[R-sig-Geo] What is the metric projection equivalent to WGS 84

[Michael Sumner]

On Mon, 25 May 2015 at 06:37 sadaoui <sadaouimahrez at outlook.com> wrote:

> Hello,
>
>
> what is the good  projection to be used, in order to calculate the area
> (square meters ) of my polygons that are found in different places of the
> world map.
>

You can always use Lambert Azimuthal Equal Area (LAEA) for any region - but
it's best if it's a generally localized area (i.e. no larger than a
hemisphere) since it's likely the approximations applied to representing
the polygons won't be robust near the edges (vertices too far apart stray
from great circles).

This applies generally though, if you have rectangular cells defined in
longlat not many projections will stay sane unless you have sufficient
intermediate vertices between the corners.

Safest is to reproject every polygon to its local centre, and use that LAEA
(+proj=laea +lon_0=centerlon +lat_0=centerlat +ellps=WGS84 for example) for
individual calculations on each polygon. But it depends on what your data
are and how they are represented and distributed in space. Given the
limitations baked into polygon representations it's not possible to give an
answer that will always work.

There are global projections that are equal area - Sinusoidal and Albers
Equal Area Conic, but still you can get bitten by topology so checking the
visually what's happening is important.

These guides might be helpful:

http://egsc.usgs.gov/isb//pubs/MapProjections/projections.html

http://www.georeference.org/doc/guide_to_selecting_map_projections.htm

Cheers, Mike.


> best regards
>
>
>
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