[Qgis-developer] Fwd: [GRASS-user] What distance is being measured and used for buffers ?

[Anita Graser]

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 <cavallini at faunalia.it>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 <mlennert at club.worldonline.be><mlennert at club.worldonline.be>  A:
> GRASS user list <grass-user at lists.osgeo.org> <grass-user at lists.osgeo.org>
>
> 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
>
>
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