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 <ash...@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.alexan...@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.hu...@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 ash...@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 ash...@msu.edu Dept of Geography http://www.msu.edu/~ashton 235 Geography Building ph (517) 432-3561 Michigan State University fx (517) 432-1671 _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo