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post #13 of Old 08-11-2009
Don Radcliffe
Join Date: May 2007
Location: Santa Cruz
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I had a friend who bought a GPS mouse from Garmin, and helped him get it to work. We had to use Fanson's GPSGATE program to create a virtual com port for the charting softwar, but it worked fine after that. BTW, you can get the USB GPS's for less than $50 on Ebay, and GPSGATE Express is $13 after your free trial runs out.

The biggest advantage of the laptop charting programs is that you are not tied to proprietary vendors who charge $$ for changes. When we got an AIS receiver, SeaClear was a free download for the laptop to give us charting with AIS targets. How much did you pay to upgrade your chartplotter to AIS??

I absolutely second oceanscapt on using every input you have for navigation-GPS waypoints, paper & electronic charts, radar, visual ranges and bearings, depthsounder, eyeball, ears, wave patterns, etc. If all those inputs are agreeing, full steam ahead. If one or more disagree, its time to slow down or stop until you figure out why.

Electronic charts are based on paper charts, most of which were made before the surveyor knew his longitude and latitude to the accuracy that your GPS does. I have seen paper charts 1.75 miles off (Tonga), but they are usually consistent errors, as the surveyor used distances and compass bearings to record features like points and reefs--if you forget the GPS and use ranges and bearings these charts are still useful.

The differences between electronic and paper charts are usually datum errors, but I did find one BSB chart of the approaches to Bermuda which was 8 miles off in latitude (put the cursor on a line of latitude and the chart program read exactly 8 miles further north).

I did a delivery on a boat where the laptop was off 0.6 miles coming into Grand Canaria. It turned out the boat had been in Croatia, and someone had reset the GPS to some obscure Croatian datum. Resetting the GPS to wgs84 had us within 0.05 miles on both electronic and paper charts.

I was on a power boat (just for cocktails) in Tobago Keys this year which had two high end chartplotters. The one downstairs was OK, but the flybridge (different brand) was off by 0.8 miles. It turned out this chartplotter had a feature down in the menus whereby you could set the datum by clicking on a known point on the chart. The really scarey thing was this chartplotter didn't give any warning that it was using a user-generated datum!

In all of these cases, a comparison of available inputs indicated something was wrong, and it was time to be very careful until we figured out why.
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