[postgis-devel] ST_Area, ST_Area2D

Obe, Regina robe.dnd at cityofboston.gov
Mon Jul 21 04:09:39 PDT 2008


Good point.  That's exactly what I was thinking that if I'm working with
3d data, most of the time if I ask for a ST_Length(..) I expect a 3d
length and 2d I would never expect to get more than 2D, but I wasn't
sure if the specs somehow required ST_Length() should return a 2D length

Is their a penalty to doing a force_2d() though that would be more
efficiently done with an 
ST_Length(geom, [dim])

If so then I would suggest we go with Paul's idea, but instead of
assuming No Dim means 2, that No Dim means the highest dimension (shall
we say -1 so we have only one backend function to contend with).


-----Original Message-----
From: postgis-devel-bounces at postgis.refractions.net
[mailto:postgis-devel-bounces at postgis.refractions.net] On Behalf Of Mark
Sent: Monday, July 21, 2008 5:07 AM
To: PostGIS Development Discussion
Subject: Re: [postgis-devel] ST_Area, ST_Area2D

Paul Ramsey wrote:
> Optional argument? :)
> st_length(geom, [dim])
> default is 2d regardless of input, [dim] = 3 allows you to get 3d if
> you need it.
> P

Hmmm that seems like a lot of extra clutter. My feeling would be that 
the result returned should reflect the highest dimension input geometry,

so as Regina suggests would be 2D length for a 2D line or 3D length for 
a 3D line. In terms of compliance, anyone inputting 3D geometries is 
already outside of the SFS 1.1 spec, and so I wouldn't feel bad about 
asking people to add a force_2d() call to switch the result if the 2D 
result is what they actually wanted. Common sense would suggest that if 
you have a 3D input, you'll get a 3D output.



Mark Cave-Ayland
Sirius Corporation - The Open Source Experts
T: +44 870 608 0063
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