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Old 17-03-2017, 10:03   #16
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

As a professional navigator of considerable vintage can some one please tell me why you are relying on electronics rather than using your eyes to run up what looks to be a perfectly adequately buoyed channel. Eyes are far more accurate and reactions faster when using them than any electronic gizzmoes, also no matter how accurate your electronica are what they give you is history not the now.

Accuracy of electronics can vary for a various reasons
1. Accuracy of GPS depends on the number of satellites in view and their position relative to the receiving aerial any less than four can mean less accurate fixes
2. The survey standard used for the survey of the area the latest is WGS84 and GPS positioning uses this, there are dozens of others and all good mapping systems should have the errors between the survey standard used and WGS84 built into them, yours may not have.

Anyway if you go back to basics and use the senses you were given not a box of electronics you will find navigation suddenly becomes an interesting pastime
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Old 17-03-2017, 10:48   #17
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

As Emouchet says, it's probably a datum issue but could also be the chart is off.

Charts for the US are extremely accurate so sailors often get in trouble when they leave the US for the first time and cruise the Bahamas.

Outside the US (and other rich countries) you can only count on charts for major ports to be accurate. For other places they may be up to a mile off, or be missing things you can hit.

Best thing to do is to stay a mile away from anything you can hit on longer sails where you are not really paying attention and then keep your wits about you when you get closer.

For the Bahamas we found the Explorer charts on Garmin really good but they were 200m off in some of the less travelled areas. 200m implies an issue calculating the grid projection in those areas.
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Old 17-03-2017, 11:04   #18
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

Also being of a vintage of pre-digital navigational aids, the advice offered by wjhutchings to place your primary confidence in your senses is worth considering. Not being familiar with charting in and around the Bahama's I can only comment on the accuracy of GPS readings along the west coast of Mexico and Sea of Cortez. There are areas that show large discrepancies in actual GPS position (which usually is a reliable estimate of where on the face of the earth you find yourself bobbing about) and the relative position of the land masses represented on your chart plotters screen. These discrepancies along the west coast of Mexico have a great deal to do with the fact that the paper charts, which many, if not all the digital representations are based upon, were last updated from decades to up to a century prior. Most of the digital chart representations are not based on satellite imagery, but are taken from the most recently available charts. Not too surprising that there are some discrepancies to be found.
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Old 17-03-2017, 13:49   #19
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

charts are ok your gps is off/
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Old 17-03-2017, 14:30   #20
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

This is going to sound really odd, but here goes.

I had the exact same problem in the exact same spot with my Lowrance. Night approach to Bimini channel in a t-storm. Could barely see the channel markers, which are lit with a couple of fireflies. I bought the Navionics upgrade and had the same problem. Basically off by about 50-100 yards, which is enough to run you aground in the Bimini channel.

I called Lowrance about another issue owing to intermittent power shutdowns and after working my way past the useful idiots in customer service, I was called back by an engineer in the engineering department.

He did not believe that the discrepancy could be the fault of the machine and he vowed to research (sent me a new machine, btw, which has the same problem)

After a few days he called me back to tell me that there is a navigational error on the British charts of Bimini of about 100 yards that was codified back in (I think he said) the 1840's that was never corrected. Apparently, electronic maps are a cumulative process and foundational errors are very difficult to fix.

This sounded like nonsense to me, but I asked a friend who is the business of digital electronic cartography and he says that there are anomalies like this around.

My latest Lowrance continues to show me anchored just outside of Captain Bobs Restaurant, while my eyes tell me that I am very much tied to a slip.

It seems unbelievable to me, but I have now stored a waypoint which my eyes tell me is midchannel, but the screen shows about 100 yds south of that point.

Bimini is the only place that I have experienced this.
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Old 17-03-2017, 14:47   #21
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

Quote:
Originally Posted by Rocinante33 View Post

I had the exact same problem in the exact same spot with my Lowrance. Night approach to Bimini channel in a t-storm. Could barely see the channel markers, which are lit with a couple of fireflies.
===

A night approach to any unfamiliar harbor is never a great idea. On a run that can't be completed in daylight I prefer to leave in late afternoon or early evening so I have a mid-day land fall and approach. If it's a harbor I've been to before I'll have saved way points and track lines which takes much of the risk out of approaching at night.
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Old 17-03-2017, 16:12   #22
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

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So then I did waste my money on these charts....
Only you can decide on that. If you got the $280 back, what would use to navigate with? Once you are out of the first world areas charts are often off by relatively small amounts and often lack important detail. It is a tool for the human navigator, not the navigator.
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Old 17-03-2017, 17:13   #23
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

I think C-Map is based on the Explorer charts, which is the best you're gonna get.
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Old 18-03-2017, 00:06   #24
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

did you position your boat at a known waypoint on your electronic chart then dead reckon with a magnetic compass / compare the lat & long / the gps system you are using takes it's data from many different satelites that are programmed to adjust and compare in case of slight earth wobbles and satellite orbit deviation / if your gps is dgps accuracy is 10mtres / if you are running normal gps accuracy is between 50-100 metres(line of sight) and meant to be a navigational aid / we are meant to visually check our position / the sounder is useful to stay in a dredged channel
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Old 18-03-2017, 06:08   #25
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

Chart plotters are a great AID but in the Bahamas nothing beats visual nav ("Bahamian day marks) and "reading" the water. I have seen many boat aground and out of the channel there.

I have cruised the Bahamas for over 10 years, starting with LORAN and paper charts, and learned to never trust just one navigation aid.

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Old 20-03-2017, 07:00   #26
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

Quote:
Originally Posted by royalstar View Post
You can sett GPS True, Magnetic or Grid North.
The only options I have is "Auto" or "Magnetic"

Quote:
Originally Posted by Emouchet View Post
Your GPS is probably on WGS84 datum , but are you sure ? Did you check that ?
Yes. its set to WGS84 datum.

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Originally Posted by Emouchet View Post
Your map may be on a different datum. I am sure you have verified that too?
I have no idea where I can locate those settings. I don't think I have the option to change it.
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Old 20-03-2017, 07:04   #27
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

I just compared the Lat/Long of the B&G chartbplotter using C-Map with my iPad using Navionics and its identical.
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Old 20-03-2017, 07:53   #28
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

If you're navigating on buoys then your GPS is fine. Buoys move a little depending on tide, current, wind, etc. If I have read your posts correctly, you are just fine and your money wasn't wasted and GPS is doing its job correctly.
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Old 20-03-2017, 08:13   #29
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

This is where I am currently anchored.

As you can see, these maps must be old. This area was dredged years ago. My maps have a date of "Winter 2016" so they must be far behind for updating the Bahamas charts.
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Old 20-03-2017, 08:15   #30
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Re: C-Map Digital Charts for Bahamas are not accurate! :(

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If you're navigating on buoys then your GPS is fine. Buoys move a little depending on tide, current, wind, etc. If I have read your posts correctly, you are just fine and your money wasn't wasted and GPS is doing its job correctly.
The Chanel coming into north Bimini is dredged and quite narrow. The buoys are right at the drop-off. There is no way the Chanel can move. Its set in stone quite literally.
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