Hi,
You can use simple SQL to extract core values which you manipulate& analyse in
PHP/Javascript, or you can do more of the work in SQL. Given your skills, it seems
likely that you would find it easier to use simple SQL& more script based
processing.
One initial point to note:
If you are dealing with global polygons, then SRID 4326 based data can give
very strange results for area calculations. eg, the polygons:
(0 0, 1 0, 1 1, 0 1, 0 1 ) and
(0 90, 1 90, 1 89, 0 89, 0 90)
are both 1 degree square, and will give you the same area when queried.
However, these are cartesian degrees, and you want the spherical area, which
will be close to 1 degree at the equator, but a small fraction of this at the
pole.
So, you need to either convert (transform) your coordinates to a suitable
coordinate system (identified by its SRID) before querying the area, or deal
with this issue some other way, but you need to deal with it.
Then you can use the Postgis SQL functions to retrieve the desired area values
of the transformed polygon geometries. The Postgis docs give you good basic
instructions on the commands, but tend to assume you already understand such
issues.
Rather than write a Postgis guide here, I recommend you buy a copy of "Postgis is
Action" (http://www.manning.com/obe/) - you can download the digital version prior
to the printed edition being available.
Otherwise a full discussion on this list could virtually replicate the contents
of the book!
Cheers,
Brent Wood
--- On Wed, 2/2/11, Louwrens du Toit<louwrence...@eva.mpg.de> wrote:
From: Louwrens du Toit<louwrence...@eva.mpg.de>
Subject: [postgis-users] Postgresql, postgis and PHP
To: postgis-users@postgis.refractions.net
Date: Wednesday, February 2, 2011, 11:24 PM
Dear Postgis-Users List,
I'm an experienced web developer (mainly PHP, Javascript,
MySQL) and have recently started a job where I need work
with geographic information, mainly in a Postgresql
database. So, I'm quite new to all this geographical data
etc and have a few questions:
So, I have a database (Postgresql) with geographical data
(mainly polygons in format "geometry" - srid 4326). Now I
can do some area (ST_Area) and intersection
(ST_Intersection) queries but would like to know the
following:
1. I have several tables with polygon data (layers) but
need to do queries across them (what I need to do in the end
is to create charts - bar,pie etc). I.e I would like to know
for instance, what percentage area of layer 1 falls within
the area in layer 2? or
2. I have raster data (in geotiff format), and would like
to know... per polygon (in layer 1 above), what is the mean
value of the raster data...
Can someone point me in a direction of where to information
like this. How should I go about this problem?
I hope this makes sense. As I said, I'm very new to all
this geographic stuff.
Thank you,
LJ du Toit
--
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