Dear all, In my investigation journey towards the optimisation of lidar data selection (pending the correct handling of NO_DATA values by the Editor), I tried to determine the coordinate reference system (CRS) used for TSW tiles. The aim was to display the corresponding grid in QGIS ('Vector' menu > 'Research Tools' > 'Create Grid...') in an accurate and practical way. I figured out the CRS of TSW seems to be based on the UTM zone corresponding to the origin of the route, but with the origin of the CRS aligned with the origin of the route. As the origin of the route lies in the middle of the [0, 0] tile, one can define two grids, or custom CRS: one that crosses at the origin of each tile (and reads (0, 0) at the origin of the route); one that delimits the boundaries of each tile. These two grids are shifted 500m in longitude and 500m in latitude. By convention, let's say the origin of datum (2) is set at the southwest corner of the (0, 0) tile. Given: [x0, y0] the coordinates of the origin of the route in the UTM system (longitude, latitude), and L the longitude (in degree) in the middle of the UTM zone (NB - West of Greenwich, L < 0), the "proj string" code for generating the CRS of the route should be (depending on where you want your origin): +proj=tmerc +datum=WGS84 +lon_0=L +k_0=0.9996 +x_0=500000-x0 +y_0=-y0 +proj=tmerc +datum=WGS84 +lon_0=L +k_0=0.9996 +x_0=500000-x0+500 +y_0=-y0+500 (Of course you will have to do the math before filling in the parameters above.) Example: The origin of my route is set at coordinates (N46.79376, E3.9693). The corresponding UTM zone is 31 North, the median longitude of which is L=+3°. The origin of my route in the UTM31N CRS is (573 974.6, 5 182 740.6). Therefore the proj string code for the custom CRS of type (2) should read: +proj=tmerc +datum=WGS84 +lon_0=3 +k_0=0.9996 +x_0=-73474.6 +y_0=-5182240.6 This seems to work just fine in QGIS, where I was able to cross-check the results with the coordinates displayed by the Editor (in degrees) of the boundaries of two tiles about 25 km apart. Apologies if this has already been covered elsewhere. Cheers, Matthieu
