-----Original Message----- From: Michael Maurice >Arlid, ( ARILD ) >A little too much detail? On the other hand you're right about the room for >inaccuracy. >If you read the BC Sailing Directions, take note of the warning in them >that most of the water inside of 20 fathoms or so, is only sketchily >"surveyed". I take this to mean that the charts are little more than >guesses inside those areas. You got it! Often the initial chart begins with aerial photos showing the shoreline. The dicision to send in survey launches depends on expected traffic and estimated potential for hazards. >Part of the problem with debates like this is that no one is knowledgeable >enough to have a handle on the "whole elephant", as in the 7 blind men and >the elephant story. >Capt. Mike Maurice Correct! When we only had paper charts towork with, most navigators would not attempt to plot inside a box about as big as a postage stamp. Common sense dictated that you simply can't expect 50 foot accuracy when plotting on a 1:40,000 scale chart. The accepted standard says that a "charted" object will be positioned with 1 mm accuracy on the paper chart. Translate that into a paper chart environment and you realize that this means the 1 mm dot on the paper chart covers 40000 millimeters in the real world. 40,000 mm = 40 meters = 132 feet. The problem is you can "Zoom in" using raster chart so that it looks much bigger. But all you have done is expand the error circle. Now add the uncertainties inherent in field work, plus the draftmanship at the cartography office, etc. When a chart is initially planned the intended scale is decided first. This dictates what scale to do the field sheets at. That in turn detemines what size of objects to actually chart. This is where many details are simply dropped as not being deemed significant. The criteria as to what is significant is often based on commercial shipping needs; NOT on recreational boat needs. The one exception being if the planned chart is specifically designated for "smal craft use" C-MAP and Navionics further muddy the waters ( pardon the pun ) by digitizing the database or an actual paper chart but they also apply their own criteria as to what to display and what to supress. I have found numerous errors in both their cartography while cruising in the Great Lakes region. Transas does use the same database provided from NDI or NOAA. This ensures better detail and accuracy but you also pay $$$ for that. And this still includes any errors that exist in the official database - see earlier note - The only time I was able to use electronic charting wtih full confidence was on board the freighter in the St. Lawrence system. We had something called "superscale charts" specifically done for the commercial ships. Scale was 1:500 < grin> With it you could literally see well enough to enter the locks without reference to other visual aids. Mind you, the ship was only 2 feet smaller than the lock so as long as you got the bow positioned, all we had to do was apply half ahead power to push the ship into the chamber. but if you forgot to cut the power you hit the end door or if you cut power too soon the water popped the ship back out again like a cork. Cheers Arild