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

[Werner Macho]

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 2¢

kind regards
Werner


On Fri, Aug 24, 2012 at 10:32 AM, Anita Graser <anitagraser at gmx.at> 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 <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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