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I'm trying to read a GeoTIFF with one band which stores a value between 0 and 3 ( 255 is no maks ). My Goal is to write a little program which takes in a latitude/longitude and returns the fitting pixel value at the geocoordinate in the geotiff.

I downloaded it from here : https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/QQHCIK And if you wanna take a look at it... heres the download link. https://drive.google.com/file/d/104De_YQN1V8tSbj2uhIPO0FfowjnInnX/view?usp=sharing

However, my code does not work. It just outputs the wrong pixel value for a set of geocoordinates every single time. I guess either my geocordinate to pixel transformation is wrong or theres a huge logic mistake.

This was my first try, it prints the wrong value for the geo-coordinate. Furthermore it crashes with some geocoordinates, a negative longitude makes it crashes because this will make pixelY negative which causes an exception in the raster method.

            GdalConfiguration.ConfigureGdal();
            var tif = "C:\\Users\\Lars\\Downloads\\pnv_biome.type_biome00k_c_1km_s0..0cm_2000..2017_v0.1.tif";
            var lat = 51.0;
            var lon = 8.3;
            using (var image = Gdal.Open(tif, Access.GA_ReadOnly)) {

                var bOneBand = image.GetRasterBand(1);
                var width = bOneBand.XSize;
                var height = bOneBand.YSize;
                
                // Geocoordinate ( lat, lon ) to pixel in the tiff ? 
                var geoTransform = new double[6];
                image.GetGeoTransform(geoTransform);
                Gdal.InvGeoTransform(geoTransform, geoTransform);
                
                var bOne = new int[1];
                Gdal.ApplyGeoTransform(geoTransform, lat, lon, out var pixelXF, out var pixelYF);

                var pixelX = (int)Math.Floor(pixelXF);
                var pixelY = (int)Math.Floor(pixelYF);
                
                // Read pixel 
                bOneBand.ReadRaster(pixelX, pixelY, 1, 1, bOne, 1, 1,0,0);
                Console.WriteLine(bOne[0]); // bOne[0] contains wrong data
            }

My second attempt looks like the following... but also outputs the wrong pixel value for the given coordinates.

GdalConfiguration.ConfigureGdal();
            var tif = "C:\\Users\\Lars\\Downloads\\pnv_biome.type_biome00k_c_1km_s0..0cm_2000..2017_v0.1.tif";
            var lat = 24.7377; // 131.5847
            var lon = 131.5847;
            
            using (var image = Gdal.Open(tif, Access.GA_ReadOnly)) {

                var bOneBand = image.GetRasterBand(1);
                var bOne = new int[1];
                
                // Spatial reference to transform latlng to map coordinates... from python code
                var point_srs = new SpatialReference("");
                var file_srs = new SpatialReference(image.GetProjection());
                point_srs.ImportFromEPSG(4326);
                point_srs.SetAxisMappingStrategy(AxisMappingStrategy.OAMS_TRADITIONAL_GIS_ORDER);
           
                var mapCoordinates = new double[2]{ lat, lon};
                var transform = new CoordinateTransformation(point_srs, file_srs);
                transform.TransformPoint(mapCoordinates);
                
                // Map coordinates to pixel coordinates ? 
                var geoTransform = new double[6];
                image.GetGeoTransform(geoTransform);
                Gdal.InvGeoTransform(geoTransform, geoTransform);
                
                Gdal.ApplyGeoTransform(geoTransform, mapCoordinates[0], mapCoordinates[1], out var pixelXF, out var pixelYF);
                var pixelX = (int)pixelXF;
                var pixelY = (int)pixelYF;
                
                bOneBand.ReadRaster(pixelX, pixelY, 1, 1, bOne, 1, 1, 0,0);
                Console.WriteLine(bOne[0]);  // bOne[0] contains wrong value
            }

Does anyone here have experience on that topic?

I simple cannot fix it after hours of searching and looking at examples.

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  • As per the help center please do not include chit chat like statements of appreciation within your posts.
    – PolyGeo
    Commented May 19, 2022 at 22:08

1 Answer 1

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Such a program "gdallocationinfo" exists already and it is one of the standard GDAL utilities. The usage is documented in https://gdal.org/programs/gdallocationinfo.html and the source code is here https://github.com/OSGeo/gdal/blob/master/apps/gdallocationinfo.cpp. You would probably use the -wgs84 option

Description

The gdallocationinfo utility provide a mechanism to query information about a pixel given its location in one of a variety of coordinate systems. Several reporting options are provided.

...

-wgs84

Indicates input x,y points are WGS84 long, lat.

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