Chart datum, GPSs and WGS84

When we were sailing with Andreas on Saturday, he mentioned that his Garmin GPSMap constantly reported himself to be somewhere else while he was sailing in the British Virgin Islands. The error was a constant shift in position.

It must be pretty disturbing to look up your position on the chart and frequently find that you are not there.

His problem reminded me of the the RYA courses that I took in the UK before moving out West. In these, it was part of out boat's checklist to ensure that the boat's GPS (which at that time only reported longitude and latitude, and didn't come with a chart plotter), agreed with the boat's chart's datums. I've never had to do such a thing in the USA and so was prompted me into a bit of revision:

WGS84

Most charts predate GPS and no widespread accurate means of establishing a latitude and longitude that accounted for the wobbles in the shape of the Earth existed at the time they were made. With the advent of GPS, a neat grid may be laid over the surface of the Earth with great accuracy. The latest, and most widely recognized standard version of this grid is called WGS84 (World Geodetic System, 1984) is the one all modern GPSs report as their default position.

Charts predating GPS plot their positions on a variety of other grids, called the chart's position datum. Over 100 datums are in regular use in marine charts, and most GPSs can be programmed with the chart's datum to correct for the disagreement with its calculated WGS84 position.

All good marine GPSs have the ability to correct the positions they report to the chart datum. This is an essential feature for a variety of reasons. Over 100 chart position datums are in use, but the agreed new standard is WGS84. Some chart datums disagree with WGS84 by as much as 30" (or one half of one nautical mile). For locals, this is the difference between being anchored in Paradise Cove, or adrift the shipping lanes.

In most parts of the world, it is essential that you check the chart's datum when plotting a latitude and longitude obtained from a GPS (or any other source).


The USA, The Centre of the Universe and WGS84

In 1884, at the International Median Conference, Washington, DC, the Greenwich Prime Meridian in London, after several centuries of use, was formally defined to be 0 degrees Longitude by a vote of the 25 countries attending (only France abstained from voting, continued to use Paris as their Meridian for several decades before seceding to the Greenwich standard). This established the Greenwich Prime Meridian as the place where time began.

Differences from the Greenwich Prime Meridian are in part what chart datums measure.

Today, if you take a GPS receiver to the bronze markers laid in concrete at the Greenwich Observatory that define the Greenwich Prime Meridian, you'll be disappointed to find that your GPS, if it is set to the its default datum of WGS84, will report that you are 100 metres East of the Meridian. (That's 336.3 feet.)

Why? Because, WGS84 (a US Department of Defense standard) was derived from the older 1927 North American Datum which in turn agrees with the Paris Meridian. Even though this has long since been abandoned by rest of the world, including the French, and disagrees with centuries of work constructing charts with Greenwich as their datum, it did mean that the USDoD didn't need to reprint all of its own charts, rather than correct their error.

There's something a little sad about this. 25 countries met to agree upon a global standard which been a cornerstone of maritime safety for centuries, while WGS84 was reached by no such consensus. With the rise in popularity of GPS, it seems like the rest of the world will just have to suck it up.

http://www.sailtrain.co.uk/gps/positions.htm
http://en.wikipedia.org/wiki/WGS84
http://en.wikipedia.org/wiki/Prime_Meridian
http://en.wikipedia.org/wiki/International_Meridian_Conference