[GRASS-dev] geodesic distances for measuring and buffers, even when working in planar coordinate system ? [was: Re: [GRASS-user] What distance is being measured and used for buffers ?]

[Moritz Lennert]

Leaving below mail as record of my original issue, I would to raise the 
fundamental question of whether it would be feasible

1) to (optionally) provide geodesic instead of planar distances when 
measuring, even if the location is in a projected coordinate system. 
E.g. QGIS provides the possibility in distance measuring to check a box 
to activate geodesic distance

2) to (optionally) allow the creation of buffers based on geodesic 
distances, again in a projected location, which would imply non-circular 
buffers.

I guess that 1) is probably easier to implement than 2) ?

Moritz


On 17/08/12 17:35, Moritz Lennert wrote:
> 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 list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user