[GRASS-user] area stats on a lon/lat projection

[maning sambale]

Thank you for the clarification.

On Mon, Jun 9, 2008 at 4:52 PM, Glynn Clements <glynn at gclements.plus.com> wrote:
>
> Maning Sambale wrote:
>
>> I know it should not be really practiced, but how accurate is the areal
>> calculations (hectares) in v.report when the projection is lon/la wgs84?
>> I have a vector data that covers several utm grids that I want to
>> compute "hectarage".
>
> AFAIK, the calculation is "exact", in the sense that it computes the
> integral with respect to latitude.
>
> The real issue is one of definition. Specifically, the polygon is
> deemed to be bounded by straight lines (in lat-lon) between the
> vertices, not geodesic lines (or straight lines in UTM). If your
> polygon edges are large, this will make a difference.
>
> If you want to know the precise calculation, see
> G_ellipsoid_polygon_area() in lib/gis/area_poly1.c.
>
> Also, bear in mind that projecting vector data (e.g. from UTM to
> lat-lon) merely projects the vertices, so you are effectively changing
> the shape of the area. In both cases (before and after projection),
> the polygon is bounded by straight lines between the vertices
> (straight lines in UTM before, straight lines in lat-lon after),
> whereas applying a non-affine projection to a straight line will not
> generally produce a straight line. I think that v.split may be of use
> here (if I understand it correctly).
>
> --
> Glynn Clements <glynn at gclements.plus.com>
>



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