[OpenLayers-Users] Determining polygon area? Trying to see through the confusion.

[Christopher Schmidt]

On Sat, Oct 18, 2008 at 10:49:57AM +0200, Pierre GIRAUD wrote:
> I think that there's something already done in OpenLayers, at least in
> the examples.
> Have a look at the measure example [1], there's kind of a conversion
> thing using distVincenty.

distVincenty is for lines, not area.

> Hope this helps,
> Pierre
> 
> [1] http://www.openlayers.org/dev/examples/measure.html
> 
> On Fri, Oct 17, 2008 at 7:25 PM, Bill Thoen <bthoen at gisnet.com> wrote:
> > plen wrote:
> >> I see that there is a feature.geometry.getArea() function.  This returns
> >> some value which I am unsure as to its meaning (square degrees?).  I have
> >> seen previous posts that sort of get into determining polygon areas but
> >> nothing seems clear.  Maybe this is a bigger type issue than can be handled
> >> by the OL API, not sure.  I am hoping that someone could answer (or lead to
> >> an answer) any of the following:
> >>
> >> 1) What unit of measurement is returned by the getArea() function?
> >>
> >> 2) How to convert that to something like square miles, meters, kilometers,
> >> etc.
> >>
> >> 3) Determining the area for regular polygons vs irrlegular polygons.
> >
> > It looks like it returns an area in terms of the units of your
> > coordinate system. If you are using decimal degrees, then the result is
> > essentially meaningless, i.e 'square degrees'.  To get a reasonable
> > "flat land" area use coordinate units in meters or feet and then apply
> > the appropriate conversion factor to get square miles, hectares, etc.
> > The method does appear to accommodate irregular polygons (ones with
> > holes in them) by subtracting the area of the holes from the area of the
> > outer ring.
> >
> > If you're up to the coding, there's a description of Girad's Algorithm
> > for calculating the surface area of a spherical triangle at
> > http://math.rice.edu/~pcmi/sphere/ and that can be extended to determine
> > the area of a polygon on a spherical surface using decimal degrees. I
> > think this would be a more appropriate method for calculating areas on
> > the earth's surface.
> >
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> >
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-- 
Christopher Schmidt
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