[gdal-dev] Re: Slovenia vector data reprojection issue

[Markus Neteler]

On Wed, Mar 31, 2010 at 6:52 PM, Frank Warmerdam <warmerdam at pobox.com> wrote:
> Markus Neteler wrote:
>>
>> Hi,
>>
>> (back to an old problem)
>>
>> On Fri, Jul 24, 2009 at 4:31 PM, Markus Neteler <neteler at osgeo.org> wrote:
>>>
>>> Hi,
>>>
>>> I am trying to reproject Slovenian country data from D48 (WKT attached)
>>> and Italian ISTAT national vector data (UTM32/ED50) into a common
>>> projection.
>>>
>>> While the map "attach" rather well in Latlong/WGS84 using GDAL 1.6.svn,
>>> they don't in EU LAEA (EPSG 3035). This is a bit suprising for me.
>>> Any trick to get this done?
>>>
>>> The input SHAPE files have each a .prj file (attached), the target SRS
>>> I request by -t_srs epsg:4326 or -t_srs epsg:3035.
>>>
>>> My preferred SRS would be EU LAEA to match the EU CORINE data.
>>
>> It appears to be a bug in GDAL.
>> If I do
>>
>> * SLO -> EU LAEA
>>  the result is shifted, apparently the TOWGS84 in epsgD48_3787.prj is
>> not applied.
>>
>> If I do instead:
>> * SLO -> LL -> EU LAEA
>>  the result is "perfect".
>
> Markus,
>
> The problem is that the eu_etrs89_laea.prj does not have TOWGS84
> parameters for the datum, so if you go directly from D48 to laea
> no datum shifting will be done (since one end is not datum enabled).

Ah - I didn't realize this.

> If you go to WGS84 first, then the D48 datum shift is applied to WGS84.
> No datum shift is applied from WGS84 to the LAEA projection, but ETRS89
> is effectively the same as WGS84 I believe.

Yes, I think so.

> So the key is to mark the ETRS89 datum as equal to WGS84 - likely with a
> TOWGS84[0,0,0] entry in the coordinate system definition.

OK, I'll prepare accordingly my ETRS89 .prj template to avoid this
problem in future
with my current GDAL 1.7.svn installation.

[ I wonder if gdal* could say that the datum shift was omitted to
  wake up the user...]

> I see that after my datum overhaul, GDAL is expanding EPSG:4258 to:
>
> GEOGCS["ETRS89",
>    DATUM["European_Terrestrial_Reference_System_1989",
>        SPHEROID["GRS 1980",6378137,298.257222101,
>            AUTHORITY["EPSG","7019"]],
>        TOWGS84[0,0,0,0,0,0,0],
>        AUTHORITY["EPSG","6258"]],
>    PRIMEM["Greenwich",0,
>        AUTHORITY["EPSG","8901"]],
>    UNIT["degree",0.01745329251994328,
>        AUTHORITY["EPSG","9122"]],
>    AUTHORITY["EPSG","4258"]]
>
> which has the TOWGS84[] entry, so we can have confidence that this
> family of problem will be reduced as the richer set of datum shifts
> comes out with GDAL 1.8.
>
> In short, I think this is an understood situation and there is no
> need for a ticket.

Perfect. Thanks for the explanations,

Markus