Re: [OpenMap Users] GeoArray.area redux

From: Don Dietrick <dfdietrick_at_gmail.com>
Date: Thu, 27 Jan 2011 18:52:22 -0500

Hi Bill,

I get an answer of 31.21463103052578. The answer provided by area()
is in radians^2, so you need to multiply that by (nm/radians)^2. What
values/algorithm did you use for your hand calculations?

Make sure the polygon coordinates are not closed, that is, that the
first point doesn't get repeated at the end. The algorithm doesn't
like that. And coords in clockwise order will generate positive
results, counter-clockwise yields negative results.

Hope this helps,
Don

On Mon, Jan 24, 2011 at 2:01 PM, Sutphin, William B CIV NUWC NWPT
<william.sutphin_at_navy.mil> wrote:
> Hi
>
> I am still having problems using area.
>
> I've simplified my shape to
>
> 29.8 -80.7
> 29.8 -80.6
> 29.6 -80.6
> 29.6 -80.7
> 29.8 -80.7
>
> This is about 83 square miles by my hand calculations, or 72 NM^2.
>
> The value returned from GeoArray.area is -1.5712.
>
> The value returned from Geo.nm(-1.5712) is -5401.4
>
> I having a hard time getting consistency with other shapes
>
> Do the points need to be in any order?  What else should I do with square radians to turn it into square NM.
>
> Thanks!
>
> Bill Sutphin
> Naval Undersea Warfare Center, Division Newport
> Bldg 1351, Room 211C
> Newport, RI  02841
> (401) 832-5813
> DSN 432-5813
>
>
>

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Received on Thu Jan 27 2011 - 18:53:49 EST

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