Joe Lovick via gdal-dev <gdal-dev@lists.osgeo.org> writes: > Could someone explain how GCP's placed into a VRT file work, i was > expecting a linear/bi-linear, or polynomial fit between the points for > the simplest EPSG:4326 projection.however that is not what i see, > adjusting a single point, effects the projection of all corner points.
(I am a newbie at VRT but find imagery registration interesting.) Before I started reading docs, I had a different expectation from you, which is that the input and output were conformal projections, and that there was a single translate/rotate/scale transformation estimated, probably via least squares, from all GCPs. I looked at: https://gdal.org/drivers/raster/vrt.html where it says GeoTransform: This element contains a six value affine geotransformation for the dataset, mapping between pixel/line coordinates and georeferenced coordinates. Typically (geotransform[0], geotransform[3]) will be the (easting, northing) of the upper-left corner of the raster, geotransform[1] the horizontal resolution in geospatial coordinates/pixel, and geotransform[5] the vertical resolution in geospatial coordinates/pixel, as a negative value if the image is north-up oriented. See Affine GeoTransform for more details about that mapping. <GeoTransform>440720.0, 60, 0.0, 3751320.0, 0.0, -60.0</GeoTransform> GCPList: This element contains a list of Ground Control Points for the dataset, mapping between pixel/line coordinates and georeferenced coordinates. The Projection attribute should contain the SRS of the georeferenced coordinates in the same format as the SRS element. The dataAxisToSRSAxisMapping attribute is the same as in the SRS element. <GCPList Projection="EPSG:4326"> <GCP Id="1" Info="a" Pixel="0.5" Line="0.5" X="0.0" Y="0.0" Z="0.0" /> <GCP Id="2" Info="b" Pixel="13.5" Line="23.5" X="1.0" Y="2.0" Z="0.0" /> </GCPList> which indicates that there are different scales for image-up anad image-right, and leaving me confused about [2] and [4]. As I was writing this, it came to me that they are the off-diagonal rotation terms.I think this is a translation (0/3) and a matrix multiply from pixel coords. map_coords = [0 + [image_east, image_north] * [ 1 2 3] 3 4] is now my understanding. So 2 and 3 are zero if the image's easting is equal to the x axis of the projected coordinate system. Thus, after reading, I still expect that GCPList is processed via least squares to find a GeoTransform translation and scale/rotation/slightly-more matrix. The basic point is that this process is starting from a belief that the imagery is consistent and needs only translation, scaling, and rotation. That's not quite true, as a general matrix multiply is looser than a rotation and scale, having 4 variables vs 2. _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/gdal-dev
