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FME supports conversions between coordinate systems using different datums. Many datum transformations are not mathematically definable and require the use of a grid of shifts. If you attempt to make a datum transformation of this kind without the appropriate grid shift file in place, FME will stop the translation. This article shows examples of how you can control which grid shift files are used and in what order.
To understand better how FME handles grid shift files, please refer to the onlinedocumentation.
Note that in many cases you need to contact the authority responsible (provincial ministry, etc.) for maintaining the grid shift file in order to obtain them. We are not always allowed to distribute them ourselves.
When converting datums, FME chooses the grid shift file to use from the list configured in Tools - FME Options - Coordinate Systems in the appropriate Grid Transformation.
When this was originally designed, there were only six HPGN files (HARN used to be called High Precision GPS Network, and thats why the files are named *HPGN.las). At the time in all cases of overlap, the results were the same from any of the overlapping files, but this is no longer true. The design was intended to make HARN look like one datum, and not preclude the conversion of data which might extend beyond or over a grid file boundary. By making a selected wihpgn.las (Wisonsin) the first entry for example, it becomes the file of choice.
If you only need high precision in one geographic region, for example the state of Wisconsin, you could remove the other entries and gain a slight performance improvement as well as not have to worry about grid shift file order.
例如,同样的原则也适用于NAD83. The alaska.las files, the conus.las files, and the hawaii.las files are all listed in Grid Transformation dialog. Users can order the files as needed. Thus, NAD83 is a single datum and prompts for which file to use are avoided.
Here is a listing of States and their associated grid shift files.
For more background on NADCON and a complete list of US grid shift files see
http://www.ngs.noaa.gov/PC_PROD/NADCON/Readme.htm
Area/State File Name ============================================ Alabama ALHPGN Arkansas ARHPGN Arizona AZHPGN California CNHPGN (Above 37 degrees latitude) California CSHPGN (Below 37 degrees latitude) Colorado COHPGN Florida FLHPGN Georgia GAHPGN Hawaii HIHPGN Idaho-Montana EMHPGN (East of 113 degrees longitude) Idaho-Montana WMHPGN (West of 113 degrees longitude) Iowa IAHPGN Illinois ILHPGN Indiana INHPGN Kansas KSHPGN Kentucky KYHPGN Louisiana LAHPGN Maryland - Delaware MDHPGN Maine MEHPGN Michigan MIHPGN Minnesota MNHPGN Mississippi MSHPGN Missouri MOHPGN Nebraska NBHPGN Nevada NVHPGN New England NEHPGN (CT,MA,NH,RI,VT) New Jersey NJHPGN New Mexico NMHPGN New York NYHPGN North Carolina NCHPGN North Dakota NDHPGN Ohio OHHPGN Oklahoma OKHPGN Pennsylvania PAHPGN Puerto Rico-Virgin Is PVHPGN South Carolina SCHPGN South Dakota SDHPGN Tennessee TNHPGN Texas ETHPGN (East of 100 degrees longitude) Texas WTHPGN (West of 100 degrees longitude) Utah UTHPGN Virginia VAHPGN Washington-Oregon WOHPGN West Virginia WVHPGN Wisconsin WIHPGN Wyoming WYHPGN
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