Hi, This "problem" doesn't sound GRASS or QGIS specific. As far as I know, all GIS act like that.
Compare http://gis.stackexchange.com/a/8031/187 for what happens if users (analysts at the Economist in that case) are not aware of it. Regards, Anita On Fri, Aug 24, 2012 at 10:16 AM, Paolo Cavallini <[email protected]>wrote: > An interesting issue from GRASS ML. > How do we deal with this issue? > All the best. > > -------- Messaggio originale -------- Oggetto: [GRASS-user] What > distance is being measured and used for buffers ? Data: Fri, 17 Aug 2012 > 17:35:00 +0200 Mittente: Moritz Lennert > <[email protected]><[email protected]> A: > GRASS user list <[email protected]> <[email protected]> > > Hello, > > I have a fundamental question about distances in GRASS, also in relation > to buffers, that I stumbled upon trying to help a student who tried to > draw buffers around cities indicating the maximum distance airplanes > could fly at different times in history. > > To test, I used the World Mercator projection: > > +proj=merc > +lon_0=0 > +k=1 > +x_0=0 > +y_0=0 > +no_defs > +a=6378137 > +rf=298.257223563 > +towgs84=0.000,0.000,0.000 > +to_meter=1 > > This projection has obvious deformations the further you get from the > equator, and thus distances are seriously distorted[1]. > > Using GRASS 6.5 to create a buffer of 6000km around Belgium gives a map > which shows a circular buffer [2]. > > Using the distance measuring tool in the wxGUI confirms that the > distance from Belgium to the buffer line in all directions is around 6000km. > > However, the actual distance is very different, especially towards the > North: According to CloudMade's routing website based on OSM data > northern tip of Norway is only ~3500km by car from Brussels, so very far > from 6000km. > > I get exactly the same results using r.grow.distance and r.cost. > > For comparison, I used a simple Plate Carree projection: > > +proj=eqc > +lat_ts=0 > +lat_0=0 > +lon_0=0 > +x_0=0 > +y_0=0 > +no_defs > +a=6371007 > +b=6371007 > +to_meter=1 > > Distortions are obviously different [3]. And the 6000km buffer is also > positioned differently compared to the land masses [4]. Again, measuring > the distances with the wxGUI distance tool gives me ~6000km in all > directions. > > So, my question: which "distance" is used for measurement and for buffer > creation ? A distance based buffer in map units should lead to deformed > buffers in most projection systems. Is there anyway to achieve such > deformed buffers and correct distance measures ? > > Moritz > > > [1] http://164.15.12.207/grass/distances/worldmercator.png > [2] http://164.15.12.207/grass/distances/buffer_worldmercator.png > [3] http://164.15.12.207/grass/distances/platecarree.png > [4] http://164.15.12.207/grass/distances/buffer_platecarree.png > > > _______________________________________________ > grass-user mailing > [email protected]http://lists.osgeo.org/mailman/listinfo/grass-user > > > _______________________________________________ > Qgis-developer mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/qgis-developer > >
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