garmin blue chart problem

I have a simple Garmin GPS that I have downloaded Garmins Blue Chart or whatever they are called. It is a download and not a card. Anyway I use it in Philippine waters for navigation and many times, perhaps all the time my position as shown on the screen is off by sometimes a mile of more. At times it shows that I am piloting the boat on land, sometimes a half mile inland. I've talk to Garmin tech's about this several times, and they have no idea why it is showing an incorrect position or how to correct the inaccuracy.
Anyone with any ideas on this?

freshalaska said:

I have a simple Garmin GPS that I have downloaded Garmins Blue Chart or whatever they are called. It is a download and not a card. Anyway I use it in Philippine waters for navigation and many times, perhaps all the time my position as shown on the screen is off by sometimes a mile of more. At times it shows that I am piloting the boat on land, sometimes a half mile inland. I've talk to Garmin tech's about this several times, and they have no idea why it is showing an incorrect position or how to correct the inaccuracy.
Anyone with any ideas on this?

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I had a similar problem a few years ago and after exhausting all the various electronic possibilities, it was found that the plotter was correct. The chart was not correctly drawn with respect to lat/lons.

Thanks Walt. That's what I guessed was the problem. Next time I'm in the Philippines I'll check the GPS lat and lon against the paper charts and plotter shown position. Anyway it makes navigating interesting and why I try and not travel after dark and thankfully it is hardly ever foggy.

This has been discussed before on this forum. The GPS is dead on accurate your charts may not be.
I've crossed the Okeechobee waterway which is about 200 feet wide and both my CMap charts and Garmins Blue charts will show the boat on land at various points along the water way but the two different charts don't always agree when I'm on land or not.

The only GPS track I'll follow in the fog is one I've previously laid. And even then, I always back it up with radar, which is real time verification. My GPS stores and displays recent tracks with the oldest being overwritten by the most recent. If I don't have a previous track, I'll place most all of my positioning dependency upon the radar. I also run multiple GPS's to provide redundancy. I run minimum three sources (i.e 2 GPS, 1 radar) and if two sources don't always agree, I stop until I can make sense of it all.

The anchor is only a push button away from deployment at any time.

While chartering in Turkey, the charts we had to use had a warning on them to the effect that the original survey on which the charts relied was done by Beaufort so not to be relied on. Beaufort died in 1857 at the age of 82. We were able to verify that the charts were off by up to a mile. The comparison was to the chart plotter supplied with electronic charts with the same origins. Apparently those waters hadn't yet been re-surveyed. Except for the heavily travelled coastlines of the US, Canada and Europe, you should expect the charts to be from old, inaccurate measurements and until re-done, should treat them all as suspect.