[postgis-users] Calculating Attributes

[Luigi Castro Cardeles]

Hi

be careful with units. This formula returns the area using projection's
units (degrees, meters, kilometers etc). If that's the case maybe convert to
some metric projetion

UPDATE table1 SET area_column = ST_AREA(ST_TRANSFORM(the_geom,
srid-of-metric-projection)); -- this returns, normally, square meters.

The problem is chose the right projection for your data (maybe more than
one).

Luigi Castro Cardeles


2009/4/3 George Silva <georger.silva at gmail.com>

> Hello Aurora,
>
> In ArcGIS it's all different. In PostGIS you can use simple/complex SQL
> queries to "calculate values" in a similar fashion of arcgis.
>
> The simplest of them all would be, in your case:
>
> ALTER TABLE table1 ADD COLUMN area_column double precision;
> UPDATE table1 SET area_column = ST_AREA(the_geom);
>
> You can in that SQL query divide, multiply, and do whatever you want with
> that result that will be written to the column area_column.
>
> I hope that helps
>
> George
>
>
> On Fri, Apr 3, 2009 at 12:55 PM, Aurora Geomatics <
> aurorageomatics at gmail.com> wrote:
>
>> Hello PostGIS users,
>>
>> I have just installed William Kyng's PostgeSQ/PostGIS packages for Mac OS
>> X.5
>>
>> I have been playing with pgAdmin and QGIS with importing shapefiles into
>> PostGIS.  Works dandy.
>>
>> Now I am wondering how to solve Attribute Calculations.
>>
>> Years of using ArcGIS and GRASS it is pretty straightforward on those
>> apps, and am stumped with PostGIS Attribute calculations..
>>
>> For example, using a polygon feature;
>>
>> Creating a new column, say AREA and filling with a calculation that is
>> Hectares.
>>
>> Thanks!
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>>
>
>
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