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 >> >>
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