[gdal-dev] GCPsToGeoTransform returns wrong values

[Bar Ori]

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?

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:

var gcps = new[]
{
new GCP(LeftUp.X, LeftUp.Y, LeftUp.Z, 0.5, 0.5, null, "1"),
new GCP(RightUp.X, RightUp.Y, RightUp.Z, Width - 0.5, 0.5, null, "2"),
new GCP(RightDown.X, RightDown.Y, RightDown.Z, Width - 0.5, Height - 0.5, null, "3"),
new GCP(LeftDown.X, LeftDown.Y, LeftDown.Z, Height - 0.5, null, "4")
};

And when I am doing as follows:

var geotransform = new double[6];
Gdal.GCPsToGeoTransform(gcps, geotransform, 0)

Or (after using dataSet.SetGCPs(gcps, projection) :

var geotransform = new double[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.


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Ori Bar | Software Developer
Applications Development
Elbit Systems Land and C4I
T  +972-9-8898900  |   M  +972-504-734296




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