GPS On Bench Marks (GPSBM) Used To Make GEOID12
Each of the below regions uses variants of the NAD 83 reference frame and a local vertical datum. Several versions of NAD 83 exist conforming to significant plates: Pacific, Mariana, and North America. Likewise, each region has its own vertical datum. It is not possible to level across water, so islands will have selected a tide gauge to serve as the local datum point and all leveling is tied to that site. The only exception to this is Hawaii. No tide gauge was selected in the Hawaiian Islands and no vertical datum has been established as of yet. Hence, GEOID12 in Hawaii transforms between NAD 83 (PA11) and the same geopotential (geoid) surface as the USGG2012 model ( W0 = 62636856.00 m**2/s**2).
Items that are listed in the below table include the final GPSBM files for each region as both Excel spreadsheets and text files as well as thumbnail images linked to larger images showing the distribution of the GPSBM's. Alaska and the island regions are more consistent, so not many points were dropped and each is provided in it's own spreadsheet/text file and identified with the appropriate ellipsoidal reference frame and level datum (see below).
The most significant work occurred in the COnterminous United States (CONUS). For CONUS, there were 24,871 points with 868 dropped. These were supplemented from the OPUS-database with 736 points of which 258 were dropped. There were also 579 (with 5 dropped) in Canada and 744 (with 70 dropped) in Mexico to provide continuity across the CONUS borders. This totals to 26930 points with 1201 dropped. Points were dropped either because the State Adviser recommended it be dropped (e.g., known subsidence region), the residual ellipsoid height errors (from the NA2011 project) indicated a point was too noisy in comparison to other points in a state/region, the orthometric height was suspect, or the residual errors during geoid modeling were too high. The corresponding error flags are 'S', 'h', 'H', and 'N' as seen on the spreadsheet and text files. These points then represent the control data that were used to define the transformation between NAD 83 and NAVD 88 for CONUS.
