<div><font>Hi,<br></font></div><div><font><br></font></div><div><font>This "problem" doesn't sound GRASS or QGIS specific. As far as I know, all GIS act like that.<br></font></div><div><font><br></font></div>
<div><font>Compare <a href="http://gis.stackexchange.com/a/8031/187">http://gis.stackexchange.com/a/8031/187</a> for what happens if users (analysts at the Economist in that case) are not aware of it.<br></font></div><div>
<font><br></font></div><div><font>Regards,<br></font></div><div><font>Anita<br></font></div><div><font><br></font></div><div><font><br></font></div><br><div class="gmail_quote">On Fri, Aug 24, 2012 at 10:16 AM, Paolo Cavallini <span dir="ltr"><<a href="mailto:cavallini@faunalia.it" target="_blank">cavallini@faunalia.it</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
An interesting issue from GRASS ML.<br>
How do we deal with this issue?<br>
All the best.<br>
<br>
-------- Messaggio originale --------
<table border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<th align="RIGHT" nowrap valign="BASELINE">Oggetto: </th>
<td>[GRASS-user] What distance is being measured and used for
buffers ?</td>
</tr>
<tr>
<th align="RIGHT" nowrap valign="BASELINE">Data: </th>
<td>Fri, 17 Aug 2012 17:35:00 +0200</td>
</tr>
<tr>
<th align="RIGHT" nowrap valign="BASELINE">Mittente:
</th>
<td>Moritz Lennert <a href="mailto:mlennert@club.worldonline.be" target="_blank"><mlennert@club.worldonline.be></a></td>
</tr>
<tr>
<th align="RIGHT" nowrap valign="BASELINE">A: </th>
<td>GRASS user list <a href="mailto:grass-user@lists.osgeo.org" target="_blank"><grass-user@lists.osgeo.org></a></td>
</tr>
</tbody>
</table>
<br>
<br>
<pre>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] <a href="http://164.15.12.207/grass/distances/worldmercator.png" target="_blank">http://164.15.12.207/grass/distances/worldmercator.png</a>
[2] <a href="http://164.15.12.207/grass/distances/buffer_worldmercator.png" target="_blank">http://164.15.12.207/grass/distances/buffer_worldmercator.png</a>
[3] <a href="http://164.15.12.207/grass/distances/platecarree.png" target="_blank">http://164.15.12.207/grass/distances/platecarree.png</a>
[4] <a href="http://164.15.12.207/grass/distances/buffer_platecarree.png" target="_blank">http://164.15.12.207/grass/distances/buffer_platecarree.png</a>
_______________________________________________
grass-user mailing list
<a href="mailto:grass-user@lists.osgeo.org" target="_blank">grass-user@lists.osgeo.org</a>
<a href="http://lists.osgeo.org/mailman/listinfo/grass-user" target="_blank">http://lists.osgeo.org/mailman/listinfo/grass-user</a>
</pre>
</div>
<br>_______________________________________________<br>
Qgis-developer mailing list<br>
<a href="mailto:Qgis-developer@lists.osgeo.org">Qgis-developer@lists.osgeo.org</a><br>
<a href="http://lists.osgeo.org/mailman/listinfo/qgis-developer" target="_blank">http://lists.osgeo.org/mailman/listinfo/qgis-developer</a><br>
<br></blockquote></div><br>