### Re: distance / bearing calculator

In article <cjc8nd$r68$1@newsg3.svr.pol.co.uk>, Stuart

<ddjuqw@ppoxcvqq.kud> writes

>

>"C L Imber" <cliff.Imber@nospam.tumble.net> wrote in message

>

>> I note in

>> another message you talk of distortion due to the curvature of the

>> Earth. Will this really make a difference?

>

>Yes... Fitting the earrths surface onto a square piece of paper dosn't work.

>Yes, locally it works very well and each ordance survey map is very accurate

>but join all britains maps together from NA to SZ and their has to be some

>corruption somewhere! I think?

>

>
There are two different aspects that affect the output. One is the scale

factor and the other is the convergence.

Scale factor.

The scale factor is the relationship of 1 unit of distance on the map

(or in the projection) to 1 unit in reality. The OS grid has a scale

factor of ~0.9996 (a metre in reality is represented by 0.9996m on the

map / projection) on the central meridian of the grid (2 degrees w). The

scale factor at a position depends mostly upon how far it is from the

central meridian of the grid. As positions move away from the central

meridian the scale factor increases - that is why a value less than 1 is

chosen for the centre of the projection as it reduces the average error

throughout the projection.

Four parts in 10000 probably does not concern most users!

Convergence.

Convergence is the angle between grid north and true north. It is zero

on the central meridian and once again increases as positions move away

from the centre.

Unlike the scale factor the convergence is significant if you are trying

to relate grid bearings to true or magnetic bearings. Convergence can be

calculated for any position in the grid but for most uses the easiest

way to determine it is by referring to the local OS map. At the top you

will find a diagram showing the relationship between true, grid and

magnetic norths.

--

Dominic Sexton

http://www.dscs.demon.co.uk/