Very interesting article in the NY Times about the San Francisco grounding/collision! Ocean Navigator (May/June 2005 issue #146) has an article Charting the way Forward by Nigel Calder. In the article, Nigel explores the inherent errors that are present in both paper charts and when converting paper charts to electronic charts and the magnification of that error when zooming the chart. Much of the error is due to initial survey accuracy and the fact that many charts are based on surveys from the early 1800s. Nigel is very knowledgeable about the subject and the article is very well written. After reading the article youll definitely be looking out the window to see what s out there while cruising and will be comparing to paper charts. Ill definitely be honing my other navigational skills to be sure where I am at all times and whats under my keel! For those who dont have access to the article I would be glad to send it to you. Andrew H. Jackson, Jr. Belleville, IL

"Andrew H. Jackson, Jr." <andrew_jacksonjr@charter.net> At 12:39 PM 5/19/05 -0500, you wrote: >chart. Much of the error is due to initial survey accuracy and the fact that >many charts are based on surveys from the early 1800s. Nigel is very If you take a piece of paper and draw a circle within a circle, then draw in a profile of mountains and oceans in between using those two circles, the inner one as the base of the deepest ocean and the outer one the tops of the highest mountains, you can imagine what a profile of the earth looks like. Next you could draw a line that averages the highest mountains and the deepest oceans such that you would get a generalized earth. Think of this 4th line as the description for an earth as envisioned by the old map makers. Suppose that we draw a 5th line, this time compensating the 4th line for the differences in gravity related to mountains that have dense rock and those which have lighter ones and for the fact that ocean water is less dense than any kind of rock. Surprise, this last line is what the GPS satellites see. In a sense the sats are deaf, dumb and blind, but they can describe the earth below by the tug of gravity and that gravity is less over the oceans and stronger over mountains like the Rockies with their granite backbones. The topography of this satellite gravity map resembles the surface below, but it is in fact the "gravity surface" as the satellite "knows nothing" about the actual surface. These contours are constantly being mapped ever more completely with every pass, as millions of passes are made over the years. The first such data set became in effect the datum we know as WGS72 (1972), and years later the accumulated data became WGS84 (1984). Now I realize that this last statement is where the confusion begins. If the datum WGS84 is a map made from all these data points collected by the satellites, where does the map/chart that I am using come from. The answer is as follows(sort of), if the map is based on WGS84, then it is based upon the position information from the GPS constellation. In other words it is where the satellite says it is, period! But, you say, the map is based upon a survey(that's where maps come from), that was made before GPS, so how can it possibly be using the WGS84 datum? Obviously the original survey must be compensated for. These compensations are estimated from measurements taken on the ground and by other means and result in an offset consisting of so much distance plus or minus for the each of the 2 major x,y axis. The vertical axis is generally not compensated this way as elevations were generally more accurate. Note: I am not aware of any compensation being done to the gravity maps to smooth them out or make them resemble the actual ground, after all, how would those corrections be applied to the signals sent out by GPS. Your GPS receiver calculates the location from the GPS signal, but the satellite knows nothing about your location. In Oregon and Washington the paper topo maps from 10 years ago were all in NAD1927 and showed on their face the legend that they needed to be offset some 100 yards of latitude and about 50 yards of longitude to be corrected to match the output of the GPS system, which defaults to WGS84. This compensation was different in other parts of the country. If you use a datum in your GPS that is other than WGS84 then this compensation is being done for you. Whether the GPS you have uses the best compensation offset for any location you pick is an entirely separate issue. But in general if you use the same datum as the chart legend specifies the accuracy will be adequate for general usage(navigation). These datum compensation choices in your GPS are in effect a table of offsets to be applied from the locations that the GPS creates. This description of these issues is a really a GROSS simplification of the issues but for the purposes of ordinary seamen, quite adequate for navigation. The Oregon/California border is offset from exactly 42-00-00 North by about 100 yards in favor of Oregon as the border on the maps is now known to be about 41-00-54 North. The border being fixed by the surveyors of the time, not being subject to appeal? Mike Capt. Mike Maurice Tualatin(Portland), Oregon