C-Map Digital Charts for Bahamas are not accurate! :(

[epiic]

I was dead centre of these markers and as you can see, its off by quite a bit.

Did I waste $280???

[Crew of Turning]

Two different position reports on your screen shot. Were you running true or magnetic? Just asking.

[conachair]

Originally Posted by epiic

I was dead centre of these markers and as you can see, its off by quite a bit.

Did I waste $280???

A quick look at CM93 on openCPN has the charts about 0.25Nm off compared with google earth image so might be an accurate chart plotter but no accurate charts for there.

[pcmm]

Originally Posted by epiic

I was dead centre of these markers and as you can see, its off by quite a bit.

Did I waste $280???

Keep in mind that GPS is not Accurate to zero feet. your chart plotter will tell you what the GPS accuracy is ( sometimes you need to add it to the display)

Navionics has a reputation for being much worse in the Bahamas (showing your track running straight through islands.

[epiic]

Originally Posted by Crew of Turning

Two different position reports on your screen shot.

Yes. One is the position of my actual boat. The other is where you see the cross-hairs.

Originally Posted by Crew of Turning

Were you running true or magnetic? Just asking.

How would I find out? I thought GPS was GPS and did not vary like compasses do.

[epiic]

Originally Posted by conachair

A quick look at CM93 on openCPN has the charts about 0.25Nm off compared with google earth image so might be an accurate chart plotter but no accurate charts for there.

So then I did waste my money on these charts....

[conachair]

Originally Posted by epiic

So then I did waste my money on these charts....

They might be as good as it gets ........

[epiic]

Navionics on iPad had me dead centre of the buoys so I am really not impressed with C-Map.

[fallingeggs]

Maybe I'm being slow, but is your boat being shown off the map and the cursor is where you actually are? Plugging the coordinates into an online calculates says that those two points are 900 feet off. Quite a bit!

You using the Zeus GPS or an external?

[epiic]

Originally Posted by fallingeggs

Maybe I'm being slow, but is your boat being shown off the map and the cursor is where you actually are? Plugging the coordinates into an online calculates says that those two points are 900 feet off. Quite a bit!

You using the Zeus GPS or an external?

My boat is off screen and I scrolled down to the obvious area to show how far off the charts are.

I am using a Zeus2 9" chartplotter

[bvander]

Been sailing Bahamas 8 years never had a problem. 2 years ago c map and Garmin both adopted Explorer chart data for Bahamas, so I believe all the datum is from Explorer, which most everyone cobuilders the gold standard for Bahamas.

[wayne.b]

Originally Posted by epiic

I was dead centre of these markers and as you can see, its off by quite a bit.

Did I waste $280???

===

C-map charting on my Furuno GPS/plotter has always been very accurate in the Bahamas. Is this a one-time issue or are you seeing consistent errors of a similar magnitude? If so, check your plotter options to make sure you're using the correct datum and ensure you don't have any offsets entered.

[royalstar]

Originally Posted by epiic

How would I find out? I thought GPS was GPS and did not vary like compasses do.

You can sett GPS True, Magnetic or Grid North.

[Captain Bill]

Sometimes it has more to do with differences in the software that draws the chart. I have the cmap charts in my chartplotter and on a laptop running Coastal Explorer. One has me in the center of the channel and the other one has me clearly outside the channel. I never trust the chartplotter to go into a channel for the first time. I find that if it renders it properly once it is consistent and I'm willing to chance it at night, but never ever the first time. I also find that if it has rendered it improperly it is also consistent and I never will go in that channel using the GPS.

[Emouchet]

Modern all-singing all-dancing MFD device make the data look so damned cool that it’s just human nature to forget that display technology has absolutely NOTHING to do with accuracy.

"How to read Nautical charts" from Nigel Calder is a book everybody should read and reread...

Your GPS is probably on WGS84 datum , but are you sure ? Did you check that ?

Your map may be on a different datum. I am sure you have verified that too?

Have you check the scale of the map ? May be you are over zooming ? Accuracy of drawing on a map at 1:40000 is 1 mm. This correspond to 40 meters on the ground, or 120 feet. This is half of your screen at the 200 feet zoom setting...

So, a map with a 1:40000 scale has a drawing precision of 120ft. Your GPS says its precision is 4m 12 feet. How do you deal with that ?

1% of the area where sounding are less than 200 meters are considerate accurate in the Bahamas. 99% is considerred needing re-survey.

Welcome to the real world.