How to use a GPS

Grid References and the Garmin Extra

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These are instructions for obtaining Ordnance Survey (OS) grid references from a Garmin extra GPS (Global Positioning System) unit. The pictures are from the 'summit' model, although the controls and functions are identical to those on the basic (yellow) extra model.


The Buttons...

Buttons for operating the unit are found on the sides. In theory, these have been designed so that the unit can be used one-handed, though in practice, many people prefer to use two. The names by which the buttons will be referred to in this document are shown below.

 GPS buttons


Powering Up & Down...

Switch the unit on by pressing the Power button once.

When you first switch on, you will probably see either one of these two screens. Which one of these is showing (i.e. the ‘view') is unimportant and doesn't affect the performance of the unit. You can change the view, but this is not described here. If you press the Power button without holding it, that just switches a screen light on which is for night-time use; so make sure you hold it for a second or two.

Startup screen

When first switched on, it can take the unit a minute or so to get a ‘fix' on the satellites and be ready to give you a grid reference. Once on, the unit can update its position every second or two, so it is normally more convenient to leave the unit on during a day in the field. A set of batteries should last at least a full day in the field and perhaps between two and three.

To check the condition of the batteries, repeatedly press the Page button until the screen looks something like that shown here.

Check screen

The battery charge is indicated near the bottom. Always carry a spare set of batteries (two AA batteries) when you are in the field.

Press the Page button again to return to the main screen.



You will notice that when the GPS unit is ready to give you a grid reference it says ‘ready to navigate' and underneath this it gives a figure, in metres, for the level of accuracy. For the Dune Helleborine survey, we are asking you to record this accuracy figure with every grid reference you take.

So just before you take a grid reference (described next) copy down this accuracy figure to your survey sheet in the space provided.


Getting a Grid Reference...

The quickest way to obtain a grid reference is simply to press and hold the Enter button. This will take you to the Mark Waypoint screen shown here.

The grid reference is shown at the bottom and looks something like this:

GPS grid ref.

SD 37407

BNG 02420

The grid reference here – which you must copy down in full - is SD 37407 02420. The BNG – which stands for British National Grid – can be ignored.

It will help legibility if you write it down with spaces between the initial letters and two sets of numbers something like this: SD  37407  02420. That way it’s also easier for you to check that you have copied down all of the numbers.

If you do not copy the grid reference down correctly at the time you collect the data, it can be difficult or even impossible to correct at a later date and risks making records unusable. So make sure that you copy it down accurately. At this point, you can either press the Page button to return to the basic navigation screen, or you can press the Enter button to save the grid reference in the GPS unit's memory (as a ‘waypoint'). We recommend that you do the latter because, in case of problems, the grid reference can be checked later.

Note that if you have used a GPS unit before, you may be used to attenuating’10 figure references’ to 8 or 6 figure references. If you use the full 10 figure grid reference you can shorten it later.


Taking Care of the GPS Unit...

These GPS units are expensive bits of equipment (about £65) so look after them carefully. It is all too easy to put a GPS unit down somewhere whilst you are busy making a record and then walk off without it. For this reason, you should always use the lanyard to hang the GPS unit around your neck and never take it off and put it down in the field.