GPS Navigation… Can We Give an Accurate Position?



Navigation… Can We Give an Accurate Position? The answer to that question is probably not! Many of us use GPS receivers in our everyday life but what is GPS? GPS or Global Positioning System refers specifically to the American NAVSTAR system; NAVSTAR being NAVigation Satellite Timing And Ranging. There are now other systems that are coming to the fore such as the Russian GLONASS, the Chinese COMPASS and the European GALILEO to name but three; collectively these are referred to as GNSS (Global Navigation Satellite Systems).

Is the Same Place Always at the Same Position?
The simple answer to this is no, it isn’t; not even when you are using the same coordinate system. The reason for this is that the Earth is constantly changing shape and although adjustments are made to align the coordinate system with the actual ground datum positions, the fact is, that the movement of tectonic plates change the position of locations relative to each other. With this being the case we are rarely in the same place twice even though we have not moved! These movements are small but as the technology responsible for positional accuracy constantly improves these discrepancies become more and more noticeable, although perhaps not too much of a concern to the overland traveller when taking the timescales involved into account.


GPS height is also subject to the same anomalies as two dimensional plane positioning in as much as the Earth deforms due to the cycles of the Sun and Moon, tectonic plate movements, a weather system’s ‘high pressure’ can deform a landmass locally and parts of northern Europe are still rising after the last ice age!


All these things along with many, many more erode the accuracy of our navigational positioning systems although engineers are constantly working to deal with a problem that will never go away. All that said, we have never been better equipped to accurately position ourselves upon the surface of our planet.

A Coordinate System

To make any sort of use out of any GNSS we must have a coordinate system and associated datum from which to work; GPS uses WGS84 which stands for World Geodetic System 1984. WGS84 is an absolute Cartesian coordinate system and is three dimensional using an associated ellipsoid. These coordinates can be expressed as either XYZ or more commonly as Latitude and Longitude along with a height relative to the associated ellipsoid. The WGS84’s datum has its origin at the Earth’s centre of mass, this is called the Geocentre. Now, all this is a bit technical and to all intents and purposes 99.9% incomplete… Don’t worry I won’t bore you with the rest! ‘How does all this affect me?’ I hear you cry; I am but a humble traveller who just wants to bimble about on this planet with the odd thought as to where I am occasionally? Well, it’s all to do with ‘mix and match’ really and if you require accurate positional information you will unfortunately have to play the game.

Mix and Match

To ensure accurate positional information from your GPS receiver you will have to match the map datum in the receiver to the datum that has been used to produce the map; this information is normally contained in the map’s legend. Failure to do this could result in positional errors in excess of 200m.

Positional Format

You will also need to set the receiver to display the positional information in the format that will be of most use to you; this may be Latitude/Longitude, UTM/UPS, MGRS and so on. A feature on many receivers is that you can even design your own grid.

Are There Formulae to Convert One Coordinate System to Another?

Yes, there are formulae that do this, but as with all formulae they are only exact when they are used in the confines of perfect, predictable conditions, but not, unfortunately, in the real world. These tend to work much better when used in a more local navigational environment.

Are we there Yet?

Almost! Now that you have set the receiver to work in conjunction with your map you will find that it coincides perfectly with the topographical details depicted on your map. All you have to do now is ensure that you give the GPS receiver what it needs to produce accurate positional information; this is no more than a clear view of the sky! Remember, deep gorges, valleys, high buildings and heavy tree cover can all reduce the positional accuracy to some extent.

Can We Trust GPS?

Yes, without doubt. We just have to make sure that we set it up properly so that it can deliver the accuracy we have come to expect. As with all electronic devices we should never rely on them 100%, always maintain your basic navigational skills to a high standard and never venture out without a map & compass; stock-in-trade you might say for the overland adventurer.

A good way of improving your map & compass work is to pick a remote featureless point and navigate to it using only your map & compass. When you think that you’re there, check it on the GPS. Then pick another and another and another!

Practice makes perfect!

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