Calculating area of polygons - R-SIG-Geo

ronggui

Tue, Feb 15, 2011 12:31 AM #

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:

Wincent Ronggui HUANG (PhD)
Department of Sociology
Fudan University
http://asrr.r-forge.r-project.org/rghuang.html

Alexandre Villers

Tue, Feb 15, 2011 3:57 AM #

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 :

ronggui

Tue, Feb 15, 2011 4:43 AM #

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:

Wincent Ronggui HUANG (PhD)
Department of Sociology
Fudan University
http://asrr.r-forge.r-project.org/rghuang.html

Ashton Shortridge

Tue, Feb 15, 2011 5:50 AM #

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:

-----
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

ronggui

Tue, Feb 15, 2011 6:09 AM #

Thans Ashton. My object is SpatialPolygonsDataFrame, which is imported
from arc GIS shapefile. It has such slots,

[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

Wincent Ronggui HUANG (PhD)
http://asrr.r-forge.r-project.org/rghuang.html

Ashton Shortridge

Tue, Feb 15, 2011 7:19 AM #

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 ?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

-----
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