[Proj] How to use +towgs84
aaron at cartopac.com
Thu Mar 5 19:14:50 EST 2009
Eric, et al,
> -----Original Message-----
> Proj4 doesn't know about the different realizations. (Actually, it
> doesn't really know that much about any datum.) I believe most of the
> towgs84 conversion parameters in epsg and elsewhere are based on the
> original realization. I'd guess that the error introduced by the 3-
> and 7-parameter datum conversion methods greatly exceeds the
> differences between the WGS84 realizations. So, if you need to retain
> the accuracy of a specific epoch you probably should avoid any datum
That was my assumption. Thanks for the clarification.
Unfortunately, with non-WGS84 data going to/from a GPS, avoiding any
datum conversion is impossible. I simply need to be aware of the
approximate error introduced in the conversion.
> The NGS has a tool for the U.S. HPGN/HARN, WGS84 and ITRS epoch
> conversions. I'm not sure how valid it is for areas outside the U.S.
> for handling just the WGS84 and ITRS. See:
I am using HTDP for NAD83 to WGS84 transforms, thanks.
FYI, HTDP does not transform HARN data. CORS96 is the earliest
NAD83 realization supported. For my use with HARN data, I simply
assume the HARN data is CORS96 (1997.0) with an additional
More information about the Proj