NAVIGATION GPS functions


Even the most basic GPS units will do far more than display your latitude and longitude. Indeed, to understand all their functions typically takes a manual about the size of this book – a manual, incidentally, which should be read, since it contains important information. But there are some common tasks that we should deal with here.


Speed over the ground (SOG) This can differ from speed through the water because the latter may be affected by tidal stream. SOG is the actual progress you are making.


Course over the ground (COG) Otherwise known as your ground track, or Course Made Good (CMG). This is often different from your compass course which may be allowing for the stream. COG is the actual direction in which you’re progressing.


Waypoints A waypoint is a position you want to head towards. It may be your final destination or it could be a point where you intend to change course – say, a course alteration off a headland. For anglers, it’s often handy to place waypoints over tasty shoals or wrecks. Waypoints can usually be named and stored in your GPS. Also, they can often be strung together as ‘routes’ and, in many cases, ‘reverse routes’, so you can follow the same track there and back. Many GPS sets have a ‘Go To’ button which will give you the course and distance to a waypoint of your choice. Then, as you head towards it, they are able to show Cross Track Error (XTE) if you stray off the ground track.


Understanding datums You might think that the concept of latitude and longitude would be uniform – that’s to say that a position on one chart would precisely match the same position on another. Regrettably, this isn’t always so. Because our planet isn’t a true sphere, nor spins on a fixed axis, a variety of reference points – datums, as they’re called – have been established over the years. Efforts to establish a universal datum still continue, and 1984 saw the arrival of WGS84 (WGS stands for World Geodetic System) which is the standard datum used by GPS and most modern charts. Until quite recently, British charts were based on Ordnance Survey Great Britain 1936 (OSGB36) and northern European charts to European Datum 1950 (ED50) and there are still a lot of these about. The difference between datums isn’t huge on a global scale – in some areas 150m or so – but that could be significant if you wanted to be more precise. For instance, if there was a wreck you wanted to fish on and you took its position from a paper chart and entered it as a waypoint, your GPS might direct you to somewhere close but not spot on. The good news is that GPS sets can be programmed to work to any known datum, but it’s important to check that your charts and GPS agree.


Before we leave this subject, it’s worth noting that charts aren’t always 100% reliable. Many of the surveys were done decades or even centuries ago and have yet to be updated. The fact is that GPS is revealing all sorts of inaccuracies, particularly in remote locations where there’s little or no commercial pressure to resurvey.


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RYA Seamanship for Sea Anglers


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