Traditional versus GPS navigation

Jim,
Please explain the following paragraph from the July 22, 2008 entry:

"There are lots of safe and deep passages through all the little islands, navigation is simple pilotage from good charts. Be careful of your electronic charts though, the charts were printed from surveys done in 1898 so the GPS and electronic charts do not line up exactly with the hard."

Some of us were confused by this paragraph. In the first sentence you refer to "good charts", but in the second sentence you say that the charts were printed from surveys done in 1898. Does "the charts were printed from surveys done in 1898" really mean "the electronic charts were derived from surveys done in 1898"?

Bill



Bill,
Clarification is due. The good charts are the paper charts based on surveys done in 1898. The headlands, heights of land, reefs, passes are all based on what those surveyors could determine, and they used primarily good visual references, e.g. mountain tops, headlands etc. You can very easily navigate through the Islands using compasses, and other visual clues, e.g. transits.

Todays charts are electronic and are based on Lat and Long as given by the GPS. In first world countries where so much commercial shipping uses autopilots, computers and GPS, the charts have been corrected to GPS Lat and Long.

In 1898 the chart surveyors used a sextant for lat and long and generally the lat is pretty accurate, it being derived simply from the sun every day at meridian passage (that is high noon). Longitude of course is less accurate in the sextant, it being heavily dependent on time, and time in 1898 being a bit more inaccurate than today. So the less commercially traveled areas of the third world have no reason to upgrade the charts to GPS Lat and Long standards, hence the 1898 survey is very accurate if used with traditional navigational tools and methods, but not corrected to todays GPS lat and long accuracy.

Here in Fiji, the Nadi Waters, or the part of the inland sea from the ocean passage to the port of Lautoka has been upgraded to GPS accuracy due to commercial shipping, but the waters past Lautoka are usually .3 NMile of longitude out of true.

Jimsh