[QGIS-Developer] Coordinates of a pixel

[Charles Dixon-Paver]

I took a cursory glance at the calculator you used and the description
you've given, and I think there's a possibility you're simply using an
easting value in the calculator. You could try multiplying your input
longitude by -1 for a quick fix (although I haven't tested this will work
at all).

On Tue, 1 Sep 2020 at 03:31, Miguel-Ángel Fas-Millán <fas.millan at gmail.com>
wrote:

> Thanks for your reply. Well, let me provide more details to explain why I
> was asking that.
> I have a DSM, with its xllcenter/yllcenter coordinates and I need to know
> the coordinates of the (in this case) center of any of the squares/cells
> (which, maybe wrongly, I called pixels) represented by this dataset. To
> that I've been using a function equivalent to the provided by this
> calculator: https://geodesy.noaa.gov/cgi-bin/Inv_Fwd/forward2.prl , which
> takes as parameters the starting point, azimuth, distance, and returns the
> ending coordinates.
>
> For instance, I want to calculate the coordinates of the first cell of the
> dataset (top left of the matrix). So I do:
>
> calculateEndingGlobalCoordinates(Ellipsoid.*WGS84*, start_coords, 0.0,
> nrows*cellsize) //0.0 for north
>
> The problem is that it returns a value with a latitude that seems ok, but
> a wrong longitude. When I place that coordinate in QGIS, it is out of the
> image, quite at left of the top left corner.
>
> Unsurprisingly, if I place the coordinates obtained with this method (or
> with the online calculator mentioned) in google maps, to check if it makes
> sense looking at what's there in the satellite images, they make no sense
> at all.
>
> However, if I take the coordinates appearing in QGIS when I hover one of
> the cells, and place them in google maps, it makes perfect sense. With a
> few meters of difference, but well, at least it is on the right track. (I
> am that sure because I took as reference a ATC tower, which is the highest
> element in a wide area).
>
> That's why I wanted to check what's the difference between the mentioned
> method I was using and whatever is made in the code to return those quite
> correct coordinates.
>
>
>
>
> El mar., 1 sept. 2020 a las 0:01, Charles Dixon-Paver (<
> charles at kartoza.com>) escribió:
>
>> Someone can correct me if I'm wrong, but I think there's a
>> misunderstanding here of how this raster data is handled in a GIS. From my
>> understanding of the question, I don't know that what you're asking for is
>> available "in the code" the way you expect.
>>
>> The coordinates are showing the position of the cursor relative to the
>> origin of the assigned coordinate reference system. The raster data is
>> "projected" onto that reference system, which assigns positions to some
>> points on the image and stretches, rotates or distorts the image in
>> accordance with the images affine parameters that ensure all the different
>> parts of the image remain spatially correct. How the GIS knows where to get
>> these parameters varies between data types and file formats.
>>
>> Playing around with the georeferencer tool in QGIS should give you a
>> pretty clear understanding of how this "projection" works.
>>
>> There are ways to get the coordinates of a position or pixel within an
>> image programmatically, the easiest of which that I can think of is using
>> values from a world file [1] with an xy position (in pixels) of the pixel
>> of interest. The code required for achieving this, however, is probably
>> going to be dependant on a wide variety of factors (not least of all the
>> CRS units and pixel size). Alternatively, you could likely grab the
>> coordinate position of a pixel from within a QGIS project, but that doesn't
>> seem to be what you're after. Perhaps a developer familiar with the GDAL or
>> QGIS code bases can point you in the direction of some wizardry that will
>> achieve what it is you are looking for without a clearer understanding of
>> your use case though.
>>
>> [1] https://en.wikipedia.org/wiki/World_file
>>
>> On Mon, 31 Aug 2020 at 19:57, Miguel-Ángel Fas-Millán <
>> fas.millan at gmail.com> wrote:
>>
>>> Hi all,
>>> New around here and as a QGIS user. I need to find something in the
>>> code, I hope you can help me.
>>> When you open a DSM and place the mouse on any pixel, you can see the
>>> coordinates of that point. Could someone tell me where in the code is made
>>> the calculation of those coordinates?
>>>
>>> Regards,
>>>
>>> Angel
>>>
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>>
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