Well, thinking about it, I realize this question was a bit weird/silly 
in the real world... Decimal degree coordinates does not tell us 
anything about the ellipsoid/datum they refer to and this must be done 
within a CRS framework. If we want a reasonable precision on small 
areas, sound mathematics to get reasonable projections in an euclidean 
space are likely unavoidable (which is easy within rgdal and 
sptranform()). Except if one think that the earth is a perfect sphere. 
Would even be easier if still flat...


Roger Bivand a écrit :
> On Wed, 31 May 2006, Patrick Giraudoux wrote:
>
>   
>> Dear Listers,
>>
>> The function areapl() of the package splancs computes polygon areas  in 
>> the coordinate units. This means in square meters when using UTM or 
>> Lambert projections but in "square degrees" when using longlat degrees.
>>
>> Does anybody knows a R function (or an algorithm) to compute the area 
>> directly (without projecting or projecting internally), eg in square 
>> meters, of a polygon whose node coordinates are given in decimal degrees?
>>     
>
> I think it could be done internally in the sp/src/Rcentroid.c function if 
> Area2() was rewritten to call sp_gcdist() on each of the four segments in:
>
>   area = (b[0] - a[0]) * (c[1] - a[1]) - (c[0] - a[0]) * (b[1] - a[1]);
>
> but it would only be acceptable for larger areas. As it is, the area of 
> SpatialPolygons objects is really only used to create the plot order (to 
> plot larger polygons before smaller ones). In fact we only have the 
> measurement at the Polygon object level sensibly - above that we don't 
> really know whether multiple polygons in a Polygons object are holes or 
> not, so summing the component Polygon object areas in a Polygons object 
> may be wrong.
>
> Roger
>
>   
>> Thanks for any hint,
>>
>> Patrick
>>
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>>
>>     
>
>   

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