[postgis] new Functions/ Function changes(Calculating centroids)

[Hisaji ONO]

Hi.

 I don't know OGC spec.

 But I have a couple of questions about your getting centroids method.

 Do you consider concave polygons for calculating centroids? For some of
them, centroids by "average location " maybe exist outside of  polygons.

 For multipolygons, "average all the points on all the outer rings" doesn't
seem to always exist within multipolygons. If not, in some cases, centroids
will exist within smaller ones of multipolygons. I don't think smaller
polygon's centroid is representative  of multipolygon without any other info
or knowledge, for example, Vancouver Island, which has Province capital,
Victoria, in BC province .

 I think larger polygon should has a centroid within a multipolygon.


   How do you think?


Dave Blasby wrote:

>from section 2.1.9.1/3.2.18.2
>--------------------
>
>centroid(geometry) :- if geometry is a polygon (or multipolygon), return
>the mathematical centroid (no guaranteed to be on polygon), otherwise
>return NULL. I define centroid as the average location of all the points
>in the polygon (outer ring only).  For multipolygons, average all the
>points on all the outer rings.
>


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20010803/a17b658e/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: za547lu68&atlas=montana.url
Type: application/octet-stream
Size: 177 bytes
Desc: not available
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20010803/a17b658e/attachment.obj>