I'm trying to use NOAA's VDatum to verify the NAVD88 -> WGS84 datum conversion in a c++ datum conversion library that I've written.
The answers didn't agree, so I attempted to check a simple ITRF2008 -> ITRF2005 conversion, which should just use a single helmert transformation, and again the results don't agree.
Our helmert transform in question was written using the 7 parameter method in EPSG guidance 7.2 (using the position vector transform notation, the same as IERS), and tested against the GIGS dataset.
Using the (7 parameter) 2008 to 2000 Parameters from IERS Conventions (2010) page 41 yields the following transform*:
- X: 1513460.90425574 -> 1513460.90372061
- Y: -4463118.57366578 -> -4463118.58057589
- Z: 4283412.81104103 -> 4283412.81065335
(* small angle approximation, position vector rotation)
but the VDatum results are several centimeters offset in Z and several millimeters offset in X/Y, which I wouldn't expect if we were using the same transform and parameters:
So my question is, does anyone know whether VDatum uses the IERS specified transformation parameters for ITRFXX to ITRFYY conversions, or even better, what parameters at what epochs are being used?
NOTE: I realize the precision shown here is substantially greater than the accuracy in the parameters themselves, but I'm just trying to verify the math is done correctly, not that the results are accurate to the given number of decimal places. The same transform with the same parameters should match much closer than the second decimal place.