[QGIS-Developer] Coordinates of a pixel

Tue Sep 1 06:32:41 PDT 2020

Hi

Also you can get the extents of the layer in the layer properties ->
Information tab e.g.:

Name DEM_10m
Path /home/timlinux/gisdata/Rwanda/DEM_10m.tif
CRS EPSG:4326 - WGS 84 - Geographic
Extent 28.9592194665249139,-2.8404830825979692 :
30.8994723851809887,-1.0469841014800894
Unit degrees
Width 21524
Height 19896
Data type Int16 - Sixteen bit signed integer
GDAL Driver Description GTiff
GDAL Driver Metadata GeoTIFF
Dataset Description /home/timlinux/gisdata/Rwanda/DEM_10m.tif
Compression
Band 1
RepresentationType=THEMATIC
STATISTICS_APPROXIMATE=YES
STATISTICS_MAXIMUM=3821
STATISTICS_MEAN=1697.9520539082
STATISTICS_MINIMUM=15
STATISTICS_STDDEV=341.19933119234
STATISTICS_VALID_PERCENT=56.38
More information
AREA_OR_POINT=Area
DataType=Generic
Dimensions X: 21524 Y: 19896 Bands: 1
Origin 28.9592,-1.04698
Pixel Size 9.01436962765319623e-05,-9.01436962765319623e-05

Regards

Tim

On Tue, Sep 1, 2020 at 9:51 AM Charles Dixon-Paver <charles at kartoza.com>
wrote:

> 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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Tim Sutton
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