The spherical algorithm is NOT the proper way. The UK
National Grid is based on the Gauss-Kruger Transverse
Mercator fully-conformal projection.
Therefore, you:
(1.) MUST use the cartesian algorithm, and
(2.) MUST correct for the scale factor squared. (Length x
scale factor x width x scale factor = area x scale factor
squared.)
If the referenced ArcView data gives the identical answer
to what you get with step (1.) above, then the ArcView data
is incorrect. You must correct for this non-trivial source
of systematic error each time you measure an area, whether
it by a fancy software package or with your own two hands
on a paper map. (Computers just make it easier to generate
garbage data in a more efficient manner.)
Yes this should (and will) compare favorably with the
methods used with paper maps. (Such as with a compensating
polar planimeter.)
Consider:
(1.) Purchasing a copy of "Measurements from Maps," by
D.H.Mailing, Pergamon Press, ISBN 0-08-030289-0, and
(2.) Reading it.
The folks at Ordnance Survey will be happy to help you
also, they just won't be familiar with the esoterics of MI.
(Nor am I.)
Good luck!
Clifford J. Mugnier ([EMAIL PROTECTED])
The Topographic Engineering Laboratory
Department of Civil and Environmental Engineering
UNIVERSITY OF NEW ORLEANS
New Orleans, Louisiana 70148
Voice and Facsimilie: (504) 280-7095
On Thursday, 23 September, 1999 4:46 AM, Ian Macey
[SMTP:[EMAIL PROTECTED]] wrote:
> I have been using the spherical algorithm to calculate
the
> area of field
> polygons since upgrading to MI Pro 5.5. Is this the
> correct way for use with
> Ordnance Survey data which is tied in to the British
> National Grid? The
> reason that I ask is because I have been sent an Arc View
> shape file where
> the person quoted the area as 546.01 ha. This is the area
> that I get when
> using the cartesian algorithm. When using the spherical
> option I get an area
> of 544.48 ha which is causing some contention.
>
> Also how should these compare with the traditional ways
> that the OS use
> calculate their areas on old paper plans.
>
> Any help is much appreciated
>
> IAN
> ---------------------------------------
> Ian Macey
> Dreweatt Neate
>
> mailto:[EMAIL PROTECTED]
> http://www.dreweatt-neate.co.uk
> Telephone: 01635 263050
> Facsimile: 01635 263090
>
>
>
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