How to calculate OS grid refs

Questions..

1) I am learning GPS basics and would like to know how you end up with
a 5 fig Easting and a 5 fig Northing ........ I can do a six figure grid
ref no problem.
I have been advised to stick a couple of zeros on the end of each of N'
and E's, and this does work ...... but I would like to know a little
more.

2) In Mallorca .... what do I set my Garmin Etrex Vista C to ......
Position format .. map datum?

Thanks

Mike

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Mike wrote...

1) The OS grid reference gives you the distance in metres from the SW corner
of the local 100km square zone. Thus the number of digits you use in the
Northings and Eastings just determines the precision of your position. For
my area (Worcestershire) the zone is SO and a six figure grid reference
SO776458 gives the position of my local theatre to the nearest 100 metres in
Northings and Eastings (the left-most digit of northings and eastings is
always tens of km). An eight figure reference fixes the position to the
nearest 10 metres and ten figure reference to the nearest metre.

To generate a grid reference from an Ordnance Survey map (1:50,000 or
1:25,000) you read the coordinates of the SW corner of the kilometre square
containing your point to give you a four figure grid reference, egSO7745,
you then have to estimate or measure the position of the point within the
square, from its SW corner, to the nearest tenth, hundredth or thousandth
(!) part to get the extra digits for a more precise reference.

2) If you are only going to be using your GPS receiver with its own maps
then it doesn't matter how it is set up - it will always work with its own
datum (WGS84) whatever you set it to display and show you your position on
its map. If you are intending to use it with paper maps then you must set
the datum and position format to whatever is used on the map. This
information should be printed in the map margin but if it is a map from the
Spanish national mapping agency then as far as I know the Datum should be
European 1950 and position format UTM (it will be "UTM UPS" in your GPS but
UPS refers to the polar regions!) Whilst I'm fairly sure about this you
MUST check your paper map before taking my word for it.

David

On this great day..Sat, 16 Apr 2005 10:26:53 +0100, "David Lee"

My Mallorca map has Datum WGS84 printed on it. I do not have that
selection in the Etrex Vista ...... but I do have "UTM UPS" ... so I
will give that a try.

Thanks for the clear explanation re grid references.

Mike

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Mike wrote...

Your Vista will have WGS84 - it's the standard GPS datum (it's near the
bottom of the datum list). If you select "UTM UPS" position format then the
Vista should set the WGS84 datum automatically - at least it does on mine.

Cheers

David

In an earlier contribution to this discussion,

To summarise the more detailed answers given by others:

A 6-figure reference gives 3 digits for northing and 3 digits for easting -
the least significant digit in each case representing 100 metres.

When you have 5 digits for each direction, the least significant digit
represents *one* metre.

Adding zeros results in 5 digit numbers representing exactly the same
location as the 3 digit numbers. However, adding additional non-zero digits
enables you to specify a point to a greater level of precision.

You will probably find that your GPS displays 5 digits for each axis - thus
giving a *resolution* of one metre. Don't confuse that with *accuracy*
though - since most GPS's are only accurate to about 15 metres at best.

HTH.
--
Cheers,
Set Square
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