[Qgis-user] Difference: EPSG 3004 - EPSG 102092

[Paolo]

Hello,
this last piece of information made everything even clearer.
Really thanks to everybody who took time to anser.
Regards
Paolo

Il 07/11/2013 16:39, b.j.kobben at utwente.nl ha scritto:
> As an added note: be aware that 102092 is NOT an actual EPSG code. EPSG
> codes with no's > 32768 are not in the official EPSG database, and are in
> actual fact therefore not EPSG codes really.
>
> It's just that many softwares allow for the creation of user defined
> Spatial Reference Systems, and to make them work in software that works
> with EPSG codes, you can use an "unofficial" code > 32768 to define them.
>
> The best-known example of that is the Pseudo-Mercator used by Google and
> others which was given the user defined code 900913. When EPSG finally
> officially sanctioned that SRS and put it into their database, it got the
> EPSG code 3857...
>
> Yours,
>
> --
> Barend Köbben
> ITC - University of Twente
> PO Box 217, 7500AE Enschede (The Netherlands)
> +31-(0)53 4874 253
> @barendkobben
>
>
>
> On 07-11-13 16:09, "Paolo" <e-paul at tiscali.it> wrote:
>
>>
>>
>> I realize I still need to learn much.
>> Thanks for taking time to explain.
>> Paolo
>>
>> Il 07/11/2013 14:59, G. Allegri ha scritto:
>>
>>
>>
>>
>>
>> If I understand well, the datum shift parameters are required in order to
>> improve the accuracy of the conversion between 3004 and other RS as ED50
>> UTM33, while they are not needed for conversion between 102092 and e.g.
>> WGS84?
>>
>>
>>
>>
>>
>>
>>
>> The shift parameters are alway needed to obtain higher precision for CRS
>> trasformations where the datums are different. When a transform from
>> datum A to datum B is required, Proj4 (the engine underlying SRS and
>> projections) does  datum A -> WGS84 -> datum
>> B. The shift parameters (+towgs84) are used my Proj4 to manage the
>> intermediated transformations to (and from) WGS84, which is kind of a
>> "pivot CRS".
>> So, they're should be always employed. 102092 does not define them,
>> that's why you get worst results.
>>
>>
>> giovanni
>>
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