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
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