RECORD receives an increasing number of records from the Cheshire region for which recorders are using hand-held Global Positioning System (GPS) devices. These GPS devices are used to help locate the position (grid-reference) for records more accurately and they are proving to be very useful. However, a few pointers may help to improve their usage.

The accuracy of most of these devices is greatly improved following the shut-down of the signal scrambling and deliberately introduced errors which used to happen prior to May 2000. The American satellites introduced these errors to assist in combating terrorism but it was eventually realised that it did little in such areas and only caused frustration from people wanting to use the signals for valid reasons. Many GPS devices will give 8-figure grid references or better and this implies an accuracy of 10 metres, whereas, for handheld units, the quoted accuracy in good conditions seems to be about 15 metres. However, this still makes an 8-figure reference more accurate than a 6-figure, which only gives you a 100m x 100m square. For the majority of records a 6-figure reference is sufficient, unless it is a rare plant or an ancient tree that you wish to pinpoint more accurately. If the extra accuracy really does seem important then it may be worthwhile contacting us for use of the MapInfo GIS system at RECORD or as a means of obtaining access to an aerial photograph of that part of Cheshire (N.B. Please note, we do not currently possess copies of the aerial photographs for Cheshire but the County Council do, as do a number of the Local Authorities, and we may be able to assist in gaining access to help your recording efforts).

To save battery life many users switch off the receiver between readings. However, experience suggests that it does take a few minutes to get the best readings and for the reading to stabilise. Just watch the drift in numbers as you stand and watch the receiver after switch on and once the grid ref. display first shows on-screen.

Readings in valleys (e.g. cloughs) and in woodlands are still difficult due to either the valley sides or the woodland canopy blocking the receivers access to signals from the various satellites. Essentially, the more satellites the receiver can connect to the better the triangulation information from which it calculates your position. If using the receiver is a valley or in woodland it is better to leave it for as long as you can before taking a reading in order to try to improve the number of satellites used and the accuracy of the final reading.

Please Note: If you have an 8-figure reference, and you only want a 6-figure reference for your records, please truncate and cut off the numbers you do not require (e.g. SJ 5405 8235 would become SJ 540 823) do not round Down and certainly NOT UP, because rounding up puts the record into the next 100 metre square.

Tip: Make sure that you have the correct datum line set within the parameters of your GPS (note this is an absolute necessity for Garmin products). If you do not have the correct one set then your readings will be constantly out by the same amount each time.  You will need to go to your Set-Up menu and ensure that you have: Set the Country to 'UK-England', Set the Position Format to 'British Grid' and Set the Map Datum to 'Ord Srvy GB', 'Brt Ord Svy' or 'OSGB36'. The default Map Datum setting in many makes of GPS is 'WGS84' and, if used, this will give readings which are consistently between 400 and 500 metres too far north. Change this value to one of the above (Ord Srvy GB if you have it). The technical description of this problem is available on the Ordnance Survey web-site here: www.gps.gov.uk/additionalInfo/GPSandOS.asp

For further information on GPS and its usage please take a look at the Ordnance Survey GPS pages by clicking the button below:



At RECORD, as from January 2003, we have 6 Garmin GPS-12 receivers which we are prepared to offer for short-term loans to recorders in the Cheshire region who are providing records to the local record centre and who wish to make attempts to improve the accuracy of their data.

Garmin GPS Examples:

There are many different GPS receivers now available with a range of prices to suit most pockets. The ones we currently hold at RECORD are from Garmin and they are model GPS-12 - when purchased (2003) these were £160 each but a GPS-12XL model was obtained 2 years earlier for £99. Garmin's E-Trex models are cheaper but less sturdily built.

When you first switch on your reciever you will get a display which shows a number of items. The first is that the system is communicating with the satellites and acquiring an EPE value - this indicates that the GPS is automatically working out which country it is being operated in. Also shown on the same screen are the satellites which have been identified (in the concentric circles of the display) - un-attached/disconnected satellites are shown with a dark rectangular background; connected/attached satellites are shown with a clear background. At the bottom of the screen are shown the signal strengths of each of the connected satellites (the vertical bars) - Note: The more connected satellites and the higher their signal strength the greater the accuracy of the triangulation and hence of the Grid Ref provided by the unit.  To the left of the display is a dark vertical bar which shows the total battery power remaining available.

When enough satellites have been connected to allow the unit to work out a grid reference via triangulation the display changes to that shown at the left. As can be seen, under the heading 'Position' are provided the 100Km square two-letter designation (here: 'SJ') with the figures of the grid ref. split into eastings and northings (both of 5 characters each) thereby providing a 10 figure grid ref. The eastings are shown at the top (here: 41433) and the northings immediately below (here showing a reading of: 70158). These should be truncated down to 8 characters at best, and this means removing the last number in the eastings and in the northings (the two numbers now read: 4143 and 7015).  Put these together with the 100Km square reference (the 2 alphabetic characters at the start of the Grid Reference) and the grid reference we have is: SJ41437015.