GRASS Geographic Information System

Moritz Lennert | 17 Aug 2012 17:35

What distance is being measured and used for buffers ?

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

(Continue reading)

headers

Paolo Cavallini | 24 Aug 2012 10:16

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

An interesting issue from GRASS ML.
How do we deal with this issue?
All the best.

-------- Messaggio originale -------- Oggetto: Data: Mittente: A:

[GRASS-user] What distance is being measured and used for buffers ?

Fri, 17 Aug 2012 17:35:00 +0200

Moritz Lennert <mlennert-48KcJ953GH/Z3NRrNVlbw4ble9XqW/aP@public.gmane.org>

GRASS user list <grass-user-qjLDD68F18NyQMAyxoHuMA@public.gmane.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 _______________________________________________ grass-user mailing list grass-user-qjLDD68F18NyQMAyxoHuMA@public.gmane.org http://lists.osgeo.org/mailman/listinfo/grass-user

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Anita Graser | 24 Aug 2012 10:32

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

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-h12OV65+u/41GQ1Ptb7lUw@public.gmane.org> wrote:

An interesting issue from GRASS ML.
How do we deal with this issue?
All the best.

-------- Messaggio originale -------- Oggetto: Data: Mittente: A:

[GRASS-user] What distance is being measured and used for buffers ?

Fri, 17 Aug 2012 17:35:00 +0200

Moritz Lennert <mlennert-48KcJ953GH/Z3NRrNVlbw4ble9XqW/aP@public.gmane.org>

GRASS user list <grass-user-qjLDD68F18NyQMAyxoHuMA@public.gmane.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 _______________________________________________ grass-user mailing list grass-user <at> lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user

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Werner Macho | 24 Aug 2012 10:42

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

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-RbZlAiThDcE@public.gmane.org> 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-h12OV65+u/41GQ1Ptb7lUw@public.gmane.org> wrote:

An interesting issue from GRASS ML.
How do we deal with this issue?
All the best.

-------- Messaggio originale -------- Oggetto: Data: Mittente: A:

[GRASS-user] What distance is being measured and used for buffers ?

Fri, 17 Aug 2012 17:35:00 +0200

Moritz Lennert <mlennert-48KcJ953GH/Z3NRrNVlbw4ble9XqW/aP@public.gmane.org>

GRASS user list <grass-user-qjLDD68F18NyQMAyxoHuMA@public.gmane.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 _______________________________________________ grass-user mailing list grass-user <at> lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user

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Moritz Lennert | 1 Sep 2012 18:02

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 ?]

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

headers

Moritz Lennert | 4 Sep 2012 18:57

Re: 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 ?]

On 01/09/12 18:02, Moritz Lennert wrote:
> 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.
>

Exploring my exploration of this in the hope that someone might share an 
interest:

r.buffer actually provides the possibility of geodesic buffering when 
used in a lat-long location. Would it be difficult to implement the same 
in v.buffer ?

Moritz

headers

Markus Metz | 7 Sep 2012 09:05

Re: 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 ?]

On Tue, Sep 4, 2012 at 6:57 PM, Moritz Lennert
<mlennert <at> club.worldonline.be> wrote:
> On 01/09/12 18:02, Moritz Lennert wrote:
>>
>> 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.
>>
>
> Exploring my exploration of this in the hope that someone might share an
> interest:
>
> r.buffer actually provides the possibility of geodesic buffering when used
> in a lat-long location. Would it be difficult to implement the same in
> v.buffer ?

The short answer is yes, it will be difficult. The GRASS-internal
vector buffering algorithm has a number of bugs, the only vector
buffering method that is AFAICT bug-free is v.buffer in trunk with
GEOS support which uses the GEOS buffering algorithm which in turn
does not (yet?) support geodesic distances in latlong.

Markus M

headers

Moritz Lennert | 7 Sep 2012 09:45

Re: 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 ?]

On 07/09/12 09:05, Markus Metz wrote:
> On Tue, Sep 4, 2012 at 6:57 PM, Moritz Lennert
> <mlennert <at> club.worldonline.be>  wrote:
>> On 01/09/12 18:02, Moritz Lennert wrote:
>>>
>>> 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.
>>>
>>
>> Exploring my exploration of this in the hope that someone might share an
>> interest:
>>
>> r.buffer actually provides the possibility of geodesic buffering when used
>> in a lat-long location. Would it be difficult to implement the same in
>> v.buffer ?
>
> The short answer is yes, it will be difficult. The GRASS-internal
> vector buffering algorithm has a number of bugs, the only vector
> buffering method that is AFAICT bug-free is v.buffer in trunk with
> GEOS support which uses the GEOS buffering algorithm which in turn
> does not (yet?) support geodesic distances in latlong.

Ok, thanks for the answer. This means that efforts should be put into 
including this into GEOS and in the mean time, maybe write a small 
script v.buffer.geodesic that uses r.buffer.

Since we're on it: any idea about question 1) ?

Moritz

headers

Markus Metz | 9 Sep 2012 16:34

Re: 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 ?]

On Fri, Sep 7, 2012 at 9:45 AM, Moritz Lennert
<mlennert <at> club.worldonline.be> wrote:
> On 07/09/12 09:05, Markus Metz wrote:
>>
>> On Tue, Sep 4, 2012 at 6:57 PM, Moritz Lennert
>> <mlennert <at> club.worldonline.be>  wrote:
>>>
>>> On 01/09/12 18:02, Moritz Lennert wrote:
>>>>
>>>>
>>>> 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.
>>>>
>>>
>>> Exploring my exploration of this in the hope that someone might share an
>>> interest:
>>>
>>> r.buffer actually provides the possibility of geodesic buffering when
>>> used
>>> in a lat-long location. Would it be difficult to implement the same in
>>> v.buffer ?
>>
>>
>> The short answer is yes, it will be difficult. The GRASS-internal
>> vector buffering algorithm has a number of bugs, the only vector
>> buffering method that is AFAICT bug-free is v.buffer in trunk with
>> GEOS support which uses the GEOS buffering algorithm which in turn
>> does not (yet?) support geodesic distances in latlong.
>
>
> Ok, thanks for the answer. This means that efforts should be put into
> including this into GEOS and in the mean time, maybe write a small script
> v.buffer.geodesic that uses r.buffer.
>
> Since we're on it: any idea about question 1) ?

I guess this feature would need to be implemented on both library and
module level. A library function to first project to latlong, then
calculate geodesic distance would be needed. Modules could then get an
option to use geodesic distance, as long as it is not a pseudo xy
projection, and make use of the new library function if requested.
There may be a lot of pitfalls with such a feature.

Markus M

headers

Moritz Lennert | 10 Sep 2012 14:48

Re: 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 ?]

On 09/09/12 16:34, Markus Metz wrote:
> On Fri, Sep 7, 2012 at 9:45 AM, Moritz Lennert
> <mlennert <at> club.worldonline.be>  wrote:
>> On 07/09/12 09:05, Markus Metz wrote:
>>>
>>> On Tue, Sep 4, 2012 at 6:57 PM, Moritz Lennert
>>> <mlennert <at> club.worldonline.be>   wrote:
>>>>
>>>> On 01/09/12 18:02, Moritz Lennert wrote:
>>>>>
>>>>>
>>>>> 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.
>>>>>
>>>>
>>>> Exploring my exploration of this in the hope that someone might share an
>>>> interest:
>>>>
>>>> r.buffer actually provides the possibility of geodesic buffering when
>>>> used
>>>> in a lat-long location. Would it be difficult to implement the same in
>>>> v.buffer ?
>>>
>>>
>>> The short answer is yes, it will be difficult. The GRASS-internal
>>> vector buffering algorithm has a number of bugs, the only vector
>>> buffering method that is AFAICT bug-free is v.buffer in trunk with
>>> GEOS support which uses the GEOS buffering algorithm which in turn
>>> does not (yet?) support geodesic distances in latlong.
>>
>>
>> Ok, thanks for the answer. This means that efforts should be put into
>> including this into GEOS and in the mean time, maybe write a small script
>> v.buffer.geodesic that uses r.buffer.
>>
>> Since we're on it: any idea about question 1) ?
>
> I guess this feature would need to be implemented on both library and
> module level. A library function to first project to latlong, then
> calculate geodesic distance would be needed. Modules could then get an
> option to use geodesic distance, as long as it is not a pseudo xy
> projection, and make use of the new library function if requested.
> There may be a lot of pitfalls with such a feature.

Would it be at least possible to enable this in the measurement tool in 
the GUI without having to modify many modules or libraries ?

Just brainstorming...

Moritz

headers

Markus Metz | 12 Sep 2012 13:48

Re: 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 ?]

On Mon, Sep 10, 2012 at 2:48 PM, Moritz Lennert
<mlennert <at> club.worldonline.be> wrote:
> On 09/09/12 16:34, Markus Metz wrote:
>>
>> On Fri, Sep 7, 2012 at 9:45 AM, Moritz Lennert
>> <mlennert <at> club.worldonline.be>  wrote:
>>>
>>> On 07/09/12 09:05, Markus Metz wrote:
>>>>
>>>>
>>>> On Tue, Sep 4, 2012 at 6:57 PM, Moritz Lennert
>>>> <mlennert <at> club.worldonline.be>   wrote:
>>>>>
>>>>>
>>>>> On 01/09/12 18:02, Moritz Lennert wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> 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.
>>>>>>
>>>>>
>>>>> Exploring my exploration of this in the hope that someone might share
>>>>> an
>>>>> interest:
>>>>>
>>>>> r.buffer actually provides the possibility of geodesic buffering when
>>>>> used
>>>>> in a lat-long location. Would it be difficult to implement the same in
>>>>> v.buffer ?
>>>>
>>>>
>>>>
>>>> The short answer is yes, it will be difficult. The GRASS-internal
>>>> vector buffering algorithm has a number of bugs, the only vector
>>>> buffering method that is AFAICT bug-free is v.buffer in trunk with
>>>> GEOS support which uses the GEOS buffering algorithm which in turn
>>>> does not (yet?) support geodesic distances in latlong.
>>>
>>>
>>>
>>> Ok, thanks for the answer. This means that efforts should be put into
>>> including this into GEOS and in the mean time, maybe write a small script
>>> v.buffer.geodesic that uses r.buffer.
>>>
>>> Since we're on it: any idea about question 1) ?
>>
>>
>> I guess this feature would need to be implemented on both library and
>> module level. A library function to first project to latlong, then
>> calculate geodesic distance would be needed. Modules could then get an
>> option to use geodesic distance, as long as it is not a pseudo xy
>> projection, and make use of the new library function if requested.
>> There may be a lot of pitfalls with such a feature.
>
>
> Would it be at least possible to enable this in the measurement tool in the
> GUI without having to modify many modules or libraries ?

The GUI could use cs2cs to translate coordinates to latlong. Geodesic
distance calculations are done by libgis with
G_begin_geodesic_distance() and G_geodesic_distance(). The GUI could
duplicate the relevant code, but code duplication is IMHO not a good
idea. Maybe a dedicated module to calculate distances and a flag to
calculate geodesic distances would help?

Markus M

>
> Just brainstorming...
>
> Moritz