Calculating area of polygons

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 ?crit :

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