[Qgis-user] contour plugin

[Micha Silver]

On 07/12/2012 05:23 PM, David M. Lawrence wrote:
> Dear all:
>
> I tried all of this -- contour does work on my machine with a dataset 
> I downloaded from an online tutorial, so I think the problem may be 
> with my data instead.  The tutorial's data were obtained from an 
> evenly spaced grid.
>
> My data are not evenly spaced. They are soundings (depth measurements) 
> from a quarry linked to geographic data obtained via a GPS receiver.  
> The measurements are irregularly spaced.

In that case I'd definitely suggest you first create an interpolated 
raster (DEM) from the points. Then use that raster to extract contour 
lines. GRASS is one possible tool for this. The relevant modules would be:

v.in.ascii     (to get your points into a GRASS point vector)
g.region     (to set the region to the area covered by the points, and 
choose a suitable resolution)
v.surf.rst     (to interpolate the points and create a DEM raster)
r.contour     (to delineate vector contour lines from the DEM )
v.out.ogr     (if you need to export the GRASS contour vector to other 
formats)

WIth only 900 points the above should be no more than a few minutes...

HTH,
Micha

>
> The latest version of the data set has more than 900 points covering 
> 75 percent of the aerial extent of a quarry.  I've been running the 
> contour procedure for 12 hours now.  The contour dialog box indicates 
> it is "not responding," but I know from other software that that may 
> mean the contour procedure is actually thinking.
>
> Every time I check Task Manager (I'm running QGIS Wroclaw on Windows 
> 7-64) the number of processes seems to fluctuate around 50, with an 
> observed range of 42 to 52.  The amount of memory consumed ranges from 
> 68,000 Kb to a little more than 100,000 Kb.
>
> From this, I consider the hypothesis that contour is working more 
> likely than the hypothesis that it has frozen.  Comments, anyone?
>
> Thanks again for your time.
>
> Sincerely,
>
> Dave Lawrence
>
> On 7/10/2012 3:58 AM, Alister Hood wrote:
>> Anyway, since you seem to be able to start the plugin, your problem 
>> doesn't appear to be with getting it all installed.  Some ideas:
>> - Try  the plugin on an even smaller dataset to see if it works (If I 
>> remember correctly, I think it might be quite slow, but several hours 
>> for 400 points does seem like a very long time).
>> - When the plugin appears to be frozen, can you look in the windows 
>> process manager to see if it is actually doing any work?
>> - Remove the separate Python and all the associated junk to see if 
>> that is affecting QGIS.  (But I doubt that it is)
>> - Provide your data for someone else to check that it works fine on 
>> their system.
>> Also, I could be wrong, but I think I remember this plugin freezing 
>> for me, and then somehow working fine when I tried it again with 
>> exactly the same options... have you tried it a second time?
>>
>> Regards,
>> Alister
>>
>


-- 
Micha Silver
GIS Consultant, Arava Development Co.
http://www.surfaces.co.il