Hi! To my knowledge - if you are doing some kind of "global" investigations you have to know which projection you choose (some fit better - some fit less) .. Anita is right when saying you have to be aware of the "specialities" of GIS. I'd say that's why there are GIS-Specialists .. Otherwise everybody could just take a GIS Software and there would not be any need for experienced people .. IIRC the first thing I learned in GIS was the difference between direct and "direct" connection of points depending on projection ..
So for the answer to the question .. probably a "self defined" equidistant projection might be a solution (depending on the point of view). with a global view it might be hard to find a "correct" solution for different towns worldwide .. just my 2ยข kind regards Werner On Fri, Aug 24, 2012 at 10:32 AM, Anita Graser <[email protected]> wrote: > 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 >> >> > > _______________________________________________ > Qgis-developer mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/qgis-developer > >
_______________________________________________ Qgis-developer mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-developer
