Configuring Datum Grid Shift Files for the USA

Overview

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 online documentation.

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.
 

How Grid Shifts are Configured in FME

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.
 

HARN / HPGN Network in North America

When this was originally designed, there were only six HPGN files (HARN used to be called High Precision GPS Network, and that's 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 (Wisconsin) 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.

This same principle applies to, for example, NAD83. The alaska.las files, the conus.las files, and the hawaii.las files are all listed in the 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.
 

Grid Shift Files by State

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

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