I am attempting to use r.sunmask to identify the time at which an oblique-view picture was taken from a webcam in a mountain area by trying to match the projected shadow pattern. I thus first compute the elevetion/azimuth of the sun for my location (as provided by the USNO web site) and the use the A-mode projection of r.sunmask to observe the shadow pattern. I was surprised to observe litte (or actually no) effect of the sun elevation on the cast shadow pattern, so I tried the following synthetic example: a DEM is defined by the following elevation file
north: 0.5 south: -0.5 east: 0.5 west: -0.5 rows: 10 cols: 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2000 2000 0 0 0 0 0 0 0 2000 5000 5000 2000 0 0 0 0 0 0 2000 5000 5000 2000 0 0 0 0 0 0 0 2000 2000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 which is inserted on a GRASS region centered around (0,0) and hence wih a 12.3 km cell size (111 km/9 cells). The resulting DEM is also found at http://sequanux.org/jmfriedt/t/sun1.png My synthetic mountain is 5000 m high in the first cell which hence exhibits a slope of 22 degrees, and the second half of the mountain rises by 2000 m over another cell so exhibits a slope of 10 degrees. The average tilt of the whole mountain slope spanning two cells is 11 degrees. I then generate the sun mask using r.sunmask for sun elevations of 20 and 40 degrees and different azimuths to differenciate the simulations (as found at http://sequanux.org/jmfriedt/t/sun2.png -- both png files also come with an associated world file): the 20-degree case should cast a shadow by the steepest slope, and the 40 degree elevation should not cast any shadow at all. As observed on the picture, a shadow is always cast all the way to the limit of my DEM, which seems incorrect if we are indeed looking at the projected shadow due to the topography. Any sun elevation above 22 degrees should not generate any cast shadow pattern, and I went up to testing a sun elevation of 89 degrees which also generated a cast shadow all the way to the limit of my DEM. Am I misunderstanding how to use r.sunmask ? Somehow r.sun does not seem to be working on my installation so I am unable to compare both outputs. All this was done with GRASS 6.4 called from QGis 2.0.1. Thank you, Jean-Michel -- JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user