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

Markus Metz markus.metz.giswork at gmail.com
Thu Sep 19 05:32:25 PDT 2013

On Thu, Sep 19, 2013 at 9:15 AM, Moritz Lennert
<mlennert at club.worldonline.be> wrote:
> On 18/09/13 16:24, Markus Metz wrote:
>> Moritz Lennert  wrote:

>>> 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...
> When I look at the numbers coming out of v.to.db, there are a series of
> points at X=638648,5 around the 3000 limit, and a series of points at
> Y=220378,5, so I do believe that numpy is right.

Right, db.univar was printing out with insufficient precision, fixed in r57750.

>> 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.
> And these alternatives are better ?

I am not sure. For areas without isles, there is a faster alternative.
For areas with isles, the current approach is fast, but the centroids
might be placed at somewhat unexpected locations (the next best
location inside the area, outside its isles). Here, alternatives would
be much slower.

Markus M

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