Hi Jacqui,
I don't know how you'd join all three tables within a single SQL, but
the following query should help calculating the overlap without doing
the cookie-cutter thing. You'll just have to run it twice - once for
hard veg and once for soft.
set area units "hectare"
select
Bird_Zone.ID,
Vegetation.ID,
AreaOverLap(Bird_Zone.obj, Vegetation.obj)
from Bird_Zone,
Vegetation
where Bird_Zone.obj intersects Vegetation.obj
group by Bird_Zone.ID, Vegetation.ID
order by Bird_Zone.ID, Vegetation.ID
into Selection
(where Bird_Zone.ID and Vegetation.ID are the field names that you would
use to identify the zones)
I discovered the AreaOverLap function a while ago at
http://www.spatialplus.com/quests/qsql009.htm. This is a fantastic list
of all available SQL statements.
Hope this helps.
Cheers,
Simon
-------------------------------
Simon O'Keefe
Groundtruth GIS
Australia
-----Original Message-----
Author: Chichester Harbour Conservancy (194.242.131.---)
Date: 11-14-2001 04:28
Dear List
I have two tables, each consisting of several thousand polygons - one of
"hard" intertidal vegetation and the other of "soft" intertidal
vegetation
and sediments. I have another table of bird count zones. I would like to
know if it is possible to use SQL to find out the precise hectarage (not
including overlaps of poygons across the bird count zones) of the hard
and
soft vegetation within each bird-count zone.
So far I have tended to use the Cookie Cutter to make new tables - to
make
sure that there is no overlap (and therefore incorrect areal coverages).
However, I have a lot of bird count zones and so would rather not have
to do
individual hard and soft vege tables for each zone. A new but accurate
method would therefore be gratefully received.
Will sum
Best wishes
Jacqui Foskett
(GIS Officer, Chichester Harbour Conservancy)
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