Configuring Datum Grid shift files

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To understand better how FME handles grid shift files, go to FME Workbench Help menu and choose Coordinate Systems help - Datums - Grid shift files. It used to be that you needed to manually edit some grid shift files or unse Universal Translator to edit others but everything can now be managed from within Workbench.

You can also see the info at: http://www.safe.com/support/gridshift/index.php which includes a link to NRCan which in turn has links to the provincial websites of the ministries responsible for distributing these.

When converting datums, FME chooses the grid shift file to use from the list provided in the GDC configuration file (e.g. Nad83ToHarn.gdc for NAD83 to HPGN). During the search, it looks for the grid shift file which covers the point to be converted which has the highest grid density (i.e. smallest grid cell size). If more than one is found, it uses the first one that it finds (it searches in the order listed in the gdc file).

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.l?s). At the time in all cases of overlap, the results were the same from any of the overlapping files. This is no longer true (obviously). The design was intended to make HARN look one datum, and not preclude the conversion of data which might extend beyond or over a grid file boundary. By making a selected wihpgn.l?s (Wisonsin) the first entry for example, it becomes the file of choice assuming that there are no other files which also cover the region being converted with a smaller grid cell size.

If you only need high precision in one geographic region, for example the state of Wisconsin, you could remove or comment out all the other entries and gain a slight performance improvement as well as not have to worry about grid shift file order.

Normally you would edit the .GDC file by using the Workbench - Tools - Options - Datum Grid Shift option OR Universal Translator - Settings - Datum Grid Files option to select particular datum shift and then use the interface to reorder the list. However, some datum conversion such as Nad83ToHarn are not available in this list. If so, since .GDC files are simple text files, they can be easily edited with Notepad. Additional customizations are available when edting the GDC files directly. For example, you can assign your own density level to override the actual density level of the file. This can be accomplished in the text editor as follows:

./USA/HARN/wihpgn.l?s,,,1.0

The value 1.0 indicates that FME is to consider this file to have a grid cell size of 1 minute, regardless of the grid cell denisty of the file. Thus, the above would accomplish the same thing as listing Wisconsin first in the file. Since 1 minute is smaller than the density of any of the other HPGN files, the Wisconsin file would be choosen over all the others, even if it were the last entry in the file (assuming that file did indeed cover the point to be converted).

Thus, between ordering the HPGN file references, removing references, and/or assigning density values to the references, you should be able to get what you need. There is no need to be prompted for which file to use, nor do we need to have 30 different HARN datum definitions, nor is the conversion hindered when the coordinates wander outside the extents of a single grid file.

This same principle applies to, for example, NAD83. The alaska.l?s files, the conus.l?s files, the hawaii.l?s files, and the Canadian NTv2.gsb files are all listed in the .gdc file. Users can then use the above techniques to order the files as needed. Thus, NAD83 is a single datum and prompts for which file to use are avoided. (This whole business of .gdc files was invented for the Australians when they had separate files for each of their states. They have a single national file now, so it no longer needed for Australia.)

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