Before choosing a suitable GPS for your hiking needs, it is important to have a reasonable understanding of what is on offer along with how they work.
There are several components that need to be looked at. There is the GPS receiver (or antenna) itself, which plays an important part. Then there is the portion of the system which uses the received information to estimate where the device is located on an internal map.
Then there are the various capabilities for route planning and recording, which vary from unit to unit. Finally, the display portion of each solution type can vary considerably, from limited location information to full color three dimensional maps of the immediate surroundings and advised route.
How it Works
The actual principal of GPS is very easy to appreciate, since it produces similar results to traditional "triangulation" although GPS does not use angles. If one imagines an orienteer needing to locate themselves on a map, they first need to be able to find at least three points that they recognize in the real world, which allows them to pinpoint their location on the map.
They can then measure, using a compass, the azimuth (This is the direction of a celestial object, measured clockwise around the observer's horizon from north) that would be needed to take them from the point on the map to their current position. A line is then drawn from each of the three points, and where the three lines meet is where they are on the map.
Translating this into the GPS world, we can replace the known points with satellites, and the azimuth with time taken for a signal to travel from each of the known points to the GPS receiver. This enables the system to work out roughly where it is located, it is where the circles representing the distance from the satellite, calculated on the basis of the travel time of the signal, intersect.
Of course, this requires that the GPS locator has the same coordinated time as the satellites, which have atomic clocks on board. To do this, it cross checks the intersection of the three circles with a fourth circle, which it acquires from another satellite.
If the four circles no longer intersect at the same point, then the GPS system knows that there is an error in it’s clock, and can adjust it by finding one common value (one second, half a second and so on) that can be applied to the three initial signals which would cause the circles to intersect in the same place.
Behind the scenes, there are also many complex calculations taking place which enable the system to compensate for atmospheric distortion of the signals, and so forth, but the principle remains the same.
Updates & Mapping
When choosing a GPS, it is important to bear in mind that each type will be updated in a different way. For example, dedicated in-vehicle units tend to be updated via a CD, which has to be purchased from the road map data supplier.
Those which are attached to a PDA, as well as most handheld dedicated GPS devices, are usually updated via a PC. They also need maps, but said maps can usually be acquired at a much lower price than the dedicated branded ones needed for other systems.
Being able to update the device easily and at a reasonable cost is a very important part of the decision process; unlike other consumer devices that you purchase, the cost of ownership of a GPS unit is proportional to its usefulness.
The less you spend on maintaining it, the less useful it becomes since the road networks are always changing. This is perhaps less important for devices aimed at hiking and orienteering, but could be a factor in deciding whether a multi-function device is better than one dedicated to a specific use.
Specific software vendors have solutions for preparing maps which can be uploaded into the GPS unit. For example, many are based around the mapsource system which permits users to define their own maps, perhaps scanned from a real one, in order to get the most out of their GPS.
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