I'm trying to implement the recommended WGS84 - NAD83 transformation for Alaska using PostGIS. This post: Geographic Transformations in Alaska Between NAD83 and WGS84 indicates the proper transformation in ArcGIS to be WGS_1984_(ITRF00)_To_NAD_1983. I haven't been successful at finding this in
spatial_ref_sys. I tried adding my own transformation into
spatial_ref_sys using the following proj4text:
+proj=longlat +ellps=GRS80 +datum=NAD83 +towgs84=-0.9956,1.9013,0.5215,-0.025915,-0.009246,-0.011599,-0.00062 +no_defs
towgs84 values were copied from the listed settings that ArcMap shows when you choose this transformation (reversing the sign because it's to WGS84 in PROJ4 and from WGS84 in ArcMap). In PostGIS, I use it like this, transforming WGS84 to NAD83 using my custom projection, then transforming it again into 4269 (or whatever NAD83 projection I really want, like 3338).
ST_Transform(ST_Transform(geom_wgs84, 900999), 4269)
The problem is that the transformed values don't seem to match the transformations that ArcGIS are using, so if I have a mix of WGS84 data and custom-transformed into NAD83 data, ArcGIS's WGS84 -> NAD83 transformation is different from PostGIS and the data doesn't line up. The errors are small (1-2 meters), but large enough to see.
Can anyone suggest an appropriate PROJ4 string to replicate the results of the ArcMap transformation, or something better for the state of Alaska such as using a NAD grid? Or tell me what I'm doing wrong with my custom projection?