On 18/09/13 16:24, Markus Metz wrote:
On Wed, Sep 18, 2013 at 11:41 AM, Moritz Lennert
<mlenn...@club.worldonline.be> wrote:
On 18/09/13 10:51, Luca Delucchi wrote:
On 17 September 2013 22:10, Markus Neteler<nete...@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...
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.
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
Oops, forgot the header line, so N goes up to 6001, instead of 6000.
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.
Yes, but it seems a bit overkill to have to go through the attribute
table to make such a calculation.
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 ?
Mortiz
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