[Proj] How to use +towgs84

[Eric Miller]

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.

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 

>>> On 3/5/2009 at 1:50 PM, Aaron Friesen <aaron at cartopac.com> wrote:
> Everyone,
> 
> One last +towgs84 question...
> 
> If I use Proj4 and the resulting datum is WGS84, which realization
> of WGS84 should I consider the result?
> 
> Original, G1150, ???
> 
> Thanks again,
> 
> Aaron
> 
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