[OSGeo-Discuss] Computing area using different Spatial Refences

[Stephen Woodbridge]

Wouldn't you want to use an equiarea projection to calculate the area?
or are these small enough that is does not matter. I would expect some 
numerical rounding error, does this difference fall into that?

All questions, no answers today :)

-Steve W

Eduardo Patto Kanegae wrote:
> Hi folks,
> 
> A cartographic doubt:
> 
> We have a WGS 84 dataset ( EPSG:4326 ) and the need to to calculate
> (in hectares) the area of each polygon. To do this, we are currently 
> transforming
> these objects for our local common UTM spatial reference ( SAD69, 
> EPSG:29183 ) .
> But, I was thinking if maybe the most correct procedure is to transform 
> objects
> to UTM WGS84 and then calculate area.
> 
> Is this right? Does "GCS-WGS84 to UTM-WGS84" provides LESS distortion 
> than "GCS-WGS84 to UTM-SAD69" ?
> 
> For a given geometry(described at the end of message), we calculated 
> values using "to UTM WGS84" and "to UTM SAD69"
> and have different values.
> 
> - Hectares using Transform(...,29183) = 30,9977784638335
> - Hectares using Transform(...,32723) = 30,997559727974
> 
> Note: I made this test using MsSqlSpatial ( 
> http://www.codeplex.com/MsSqlSpatial )
> which implements Transform functions using NTS engine( a C# GEOS fork)
> 
> best regards