[OSGeo-Discuss] Computing area using different Spatial Refences

[Paulo Marcondes]

2007/5/31, Eduardo Patto Kanegae <eduardo.kanegae at inflor.com.br>:

> 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
>

Hi Eduardo,

From the practical POV, you difference is in the order of 10^-4 Ha...
In you case, close to to 2m^2 (or 6.4% difference in area.).

I consider that to be pretty decent, considering that you are talking
about a fairly small area in geodetic terms. The error (in area, not
percentage) should not be much bigger for much bigger areas, like
counties or big farms for instance.

If determination of area down to submeter accuracy is crucial, then a
topographic survey is a necessity.
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