[gdal-dev] Re: GDAL drivers written in Python

[Even Rouault]

Selon Knut-Frode Dagestad <knutfrodesoppel at hotmail.com>:

> Even,
>
> Thank you. That clarifies, and motivates use of the MEM format.
> I gradually start to understand that GDAL is generally more
> file-oriented than memory-oriented.
>
> Then I have one last question for this thread:
> As mentioned in another post, for some formats (e.g. Envisat) both the
> image data and GCP's are read by GDAL.
> What is then the reason why reprojecting with API or gdalwarp does not
> work, and why this (and other) format is classified as "not
> georeferenced" in the table of supported raster formats?

Ah, the doc is sometimes incorrect or lags behind the code. If you see
inconsistencies, please file a ticket.

>
> My theory is that this is because polynomial fit between row/col and
> lon/lat does not work since there are errors (contradictions) in the
> list of GCP's (e.g.: http://trac.osgeo.org/gdal/ticket/3709).
> Or is there another explanation?

Yes, this is a possibility. If the GCPs are incorrect, the algorithm may not
manage to "invert" the transformation. But I would have assumed that this would
lead to a more explicit error message than the one you get.

Perhaps you can try playing with the -tps or -order option of gdalwarp, but if
the GCPs are incorrect, it is a bit useless of course...

>
> Best regards from Knut-Frode
>
>
>
>
> On 07.04.2011 13:31, Even Rouault wrote:
> > Selon Knut-Frode Dagestad<knutfrodesoppel at hotmail.com>:
> >
> >> Hi Even,
> >>
> >> On 06.04.2011 20:21, Even Rouault wrote:
> >>> You can certainly write a (non-GDAL) reader in Python, but of course, you
> >>> won't be able to benefit from the GDAL API with the "dataset" returned by
> >> your
> >>> reader, or using them in software that use the GDAL API.
> >>
> >> But if I use the GDAL API to create a GDAL Dataset and then insert a
> >> matrix of data (read with non-GDAL code), and further add some
> >> georeferencing or tie-points, it seems to me that I then can utilize
> >> GDAL API functionality for e.g. reprojecting, subsetting and exporting,
> >> just the same way as if it was read with a GDAL-driver?
> >
> > Ok, I didn't understand well what you meant before. You could for example
> create
> > a MEM dataset ( http://gdal.org/frmt_mem.html ) and feed it with matrix of
> data
> > and georeferencing. If your data is too big and cannot fit entirely into
> RAM,
> > then you'll have to go with a temporary file, such as geotiff.
>
>
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