[OSGeo-Discuss] Computing area using different Spatial Refences
Eduardo Patto Kanegae
eduardo.kanegae at inflor.com.br
Thu May 31 05:38:21 PDT 2007
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
--
INFLOR Consultoria e Sistemas Ltda.
Eduardo Patto Kanegae
Consultor GIS
Tel: +55(27)2122-0888
Cel: +55(27)8155-2990
Fax: +55(27)2122-0861
eduardo.kanegae at inflor.com.br <mailto:eduardo.kanegae at inflor.com.br>
www.inflor.com.br <http://www.inflor.com.br>
=============
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