[R-sig-Geo] Calculating area of polygons

[Ashton Shortridge]

Hi Rongui,

the proj4string slot has projection information. One of those parameters in 
the string should be units. It is possible that the projection information is 
wrong - I advise my students to conduct the reality check indicated in the 
last email.

Yours

Ashton

On 2011-02-15, Wincent, wrote:
> Thans Ashton. My object is SpatialPolygonsDataFrame, which is imported
> from arc GIS shapefile. It has such slots,
> 
> > slotNames(obj)
> 
> [1] "data"        "polygons"    "plotOrder"   "bbox"        "proj4string"
> 
> How can I figure out if this object has info about SRID?
> 
> Yeah, I may follow your suggestion by comparing the area attributes
> and actual size of the area.
> 
> Best
> Ronggui
> 
> On 15 February 2011 21:50, Ashton Shortridge <ashton at msu.edu> wrote:
> > 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


-----
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