How Accurate is your GPS?

The short answer is “about 5-10M, depending on the number of satellites visible to the unit”, but
the truth of how accurate a GPS is depends on what we are comparing it against. If we were to place a GPS outside, its position would not move more than a couple of meters, sometimes for over an hour. In this case it is very accurate - in Source Datarelation to itself. Most of the time however we want to compare GPS against something more useful like a Nautical Chart. This is where problems arise. Development of GPS and accurate depth sounders has left paper chart accuracy way behind. To get an idea of how far behind they have been left, all we have to do is open a paper chart and check the survey dates. It doesn't take long to find some very old cartography – one of the first charts I opened was NZ64 – East coast, South Island which proudly states that vast areas were surveyed between 1849 and 1851 by HMS Acheron – a whopping 160 years ago. Even more alarming than charting from during the Maori land wars is that more information on that particular chart has come from “Random soundings from various sources”. Very reassuring.
It should be noted though – before GPS, the charting only needed to be as accurate as triangulation or a sextant could be – Navigators couldn't get any more accurate anyway so there was no need for more accurate charts.
And here lies the problem. Although your GPS is accurate to 5 or 10 meters, your super duper 3D with google overlay charts are using data from the 1800's, and may have a much lower degree of accuracy.
Even charts within the last century can be out by quite a margin, before GPS (actually D-GPS but that's a story for another day) was used to get accurate positions for charting, cartographers had to get their positions the same way that navigators did – by the stars or by triangulation. As such there is some variation in their work. Until recently most boaties passing through Man O War passage at Great Barrier Island could see this first hand as their boats appeared to carve their way through the northern rocks on the chartplotter. The charts of the area were re surveyed in 2009, and I haven't passed through there since, but I assume the digital boat now remains wet. Closer to home, Brewis shoal is well known to be a wee way off – the charts are from 1960, so again before accurate positions could be reliably be calculated.
Taking these chart inaccuracies into account it is no real surprise that computer chart programs which update the charts as you move have become so popular with commercial and keen recreational fisherman. Piscatus  and Maxsea take position information from the boats GPS and depth information from the sounder and combine it to make updates to the charts which are being displayed. The results can be quite amazing, as not only are the charts more accurate, but also a lot more detailed.

Chart DescrepancySo how do you check the accuracy of the charts and chartplotter? The first step is to make sure your chartplotter is set up correctly. Although there are areas of our coast which are not accurate compared to GPS, some areas are very well surveyed, including most of Tauranga Harbour. If you are travelling out of the harbour, zoom in on the chartplotter, pass closely by one of the harbour markers and check the location of the boat in compared to the harbour marker. Your position in relation to this marker should be smack bang on the chart. If its out by more than 15M or so, then you may have incorrect settings in the chartplotter – in particular in regards to which Chart Datum it is set to, or perhaps some very old charts.
Good digital charts will hold information about the data they were derived from. Next time you are out, have a look and see if the areas you use are from recent surveys or from older surveys. Anything after about 1990 should be pretty accurate.
If there are areas which are a bit off – such as Brewis shoals, mark them with a GPS waypoint on your plotter. This will be much more accurate, and easier to return to the next time you go fishing!