[postgis-users] Area calculation

[David TECHER]

Hi Dirk 

Daniel is right if your geometries are disjoined

but if you use multipolygon joined with hole here is
an example

Suppose that I have this multipolygon= 2 polygon with
holes

MULTIPOLYGON ((
(140 220, 160 140, 200 140, 240 320, 140 300, 140
220), 
  (160 260, 180 280, 200 260, 180 220, 160 240, 160
240, 160 260), 
  (160 200, 180 200, 180 180, 180 160, 160 180, 160
180, 160 200)), 
  ((400 300, 460 260, 420 200, 360 200, 340 260, 340
300, 360 340, 400 300), 
    (380 280, 360 260, 380 220, 420 240, 420 240, 380
280)))

You have to know the number of geometries yoy have

Use numgeometries:

select numgeometries(the_geom) from <yourtable>

Then

If you want to know the aread of the first polygon
that have 2 holes, use area2d and geometryn:

select area2d(geometryn(the_geom,N)) from <yourtable>

Here N=1 is the number of the polygon . Then find the
surface  for N=2


Then add all the surfaces you find

I attached an image and a SQL file with my mail for
this example

Here poly 2 and poly 3 are the all of the first
polygone (poly 1) and poly 5 is the hole of the second
polygon (poly 4)

here is what it give  me if I do

select nomcouche,area2d(the_geom) from table where
name not like '%all%'

 nomcouche | area2d
-----------+--------
 poly 1    |  12600
 poly 2    |   1400
 poly 3    |    600
 poly 4    |  10200
 poly 5    |   1800

I find with ...area2d(geometryn(the_geom)...:

aread of (poly 1 - (poly 2 + poly 3)) = 10600
aread of (poly 4 - (poly 2 + poly 3)) = 8400

Regards

David

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