[Gdal-dev] Re: [whatever] toWGS defintions

[Tomas R]

So for now I need to find a WKT for every projections I wish to support?

To bad, for me, but hopefully I can get a big set of users to help me 
getting the correct WKT with a toWGS for their maps. I have two working 
now, RT90 and CH1903+. Working really good with no visible offset when 
drawing a GPS-track on the map.

or wait for you to do what you *ought* to do :)
No, that will not do.

No tip on a source for WKT-strings? Perhaps one with all UTM-zones? Or 
one with the different projections used in Europe?


Big thanks
Tomas

Frank Warmerdam skrev:
> Tomas R wrote:
>> Do there exist any general source for correct [whatever] to WGS 
>> definitions?
>>
>> I have an application which by use of GDAL translates WGS coordinates 
>> to and from [whatever] projection. I have it working with a few fully 
>> defined WKTs but it is no fun searching the web after all WKTs I 
>> possibly would need.
>>
>> So I tried instead to define the Spatialreferences with EPSG-codes 
>> instead of WKT. And as suspected the result wasn't what I had hoped 
>> for since the EPSG lacked the toWGS information. The result in the 
>> projection I tried (RT90 2.5 gon V, EPSG 3020) the transformation was 
>> offset by approximately 200 meters west and 50 meters north.
>>
>> Am I correct in that the error occurs because the EPSG-code do not 
>> specify the toWGS? If I am correct - what approach do you suggest?
>>
>> The goal of the application, which is written in C#, is to be able to 
>> take any map the user provides, with or without geodata in it, and 
>> serve it to another application which internally works with WGS84. 
>> That is my application is to be able to set up and translate from a 
>> projection the user selects to and from WGS84.
>>
>> The Q is simple - any source for correct definitions of [whatever] 
>> toWGS that I can use?
>> Or can I make the EPSG-codes work? Or do they work but I have made an 
>> error?
>
> Tomas,
>
> Some EPSG datum definitions do not have mappings to EPSG, but many do.
> If EPSG has exactly one transformation to get to WGS84 for a datum, then
> it should be incorporated into the WKT generated by OGRSpatialReference.
> However, if there are more than one possibility the initialize-from-epsg
> code chooses to play it safe, and use none of them rather than risk
> picking one that is inappropriate to the situation and hide the fact that
> it is guessing.
>
> This is a questionable choice on my part.
>
> Libraries like GeoTools take the approach of picking one of the available
> transformations with some jiffy rules that they hope will pick better
> ones.
>
> But the core of the problem is that every TOWGS84 transformation for a
> datum is at best an approximation.  Often several transformations
> exist, and are applicable to particular regions, minimizing error in
> that region.
>
> Think that I *ought* to do:
>  o Implement methods to get a list of all known transformations for
>    a given datum with the descriptive name so that applications could
>    offer the choice to the user.
>  o Pick one by default when generating WKT for an EPSG coordinate system,
>    ideally with some sort of hint that the towgs84 value is just one of
>    several possibilities.
>
> Best regards,