[Qgis-user] contour plugin

[David J. Bakeman]

Micha Silver wrote:
> 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...
I've had trouble using grass (my fault not grass's:) but once you have 
your vector layer in qgis you can also use the Raster/Analysis/Grid tool 
to create the raster (this uses the gdal tool gdal_grid) and then use 
the Raster/Extraction/Contour (again this uses another gdal tool).  I've 
had pretty good luck with this method.  You probably have to save your 
vector data as a shapefile first but gdal also has direct support for 
csv so maybe not.
>
> 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
>>>
>>
>
>