Ian,
I contacted our expert on geodesy, Dr Phil Davies, and he replies:
"Yes, a 2D Cartesian method would usually be used when working on the
National Grid. However, this will cause an error due to the projection
scale factor distortion which can be corrected by using the local scale
factor for the location (it must be squared to correct a measured area). An
Excel spreadsheet projcal.xls which gives the local scale factor for any
national grid easting and northing is available from our Web Site."
If you need to you can email Phil directly on [EMAIL PROTECTED] Our web
site address is http://www.ordsvy.gov.uk
I hope this is of some help,
regards,
Pete, Ordnance Survey, UK
> -----Original Message-----
> From: Thake, Peter
> Sent: 23 September 1999 13:52
> To: Davies, Phil
> Subject: FW: MI Area calculations
>
>
> Phil,
> I'd say cartesian is best for use with Nat Grid? It is a
> 'flat' projection, isn't it?
-----Original Message-----
From: Ian Macey [mailto:[EMAIL PROTECTED]]
Sent: 23 September 1999 10:46
To: 'MI List'
Subject: MI Area calculations
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 and 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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