[Proj] How to use +towgs84

[Aaron Friesen]

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
> conversion.

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:
> http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml

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
10cm error.

Aaron