Unfamiliar landfall at night

[bobwebster]

Originally Posted by hellosailor

So apparently Admiralty Chart Datum (ACD) is known to be offset from WGS84 and should not be expected to agree with GPS positioning. Did Navionics ever give you an answer?

I never did get an answer from Navionics.

The OSGB coordinates at this location are about .3 mile different from the WGS84 coordinates, which is a large difference. However, that does not explain the 1+ mile error or the fact that the Navionics data uses WGS84 and has the same error.

I decided to get to the bottom of this. Unfortunately, Google Earth doesn't show the Marietas.
However, in 2000, the space shuttle Endeavor did a radar elevation survey of the earth from 60N to 56S. You have to set some parameters, but if you go to The National Map Seamless Server you can see the locations of Marietas Islands and their lat-lon, as seen by the shuttle's radar.

Here's the screen image, with an arrow I added to show the lat-lon I requested:
http://xpda.com/minnow07/marietas/marietasSRTM.png

The lat-lon agrees with islands' location in the boat's radar, and not with the Navionics data:
http://xpda.com/minnow07/marietas/P1150869.jpg

The paper charts agree with the Navionics data. In other words, the paper charts (except for Rains's Mexico boating guide), Navionics data, and Maxsea data are off by more than a mile, even taking into account various chart datums.

I tellya, those islands are in the wrong place!

I have seen a few other "interesting" features in GPS data. For example, once I was flying off the coast of Alaska. The Garmin GPS showed only blue water to my right, but there was an island several hundred feet tall there. I notified Garmin but didn't hear back.

[hphoen]

Even when the Datum is correctly set in my chartplotter, the CMAP NT charts for the Lesser Antilles have been off by as much as 1,000 feet. I contacted CMAP about it and their response was that they made every effort to ensure accuracy, but their charts are only as good as the input data, which in many cases comes from sources many, many years old, like Admiralty charts from previous centuries. I had a similar problem with Imray Iolere charts for the BVI, even after applying the required adjustments. Maybe they all use the same ultimate source data (Admiral Lord Nelson?).

Bottom line: use charts outside the U.S. with caution; verify by eyeball navigation.

[sailingdog]

Electronic nav aids should always be confirmed by Mark I Eyeball...

[hellosailor]

Bob, haven't you heard? Due to global warming, continental drift has accellerated madly. The North Magnetic Pole has shifted radically, and those islands? Tie a string to them if you ever want to find them again.

Sometimes I think these mapping companies must be working for Stalin. He had Russian cartographers (who were among the best in the world) literally redraw whole towns, lakes, etc. many miles away from where they were, to ensure their own maps couldn't be used against them by an invading army.

Maybe the Marietas don't want tourists?

[Hartley18]

Some might be surprised how old mapping data is for even regularly-frequested parts of the globe. I make a habit of reading the Hydrographic info printed on the chart whenever I open one - it makes for an interesting history lesson sometimes!

On rare occasions, people have been known to change the topography enough that known mapping data is no longer correct. Examples: A certain property developer in the Whitsundays caused some excitement a few years ago during construction of his multi-million$ resort and I'm told it is amost impossible to obtain an accuarate chart of Singapore or Dubai without getting a subscription on a weekly basis..

[deliveryskipper]

I live in Ireland, near Dublin. Parts of the outer bay area here were last charted by Captain Bligh, of Mutiny on the Bounty fame.

What's amazing is how accurate it is, considering it was all done with a lead line.

[sailingdog]

Not to mention, how much time has passed since Bligh did the work.

Originally Posted by deliveryskipper

I live in Ireland, near Dublin. Parts of the outer bay area here were last charted by Captain Bligh, of Mutiny on the Bounty fame.

What's amazing is how accurate it is, considering it was all done with a lead line.

[billyruffn]

Says something about the Royal Navy -- training, standards, etc., eh?

[Hartley18]

Originally Posted by billyruffn

Says something about the Royal Navy -- training, standards, etc., eh?

I reckon it says more about the lack of progress in that part of the world.

"A land lost by time" is one statement I've heard to describe Ireland.

..sorry, but you asked for that - and I couldn't resist!

[catamount]

I have current Canadian Hydrographic Service charts of areas of Newfoundland that specifically say "No Datum." These are charts that were last surveyed 150 or so years ago. So Ireland is not alone.

As to entering unfamiliar ports at night, it all depends. What does the harbor and approach look like on the chart? How confident am I in the accuracy of the chart? How is the entrance described in cruising guides or pilots? How familiar am I with the type of setting (e.g rocky emergent coast, barrier island coast, estuarine rivers, etc...), if not that particular harbor? Are the aids to navigation lighted or not? (Are there even any aids to navigation at all?) What are the weather and sea conditions? What's the condition of my boat? Who do I have for crew? What's the condition of the crew? What other alternatives are there?

Tim