[GRASS-dev] How to calculate mean coordinates from big point datasets?

[Markus Metz]

On Wed, Sep 18, 2013 at 11:41 AM, Moritz Lennert
<mlennert at club.worldonline.be> wrote:
> On 18/09/13 10:51, Luca Delucchi wrote:
>>
>> On 17 September 2013 22:10, Markus Neteler<neteler at osgeo.org>  wrote:
>>>
>>> Hi,
>>>
>>> I came across this question:
>>>
>>>
>>> http://gis.stackexchange.com/questions/71734/how-to-calculate-mean-coordinates-from-big-point-datasets
>>>
>>> and wondered if this approach would be the fasted:
>>>
>>> # http://grass.osgeo.org/sampledata/north_carolina/points.las
>>> v.in.lidar input=points.las output=lidarpoints -o
>>> ...
>>> Number of points: 1287775
>>> ...
>>>
>>> Now I would use
>>> v.univar -d lidarpoints type=point
>>>
>>> (still calculating here...)
>>>
>>> Is it the best way?
>>>
>>
>> maybe v.median [0] could help?
>>
>>
>> [1]
>> http://trac.osgeo.org/grass/browser/grass-addons/grass7/vector/v.median
>
>
> Right.
>
> Here's a little test:
>
> $time v.median in=elev_lid792_randpts
> 638648.500000|220378.500000

Should be 638648|220378. It seems that numpy gets the median wrong...

>
> real    0m0.249s
> user    0m0.180s
> sys     0m0.044s
>
> $time v.to.db elev_lid792_randpts op=coor -p | awk -F'|' 'BEGIN{SUMX=0;
> SUMY=0; N=0} {N+=1;SUMX+=$2;SUMY+=$3} END{print SUMX/N, SUMY/N}'
> Reading features...
>  100%
> 638544 220339

Should be 638650 220376

>
> real    0m0.106s
> user    0m0.100s
> sys     0m0.020s
>
> Would be interesting to see results for big data. And AFAIK median is a bit
> more difficult to do in awk. I imagine that replacing the median by the mean
> in numpy is no problem (might be a flag to add to v.median).
>
The v.to.db + v.db.univar approach is working just fine, and provides
correct results.

About a little module to calculate centroid of polygon, center point
of line and centroid (possibly weighted) for points, that would be
easy because all the components are there, even though there are in
theory alternatives to the current calculation of centroids for
polygons.

Markus M