Below from David G0VZV WAB squares
MAIDENHEAD QTH LOCATORS ON THE ISLE OF WIGHT
Click map for larger image (500KB)
QTH Locator Squares Prefixed IO90xx eg. Club IO90KQ
Based on Latitude/Longitude Check on map/GPS for precision..
There are many conversion programs/sites like http://www.amsat.org/amsat/toys/gridconv.html
IW WAB Squares Click map for larger image that uses the map above. (500KB)
These are based on OSGB 10Km squares that are NOT directly based on Lat/Longitude.
It is best to refer to an Ordnance Survey map. There are conversion programs, but MAY not be absolutely precise. A suitably configured GPS will give an accurate position.
I put this map here without David's permission. The map was a copy of the one in the 2003 IW tourism booklet, I hope they do not complain!
The use of GPS
For critical navigation do not use GPS unless you are certain of what you are doing! The following are only my observations. For accuracy in conjunction with a map a GPS must be set to the same coordinate system to which the map was drawn. In the UK all the maps I have seen appear to be based on the Ordnance Survey. Therefore the GPS must be set to the OSGB grid and OSGB datum. This involves two settings on my Garmin Etrek. (For maps with a Lat/Long grid the datum should still be OSGB) I have noticed a discrepancy of around 200 yards (Metres) when my GPS was set to the WGS84 (Default) datum.
The GPS is a useful tool for navigating on land, while walking it keeps locked when in my shirt top pocket, except under heavy tree cover when holding up in my hand improves things. It will sometimes lock indoors when there is not too much above. Except when surrounded by lots of tall buildings it works well in the car under the windscreen.
An unknown destination may be reached with it's help by keying in, or transferring from a computer, the coordinates or route. The coordinates may be found by looking at a map or if you know the postcode going to a site like http://www.streetmap.co.uk/ which will give a map and coordinates. Note that a location obtained like this is not precise. A postcode derived location can be perhaps half a mile out! Most GPS units may be used with a computer running an accessory mapping CD, which should make things very easy.