[OSGeo-Discuss] Computing area using different Spatial Refences

[Eduardo Patto Kanegae]

Thanks Steve. I forgot to THINK before writing. :-)

With this error sizing for a milion of hectares, just 6 hectares will be 
the difference.

best

Eduardo

Stephen Woodbridge wrote:
> 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
>
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Eduardo Patto Kanegae
INFLOR Consultoria e Sistemas Ltda
http://www.inflor.com.br