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 <[email protected]> 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 <[email protected]> > 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 (< >> [email protected]>) 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 < >>> [email protected]> 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 >>>> >>>> _______________________________________________ >>>> QGIS-Developer mailing list >>>> [email protected] >>>> List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer >>>> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer >>> >>> _______________________________________________ > QGIS-Developer mailing list > [email protected] > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer -- ------------------------------------------------------------------------------------------ Tim Sutton Visit http://kartoza.com to find out about open source: * Desktop GIS programming services * Geospatial web development * GIS Training * Consulting Services Skype: timlinux Irc: timlinux on #qgis at freenode.net Tim is a member of the QGIS Project Steering Committee ------------------------------------------------------------------------------------------- Kartoza is a merger between Linfiniti and Afrispatial
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