Howdy All, Having read all the recent posts about GPS chart accuracy in Mexico and elsewhere I started wondering. If GPS cart datum's themselves are inaccurate due to their being produced from old, inaccurate "official" chart information, why not just forget about the chartplotting function, use the GPS for lat & long data and plot off vellum paper overlays on your cruising guides? From what I learned here, there are certain well-respected cruising guides for the popular regions that many of the listees rely on. I've checked out all the cruising guides I could at W-marine and they look to offer good value vs official charts, cd-rom's and data cartridges. I'm sure most people would not purchase a product if they knew in advance of major flaws that compromised its overall safety and utility. Why are digital cartography products treated differently? Do most GPS chartplotters allow you to enter and store modifications to the source database? Or, can you build an offset table and store it elsewhere on the system where the chartplotter could integrate your changes? Somehow, there must be a way to provide for correcting data that places your floating ship thousands of feet inland. I couldn't learn anything about this from the manufacturers websites. IMHO, Manufacturers SHOULD accept responsibility for providing the most accurate data possible AS WELL AS pushing corrected data back at the gov't agencies from which they receive their gross information. The profit motive and disclaimers about relying on their data aside, you'd think the few major manufacturers of digital cartography products would put together a clearinghouse of some sort for vetted datum updates so we could all benefit from thousands of users' experience. I imagine I always expect too much. I don't think something like the list would be appropriate for storing and disseminating such information because someone would surely run into trouble and, instead of taking responsibility for their choice, sue the pants off the list master; not a pleasant thought. Another question: at opportune moments, couldn't you use your radar for ranging data off a couple known objects at an anchorage or near shore "static" location and enter a LAT/LONG offset into your chartplotter? Over time you'd create your own, reasonably accurate database that would be reliable for night and poor visibility conditions. I assume this is what the cruising guide folks do. Regards to all... Frank & Claudette Weismantel Elverta, CA Boatless for a little while longer

frank weismantel wrote: > > Howdy All, > > Having read all the recent posts about GPS chart accuracy in Mexico and > elsewhere I started wondering. If GPS cart datum's themselves are > inaccurate due to their being produced from old, inaccurate "official" chart > information, <snip> Check out this site on Datum: http://164.214.2.59/GandG/datums/datum.htm Interesting reading. -- __________________ Keith The probability of a boat's survival is inversely proportional to the speed of arrival. High speed arrivals, small probability of boat survival and vice versa.

Frank wrote: > IMHO, Manufacturers SHOULD accept responsibility for providing the most > accurate data possible AS WELL AS pushing corrected data back at the gov't > agencies from which they receive their gross information. The profit motive and > disclaimers about relying on their data aside, you'd think the few major > manufacturers of digital cartography products would put together a clearinghouse > of some sort for vetted datum updates so we could all benefit from thousands of > users' experience. I imagine I always expect too much. REPLY Yes, you are expecting a bit too much in this case. In many cases the real difficulty lies in the fact the chart is done to an unknown datum. In other words: while the relative distances and spacing between geographic features is correct and accurate, the record of what original datum point was used is lost. the only way to fix this is to resurvey the area and establish a new datum and reference known features to this corrected datum. This is not something you can do with a couple of ranges with radar or a consumer GPS. > Another question: at opportune moments, couldn't you use your radar for > ranging data off a couple known objects at an anchorage or near shore > "static" location and enter a LAT/LONG offset into your chartplotter? Over time > you'd create your own, reasonably accurate database that would be reliable for > night and poor visibility conditions. I assume this is what the cruising guide > folks do. > > Frank & Claudette Weismantel > Elverta, CA REPLY Again the short answer is NO. The degree of precision you can achieve with this method is not sufficient. I might be enough to place you somewhere in the center of the channel. However it certainly isn't enough for any reputable authority to accept the data as "Accurate" within the meaning of the term as used for their chart products. And then they really would get sued. I'm not putting downbelittleling your questions. I asked the very same questions of the Canadian Hydrographic Service. Eventually they answered my questions in full. I spent many days at one of the regional centers and their department managers took the time to explain every step of the chart making process. I also attended a number of hydrographic conferences and received training courses on how to use a number of state of the art survey instruments. Then I was sent out with a field crew for a full training course. The same course all their staff has to take as part of their job experience. Two months later they sent a Hydrographer out to administer the final exam to my partner and myself. We passed. Subsequently my partner and I have been tasked with the revisory survey of a small lake system in central Ontario and our survey results were published the following year. The area where I lived was one of those "unknown datum" areas I mentioned earlier. While you could position yourself accurately by visual means, trying to get that sort of precision with GPS just didn't work out right. The grid didn't line up. When SA was cut off the problem became more apparent. Whereas before you expected a certain amount of error due to SA, afterwards, people now expected pinpoint precision down to the last foot. I spoke to one land surveyor about this problem. He allowed that the property survey work he was involved in was just as much in disarray as the charting was for the mariners. He estimated it would take a couple of decades before all the questionable areas were fully resurveyed.

Arild Jensen wrote: > > When SA was cut off the problem became more apparent. Whereas before you > expected a certain amount of error due to SA, afterwards, people now expected > pinpoint precision down to the last foot. I'll verify that... when SA was turned off, my boat began sitting on land when docked, and I cut across a set of condo's every time I leave the marina. > _______________________________________________ > http://lists.samurai.com/mailman/listinfo/trawler-world-list -- __________________ Keith Not one shred of evidence supports the notion that life is serious.