[gdal-dev] GCPsToGeoTransform returns wrong values

[Ari Jolma]

On 18.11.2015 11:59, Bar Ori wrote:
>
> Thanks Ari for your reply.
>
> In my situation, sometimes I am working with rectangles and sometimes 
> not. Depends the source of information.
>
> I thought to always use the GCPs as follows:
>
> In case my shape has rectangular form, so the dataSet.GetGCPCount() 
> will return me “0” and I will use the dataSet.GetGeoTransform(), 
> otherwise, will use the dataSet.GetGCPs().
>
> The issue is that for setting the GCPs, I must put the coordinate in 
> the middle of pixel (that is why I  subtract “0.5”). If I wouldn’t, so 
> yes, the geotransform values would be correct.
>
> Is it a bad practice? Should I figure out the shape from start and to 
> take two different steps?
>

I don't know the best answer to that. I'm just assuming if you do it 
with some GUI tool for example it will report you pixel coordinates in 
integer values.

Ari

> *From:*gdal-dev [mailto:gdal-dev-bounces at lists.osgeo.org] *On Behalf 
> Of *Ari Jolma
> *Sent:* 18 November 2015 11:35
> *To:* gdal-dev at lists.osgeo.org
> *Subject:* Re: [gdal-dev] GCPsToGeoTransform returns wrong values
>
> Bar,
>
> I think you are setting the GCPs up wrongly if you want them to be at 
> the corners.
>
> GCP(LeftDown.X, LeftDown.Y, LeftDown.Z, Height - 0.5, null, "4") does 
> not have 0 before Height-0.5.
>
> GCPsToGeoTransform checks the special case of GCPs in rectangular 
> shape and in top-left, top-right, bottom-right, bottom-left order and 
> returns the exact transform the dataset has.
>
> I'm not sure if it makes sense to have real values for the pixel 
> coordinates although they are real values.
>
> Best,
>
> Ari
>
> On 18.11.2015 09:32, Bar Ori wrote:
>
>     Hi,
>
>     I noticed something that for me looks weird and I would like to
>     know maybe I am doing something wrong.
>
>     If not, how should I overcome this behavior.
>
>     I set 4 GCPs as follows:
>
>     vargcps = new[]
>
>     {
>
>     newGCP(LeftUp.X, LeftUp.Y, LeftUp.Z, 0.5, 0.5, null, "1"),
>
>     newGCP(RightUp.X, RightUp.Y, RightUp.Z, Width - 0.5, 0.5, null, "2"),
>
>     newGCP(RightDown.X, RightDown.Y, RightDown.Z, Width - 0.5, Height
>     - 0.5, null, "3"),
>
>     newGCP(LeftDown.X, LeftDown.Y, LeftDown.Z, Height - 0.5, null, "4")
>
>     };
>
>     And when I am doing as follows:
>
>     vargeotransform = newdouble[6];
>
>     Gdal.GCPsToGeoTransform(gcps, geotransform, 0)
>
>     Or (after using dataSet.SetGCPs(gcps, projection) :
>
>     vargeotransform = newdouble[6];
>
>     dataSet.GetGeoTransform(geotransform);
>
>     So the values returned, are wrong.
>
>     For example, if the LeftUp.X was 35.01050 so the value of
>     geotransform[0], which supposed to be the same value, can be 35.0082.
>
>     The same with all the geotransform values.
>
>     If I am getting from the dataset and setting again, every time it
>     is changing more and more.
>
>     Is there any connection to the use of the “0.5”?
>
>     Thanks.
>
>     **
>
>     **
>
>     	
>
>     *Ori Bar |***Software Developer
>
>     Applications Development
>
>     Elbit Systems Land and C4I
>
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