[postgis-users] RE: RE: calculating area from geometry intersections

[Nelson Guda]

Ah, thank you, Bruce and Steve!

I had thought that the area problem might be related to the SRID.  Do  
you think that it would be more efficient to convert the entire  
database to an equiarea projection, or just convert for the query?   
My database covers the entire US and most of the queries are point  
queries by lat,lng.

I'm also not quite sure what SRID I should use.  Any recommendations?

Many thanks
nelson


> Nelson,
>
> Bruces query looks good, except if your data is in lat/long degrees  
> in which case you will get area in square degrees, which is  
> probably not useful. You will probably want to transform you data  
> to an equiarea projection and compute area in terms of square feet  
> or meters depending on the projection you use and then convert that  
> to acres.
>
> -Steve W
>
> Bruce Rindahl wrote:
>
>> Try:
>> SELECT county.name , roadless.name, roadless.type1,  
>> roadless.type2....,
>> sum(area2d(intersection(roadless.the_geom,county.the_geom))) group by
>> county.name, roadless.name, roadless.type1, roadless.type2....
>> Adjust for your tables and column names.
>> Bruce Rindahl