read this:
So how do you choose the geographic transformation that should be used? Here are two Esri Knowledge Base articles that can help you:
HowTo: Select the correct geographic (datum) transformation when projecting between datums. This article contains links to downloadable zip files (for different versions of the software) that contain a list of all available datum transformations and their appropriate geographic areas of use.
HowTo: Determine which NAD_1983_To_WGS_1984 transformation to use. The gist of this article is summarized below:
- NAD_1983_To_WGS_1984_1 - for the entire North American continent.
- NAD_1983_To_WGS_1984_2 - for the Aleutian islands.
- NAD_1983_To_WGS_1984_3 - for Hawai'i.
- NAD_1983_To_WGS_1984_4 - superseded by _5; this transformation method should no longer be used!
- NAD_1983_To_WGS_1984_5 - for the 48 contiguous United States.
- NAD_1983_To_WGS_1984_6 - for the Canadian province of Quebec.
- NAD_1983_To_WGS_1984_7 - for the Canadian province of Saskatchewan.
- NAD_1983_To_WGS_1984_8 - for the Canadian province of Alberta.
Note that geographic transformations work in either direction. For example, the transformation listed as NAD_1983_To_WGS_1984_5 transforms from NAD 1983 to WGS 1984, as well as from WGS 1984 to NAD 1983. When using the Project Tool, the geographic transformation is recorded in the metadata.
Posted by idfg-bthomas at 02/03/2011 in
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