[R-sig-Geo] Calculating area of polygons

[Ashton Shortridge]

Hi Ronggui,

the spatial units are distinct from the projection and datum component of the 
coordinate reference system.  For example, I could use data projected to 
Lambert Equal Area with particular parameters that were in meters, feet, 
miles, rods, etc.

"Hong Kong 1980 Grid System" tells me it's projected, but doesn't tell me the 
units. Your object may have a particular spatial reference ID code (SRID) 
associated with it - if you look this up on the internet, you can find out the 
units. Alternatively there may be a parameter string that includes the units. 
Finally, if you can display your data in a GIS, and you know the region fairly 
well, you can probably figure out the units (e.g. if you know your area is 
about 10 km across, and a distance check indicates the distance is 11500, then 
you have meters. If it's more like 35000, it's feet. 

Hope this helps,

Ashton

On 2011-02-15, Wincent, wrote:
> Thanks Alex.
> 
> Also, my object is projected to "Hong Kong 1980 Grid System", so the
> unit of area is square meters. Is it a confirmative answer? Thanks
> again.
> 
> Best
> Ronggui
> 
> On 15 February 2011 19:57, Alexandre Villers
> 
> <villers.alexandre at gmail.com> wrote:
> > Good afternoon,
> > 
> > I guess it will be the unit of your original SpatialPolygons object. If
> > you use a projected coordinate system, the unit should be in meters,
> > otherwise, I'm not sure of this, but it should be in squared degrees.
> > 
> > HTH
> > 
> > Alex
> > 
> > Le 15/02/2011 10:31, Wincent a écrit :
> >> The previous email was rejected, so I re-send it. Sorry for disturbing.
> >> 
> >> Regards
> >> 
> >> On 15 February 2011 16:23, Wincent<ronggui.huang at gmail.com>  wrote:
> >>> Dear all,
> >>> 
> >>> According to
> >>> https://stat.ethz.ch/pipermail/r-sig-geo/2009-May/005741.html,
> >>> the area of polygon can be accessed by
> >>> 
> >>>> areas<- sapply(slot(scot_BNG, "polygons"), function(x) sapply(slot(x,
> >>>> "Polygons"), slot, "area"))
> >>> 
> >>> My question is what is the unit of the area? Is it sq km?
> >>> Thank you very much in advance.
> >>> 
> >>> Regards


-----
Ashton Shortridge
Associate Professor			ashton at msu.edu
Dept of Geography			http://www.msu.edu/~ashton
235 Geography Building		ph (517) 432-3561
Michigan State University		fx (517) 432-1671