Some confusions in GPS concepts

People frequently become confused when setting a GPS to "Position Format" and "Map Datum". (These are often grouped under a heading of "Units")

It helps understand these if you add a third element of "Map Projection". Trust me!

I will do my best to explain. I am not a specialist in this area: I fought my way here like perhaps you are doing now.

If calculations have to be done to relate satellite signal paths to a position on the Earth's surface then you first need to know what shape and size the Earth is. The answer is that it is roughly spherical but is flattened a bit near the Poles and slightly pear-shaped. If you are keen to relate altitudes to "Mean Sea Level" then you need to know that there are then additional corrections made to the shape. For example to allow for the thinner crust in the oceans and the varying thicknesses of the Earth's crust under the continents and in the collision areas of Tectonic Plates. The geoid tends to rise above the spheroid over continents and to lie below the spheroid in the oceans. As calculations are going to be done, all this needs describing in mathematical terms. The word for such models is a Geoid. The system you will meet most often in modern map-making is WGS84. It is a Map Datum. There are others.

Now not too fast please, or we will leave out my third item and it seems to be where most people slip off the tracks. So far we have only a system for handling a large roughly spherical item. Most navigators demand maps on flat pieces of paper! To go from one to the other, map-makers use a "Map Projection" of some sort and there are lots of them in use world-wide. If you want some images then think of projecting your map details from the Geoid on to (1) a north-south cylinder of paper, (2) an east-west cylinder of paper and (3) a conical "Dunces-hat" of paper sat over either one of the Poles or perhaps over the country concerned. Is three enough enough of them for now? Just remember that all of these and some others are used by various national and international maps!

Now, having got your flat map or your spherical model, you want some way of pointing to places by using system of numbers and letters. There are many and they are referred to as a "Coordinate System" = "Grid Reference System" = "Position Format".

You will certainly have come across three of these: these common ones are "Ordnance Survey GB Grid References", "Latitude and Longitude" and "UTM/UPS" or just "UTM". (UPS is of most interest to Eskimos, since it is used in only the Polar regions)   Latitude and Longitude are not unique but depend on the values used for the lengths of the Earth's equatorial and polar axes. Each grid system is based on a pre-defined pair of such axes.

Alarmingly, all of these and more are in use and if you set the wrong one on your GPS then you are going to get errors. It helps if you try to minimise their use. Still, we need to get back to the task in hand which you may have now forgotten: it was to set the GPS appropriately. Since Latitude and Longitude involve being at home with your sixty-times tables then you may feel like using a decimal Positional Format. If so, then UTM is the obvious choice.

UTM is a system of letters and numbers for locating points on the Earth's surface. It is fairly similar to the OSGB Grid References and works for the whole Earth apart from the Polar regions, where UPS is called-in as an extra. Of course UTM works perfectly in the UK, but all our Ordnance Survey maps use the OSGB system: so, in practice, we are stuck with two systems.

Back to the GPS. We are now moving towards setting our GPS to either OSGB or UTM. How about the setting for the Map Datum? Now for some good news: if you set the Position Format on the GPS then you will find that the Map Datum has, rather magically, been changed to the correct setting without any need for action on our part. I think the usual expression is "User-friendly".

Sadly, we need to end with some much worse information. If you plan to travel world-wide and you wish to use your GPS to locate yourself on a local map, then you must set the Position Format appropriately. If you are also recording Tracks and Waypoints and you are going to pass them on to colleagues and friends then you will have to be sure that they are very fully documented and fully understood by the recipient. Failure to do so means they are junk or, worse, poison. It is better if all data intended for sharing is recorded and handled as UTM.