TWL: Fw: Maiden Post.
----- Original Message -----
From: strahan
To: TWL
Sent: 24 January 2004 1335
Subject: Maiden Post.
24 January 2004
Hi Twlers
I aplologise in advance for this post. I have been a long time lurker on the
list (2-3 years) so I should have introduced myself a lot earlier. The reason
I didn't is because I felt I really did not have much to contribute for
cruising motorboats. I have been sailing, both cruising and competitively, for
the last forty something years. During this time I have covered some 35
thousand plus miles both in Australian and foreign waters and have finally
decided to go for a boat that is a little less strenuous; i.e., no rags!
Consequently I have been lurking in the hope of learning something about the
mechanics and ancillory equipment of a vessel as opposed to just starting a
generally very neglected engine just to get out of the marina so we could put
up the sails!
I was introduced to this list by a good motorboating friend and with some
scepticism started to read the posts. What a revelation. And what a resource.
Here was this source of priceless information about every mechanical,
electrical, refrigeration etc. topic absolutely free, and provided by an
altruistic gentleman who I had never heard of and who supported the site
totally from his own expenses!
Consequently I was surprised to read some of the mean-spirited comments
regarding Georgs requesting feedback about the future direction of the TWL and
whether perhaps he may be able to defray some of the running expenses of the
list. For myself I would just like to say Thank You Georgs. Because of your
selfless contribution you have introduced possibly thousands to a priceless
resource, and I'm sure the silent majority agree. If and when the time comes
to defray your costs, by some form or other of donation, I'm sure you won't be
disappointed.
John Strahan
'Maid of Martha' 46' Huon Pine Motorsailer
Melbourne
Australia.
The recent thread concerning Loran and/or GPS and the comparative accuracy is
especially relevant in light of a recent accident in Norway.
From Lloyds List
ROCKNES (ANTIGUA & BARBUDA)
London, Jan 23 -- A press report, dated Jan 22, states: New information has
come to light as to the cause of the loss of bulk Rocknes. Seismic
recordings by the University of Bergen revealed three rather severe
contacts and one lighter over 50 seconds from 1632.20 hrs. The first
distress message was recorded by Bergen Radio at 1634.12 hrs, and the
vessel had capsized by 1636 hrs. This gives a total time of 3 minutes 40
seconds from the first impact until the vessel was lying bottom up. The
damage to the hull reveals a rift of some 12 metres, bearing proof of the
vessel coming too close to the Revskolten rock. When the area was
re-hydrographed a few years ago, a protruding rock was detected at 9 metres
depth some 55 metres off the rock. In a loaded condition Rocknes would draw
10.14 metres,
snipped for brevity<<<
Comment
In other words, in today's modern terms of precision navigation with GPS this
should be sufficient clearance given that GPS is now accurate to better than 10
meters. Except for the fact a new and previously uncharted rock just happened
to lurk below the surface.
Evidently it was not known if the pilot on board had a copy of the most recent
chart or was relying on his detailed "local knowledge" acquired over many
years but mostly predating the discovery of a new rock.
In case you think this is an unusual and rare occurrence, think again.
While doing survey work back in the Great lakes in 1998 our crew found an
additional rock near a known cluster.
The detailed chart showed two rocks. We found one more.
Three months later another crew found two more rocks for a total of five.
This was all within ten yards of a major navigation channel.
Two weeks prior to our first discovery I had been driving a client's boat
within five yards of this exact location thinking I was safely in the deep part
of the channel. And this in a channel that had been charted several times in
past decades.
This channel leads to three major marinas in the area.
I have heard of similar situations in several other locations.
Modern survey methods with side scan is revealing numerous rocks missed by
previous single beam surveys.
Kind of puts the whole issue of precision navigation and how close to a known
hazard you should plot a course.
I don't care if the chart is done to NAD 83 or WGS 84 with a survey date only
five years old.
It still does not justify placing waypoints and routes anywhere near charted
hazards.
regards
Arild
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Dang it, Arild!
You just made shivers go up and down my spine. No longer are my skills,
equipment and charts enough to protect me. Now I must rely on luck!
<grumbles>
Thanks for the informative post though. I have often wondered if they
missed any/many rocks out there. I understand Zopilote found one to her
demise. Guess we will all have to be good lil boy/girl scouts and "always
be prepared".
Good luck,
Bob Lowe
The recent thread concerning Loran and/or GPS and the comparative
accuracy is
especially relevant in light of a recent accident in Norway.
Comment
In other words, in today's modern terms of precision navigation with GPS
this
should be sufficient clearance given that GPS is now accurate to better than
10
meters. Except for the fact a new and previously uncharted rock just
happened
to lurk below the surface.
Evidently it was not known if the pilot on board had a copy of the most
recent
chart or was relying on his detailed "local knowledge" acquired over many
years but mostly predating the discovery of a new rock.
In case you think this is an unusual and rare occurrence, think again.
While doing survey work back in the Great lakes in 1998 our crew found an
additional rock near a known cluster.
The detailed chart showed two rocks. We found one more.
Three months later another crew found two more rocks for a total of five.
This was all within ten yards of a major navigation channel.
Two weeks prior to our first discovery I had been driving a client's boat
within five yards of this exact location thinking I was safely in the deep
part
of the channel. And this in a channel that had been charted several times
in
past decades.
This channel leads to three major marinas in the area.
I have heard of similar situations in several other locations.
Modern survey methods with side scan is revealing numerous rocks missed by
previous single beam surveys.
Kind of puts the whole issue of precision navigation and how close to a
known
hazard you should plot a course.
I don't care if the chart is done to NAD 83 or WGS 84 with a survey date
only
five years old.
It still does not justify placing waypoints and routes anywhere near
charted
hazards.
Arild<<<<
Not quite sure I see the relevance. I would use precision GPS to stay
in the middle of the channel as far from any obstacles as I could
reasonably get. I will certainly do things with GPS (enter a harbor in
the dark) that I would not attempt without it but I fail to see where
GPS would cause me to approach any closer to an obstacle than I would
without it.
I suppose there may be a minor increase in risk as I could hit the rock
in the dark rather than the light but I would otherwise think it is
pretty much the same.
Jim
-----Original Message-----
From: trawler-world-list-bounces@lists.samurai.com
[mailto:trawler-world-list-bounces@lists.samurai.com] On
Behalf Of Arild Jensen
Sent: Friday, January 23, 2004 10:49 PM
To: TWL
Subject: TWL: The dangers of precision navigation
The recent thread concerning Loran and/or GPS and the
comparative accuracy is especially relevant in light of a
recent accident in Norway.
From Lloyds List
In case you think this is an unusual and rare occurrence,
think again. While doing survey work back in the Great lakes
in 1998 our crew found an additional rock near a known
cluster. The detailed chart showed two rocks. We found one
more. Three months later another crew found two more rocks
for a total of five. This was all within ten yards of a major
navigation channel.
Two weeks prior to our first discovery I had been driving a
client's boat within five yards of this exact location
thinking I was safely in the deep part of the channel. And
this in a channel that had been charted several times in
past decades. This channel leads to three major marinas in the area.
I have heard of similar situations in several other
locations. Modern survey methods with side scan is revealing
numerous rocks missed by previous single beam surveys.
Kind of puts the whole issue of precision navigation and how
close to a known hazard you should plot a course. I don't
care if the chart is done to NAD 83 or WGS 84 with a survey
date only five years old. It still does not justify placing
waypoints and routes anywhere near charted hazards.
regards
Arild
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Not quite sure I see the relevance. I would use precision GPS to stay
in the middle of the channel as far from any obstacles as I could
reasonably get. I will certainly do things with GPS (enter a harbor in
the dark) that I would not attempt without it but I fail to see where
GPS would cause me to approach any closer to an obstacle than I would
without it.
I suppose there may be a minor increase in risk as I could hit the rock
in the dark rather than the light but I would otherwise think it is
pretty much the same.
Jim
-----Original Message-----
From: trawler-world-list-bounces@lists.samurai.com
[mailto:trawler-world-list-bounces@lists.samurai.com] On
Behalf Of Arild Jensen
Sent: Friday, January 23, 2004 10:49 PM
To: TWL
Subject: TWL: The dangers of precision navigation
The recent thread concerning Loran and/or GPS and the
comparative accuracy is especially relevant in light of a
recent accident in Norway.
-----Original Message-----
............................................ I have often wondered if they
missed any/many rocks out there. I understand Zopilote found one to her
demise. Guess we will all have to be good lil boy/girl scouts and "always
be prepared".
Hi Bob,
You better believe that there are a good number of uncharted rocks out there
just waiting to make your acquaintance.
Most of our charts in use today are based on old hydrologic surveys made using
individual ping type sounders (some by only a lead line soundings!), and very
few are based on the far more comprehensive side scan sonar. As such, a great
number of rocks were missed on the original surveys. Even though new editions of
the charts have new dates on them, the soundings shown are based on the old
(some times ancient) survey data.
This was the case when the QEII hit an uncharted rock in Vineyard Sound in
August of 1992. Although some wags still swear she hit another sunken Ted
Kennedy car, it turned out that she ran over a rock that was missed by the
original surveys. While the QEII was using a current chart, the chart was based
on 1929 survey data - long before the advent of side scan sonar. The rock in
question lay undetected between two parallel rows of pings on the original 1929
survey! It took the QEII accident and then a re-survey with modern side scan
sonar to find the offending rock.
Watch your butt out there! :-)
Kevin
Dream Weaver
34' Mainship III
www.BoatMoves.com
Jim Donohue wrote:
Not quite sure I see the relevance. I would use precision GPS to stay
in the middle of the channel as far from any obstacles as I could
reasonably get.
snip<<<
I suppose there may be a minor increase in risk as I could hit the rock
in the dark rather than the light but I would otherwise think it is
pretty much the same.
Jim
REPLY
The relevance, Jim, is that with navigation being so precise many navigators are
more inclined to trust the equipment to show exactly where they are. That faith
may be misplaced if the charts are not correct.
Not all of us boat in deep water with few hazards. Some of us face more
challenging conditions.
Where I come from the small craft channel only guarantees a five foot depth and
it changes direction every few hundred yards .
Pinnacle rocks abound with many of them just below the surface but rising
steeply from a bottom which may be 20 or 30 feet lower.
In other words you may think you are safely in a deep channel but stray over as
little as 5 feet and bingo you rip out the bottom of your hull.
Bent props would be the least of your worries.
When you have narrow twisted channels requiring as much as a 100 degree course
change, you cannot stray very far from a point or submerged rock without
hitting the other side of the channel.
When a rock is charted, there is an implied statement that the area had in fact
been surveyed completely. Not so!
Until recently surveys were done mostly with single beam depth sounders.
Depending on chart scale the survey tracks would be spaced anywhere from 50
feet to a quarter mile apart.
If the beam sees depths of 20 feet in successive passes on parallel tracks
the cartographer generalizes and shows this as an average depth.
Isolated pinnacle rocks can be missed.
Depending on water turbidity you may not be able to see more than 3 feet down
below the surface even from a helicopter.
In the Great Lakes a large freighter named the Edmund Fitzgerald disappeared.
in a 1971 November storm..
Eventually it was determined she hit a shoal which the existing chart had
mispositioned by a considerable distance.
The storm waves were 30 foot or more through to crest and the ship bottomed
out.
Underwater photographs revealed scouring marks on the top of the shoal that
looked like keel marks.
The Edmund Fitzgerald was eventually found in deep water totally smashed and her
back broken.
Evidently the Norwegian accident has similar conditions. The vessel steered
around a known and charted hazard.
Unfortunately in so doing they struck a hazard which was unknown until
recently.
The vessel did have a pilot on board. Pilots traditionally rely a great deal on
their personal local knowledge acquired over decades of experience. It is not
impossible that the pilot somehow forgot or missed the change in the new chart
since the new hazard was so close to the existing mark.
Cheers
Arild
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We are still talking past each other. I think you are saying Skipper
are stupid enough to believe that charts are complete and fully accurate
and there are no sins of ommission on them. I would simply point out
that Darwin will take care of those skippers.
Rationale sailors use charts to achieve at least adequate and generally
maximum clearance. Why on earth would you suggest otherwise?
Back to my point...what does any of this have to do with GPS?
I agree there are lousey areas to navigate. If in doubt you slow up
until your boat is at minimum risk from the hazards. If you are
navigating in areas where the braille system is necessary you go real
slow and careful.
What has this got to do with precise GPS?
My point is and remains this is the nature of navigation. In an area of
changing and unpredicable bottom configuration you go slow and hit
gently. Neither Radar nor celestial nor LORAN nor DR nor anything is
going to make any difference.
What does this have to do with precise GPS?
Jim
-----Original Message-----
From: Arild Jensen [mailto:elnav@uniserve.com]
Sent: Saturday, January 24, 2004 6:49 PM
To: Jim Donohue; 'TWL'
Subject: RE: The dangers of precision navigation
Jim Donohue wrote:
Not quite sure I see the relevance. I would use precision
GPS to stay in the middle of the channel as far from any
obstacles as I could reasonably get.
snip<<<
I suppose there may be a minor increase in risk as I could
hit the rock in the dark rather than the light but I would
otherwise think it is pretty much the same.
Jim
REPLY
The relevance, Jim, is that with navigation being so precise
many navigators are more inclined to trust the equipment to
show exactly where they are. That faith may be misplaced if
the charts are not correct.
Not all of us boat in deep water with few hazards. Some of us
face more challenging conditions. Where I come from the
small craft channel only guarantees a five foot depth and it
changes direction every few hundred yards . Pinnacle rocks
abound with many of them just below the surface but rising
steeply from a bottom which may be 20 or 30 feet lower. In
other words you may think you are safely in a deep channel
but stray over as little as 5 feet and bingo you rip out
the bottom of your hull.
Cheers
Arild
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I often run aground in mud on the Chesapeake's Eastern shore because I like
to gunkhole and the Chesapeake tends to shoal from West to East. So.....,
once I find the new entrance to a river or creek, and I have the
chartplotter's tracking on, I can repeat that course on the way out - with
precision using WAAS/GPS or DGPS.
Now, if that mud were rock, it would be a harder lesson. Of course, I would
be going slower. If I were in Maine and if the locals thought it was a good
idea, I'd wait for low tide so that all those rocks marked awash at low tide
were revealed on the way in to a place new to me - still using the track
plot on the chartplotter. I'd also be ticking off the obstacles on a paper
chart.
Ron Rogers
At times like that I am so busy tightening my sphincter I can't imagine
marking the offenders on a paper chart. I would not even be confindent
that I pushed the GPS buttons at the right location.
But I admire our presence of mind to leave a record for the CG when they
investigate the incident.
Jim Donohue
PS Apologize for hijacking your thread the other day. The only excuse
is that it was convenient.
From: Ron Rogers [mailto:rcrogers6@comcast.net]
Sent: Saturday, January 24, 2004 8:11 PM
still using the
track plot on the chartplotter. I'd also be ticking off the
obstacles on a paper chart.
Ron Rogers
From: Jim Donohue
We are still talking past each other. I think you are saying Skipper
are stupid enough to believe that charts are complete and fully accurate
and there are no sins of ommission on them. I would simply point out
that Darwin will take care of those skippers.
Back to my point...what does any of this have to do with GPS?
REPLY
You obviously have not kept in touch with developments in the commercial world
round the world.
In a perfect world, your statements might be valid.
Skippers are prudent, they always have back up paper charts and they always have
the latest Notices to Mariners and changes to Nav aids.
In the real world things are different.
A combination of commercial pressure, human nature and laziness, not to mention
a tendency to go with the flow, means that in the real world navigators and
skippers tend to rely on the tools provided to them.
Commercial as well as private navigators tend to use the Sea buoy located off
a harbor mouth as the destination waypoint.
As cruising guides frequently provide "safe" waypoints for the novice, these
published locations become congested with converging traffic.
Commercial ships in particular are often forced to stick to a schedule with
little or no regard for tide and weather.
Some paper pusher ashore defines the "optimum" route to follow and this now
becomes the company's bench mark.
I have often heard it said that ECDIS and computer aided navigation means the
entire fleet of ships will now be provided with a pre plotted set of routes
for all trips.
How many civilian and pleasure boaters share route information? Same thing.
When you have had several years experience with a GPS system that consistently
places your vessel on a chart display within half the width of your vessel it
is easy to become trusting enough to think the unit actually works.
Whereas in days of old you would be over cautious and steer clear by half a
mile or more now you begin to feel confident that you can plot a safe course
only 50 meters of specific waypoints.
Given the modern trend to cram as much course into as short a time as possible;
many navigation schools will now offer a crash course in only the main points
of navigating by GPS and electronic charts.
No one is interested in or have the time to take a course on the complete
history of cartography and all the possible pitfalls of relying on paper
charts.
To understand this trend just look at recent maritime disasters such as:
Two ships collide in the English channel where two routes cross.
The sunken wreck is marked with wreck buoys and a notice to Mariners
broadcast.
Over the next month the sunken wreck is rammed by several more vessels.
One can only conclude that the navigators involved must have marked the wreck
location with a waypoint and then either assumed they could safely skirt the
wreck or maybe forgot how much bigger a real ship is compared to a pin point
lat/long co-ordinate.
The alternative is to conclude that these navigators didn't bother to note the
location of the wreck and simply continued to follow their customary pre
plotted routes.
This was not an isolated incident. Over the last few years such stories are
reported more often.
In Georgian Bay the cottage associations joined forces to remove the cruising
boats away from their docks and inshore channels.
They paid to have the Coast Guard find and mark an offshore route as a short
cut across a reef strewn area.
The location of each marker buoy was posted as GPS lat/Longs.
The ongoing maintenance of the buoys is also paid for by the Cottage
association.
Having transited this route myself I can tell you that except on windless days
these buoys are hard to spot at any distance.
Without a GPS you really haven't got much chance of running safely.
Canada has officially revised their Aids to Navigation policy to limit visual
aids to navigation to an area within 10 miles of official "ports of
destination" meaning entry ports fro deep sea ships.
For areas outside such ports of destination; mariners are expected to rely on
electronic means of navigation ( meaning GPS )
As a corollary, official CHS (Canadian Hydrographic Service ) policy is to
give priority to those areas with the greatest traffic density and thus the
most risk of navigational problems.
In real terms this means the major harbours rivers and commercial routes from
port to port.
Recreational boaters being what and who they are often prefer to cruise in
less frequented waters.
Unfortunately this also means that they are using charts which may not have
been updated as frequently as those covering the commercial routes.
I'm sure that other countries with hydrographic department are forced to
rationalize their own work load along similar lines.
The sheer cost of maintaining artificial navigation aids is considerable. With
SA removed and official assurances that GPS will remain operational it is
only natural that more and more reliance be placed on GPS.
When Loran first became available much time was spent on teaching user show it
actually worked.
Same thing with GPS. However pretty soon this was short circuited to an
abbreviated summary and even this is often dropped.
No wonder such courses are often called "crash" courses. <VBG>
It isn't a case of skippers being stupid or lazy. More often than not they
have never been informed in detail of all the pitfalls involved.
Quite bluntly, you cannot guard against a danger you are not aware of nor can
you anticipate something you have never been told about.
And that is the real tragedy of our modern worlds with its headlong rush for
"instant solutions" to everything.
Cheers
Arild
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In the Army (yes, believe it or not) we were always taught to pick an offset
from an objective and follow a magnetic bearing to the offset rather than
the objective. The concept being that in the unlikely event that your land
navigation was precise, you would not stumble onto the objective until you
were ready.
With the advent of GPS and chartplotters, I have applied this concept to my
marine navigation. I pick a waypoint, let's say a buoy, and then set my
target waypoint on the channel side of that buoy and offset a little more
than the 10 meter accuracy of WASS GPS. If it is night time, or otherwise
limited visibility, I have that artifical target on the radar screen as
well. In that way I can attempt to observe other traffic in the area of the
target and between the target and my vessel. So far, this system works.
Usually, I build this course before leaving the dock. This gives me time to
insure that a rhumb line between waypoints doesn't put me on a shoal.
Pointedly,
Ron Rogers
Ron wrote:
In the Army (yes, believe it or not) we were always taught to pick an offset
from an objective and follow a magnetic bearing to the offset rather than
the objective. The concept being that in the unlikely event that your land
navigation was precise, you would not stumble onto the objective until you
were ready.
REPLY
We were no doubt trained from the same generic manual.
As part of my Officer Candidate training I was sent to an Army base to learn
(among other things) Parade drilling of a platoon, Gunnery Range Officer
training and map ( not chart ) reading.
All my prior training was about Variation and compass deviation now al of a
sudden we had declination and mils?? instead of degrees , minutes and seconds.
And of course for the orienteering exercises we were told to use offsets.
During one exercise I got separated from my unit and made it back to camp
without a map or compass.
In the process I found a pair of stragglers who managed to get lost despite
having both map and compass.
I told them to put away the paper and compass and follow me.
We marched into camp just as they were organizing a team to go look for us.
The Officer in Charge refused to believe that I had managed the course without
a map.
Just goes to show us country boys knows a thing or two about cross country
trekking < VBG>
I knew I had a safety bearing by the sun and the southern perimeter fence of
the army base we were on.
Even if totally lost, all I had to do was maintain a straight line until I hit
the perimeter.
Also keeping east of the river reduced the chance of going too far astray.
Just goes to show precision navigation was possible before GPS. < grin >
Reminds me of a delivery trip in pea soup fog for over 100 miles without any
instruments that worked.
But that's another story.
Arild
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Hehe! I've watched my boat cut right through a set of apartments on my old
GPS/Chartplotter several times while leaving the marina. I know better than
to trust that thing for close work! Either that, or I've got something
patentable on my boat that allows it to be a bulldozer occasionally.
Keith
__
When in doubt, stay out to sea. No one has ever gone aground on a wave.
----- Original Message -----
From: "Arild Jensen" elnav@uniserve.com
It isn't a case of skippers being stupid or lazy. More often than not
they
have never been informed in detail of all the pitfalls involved.
Quite bluntly, you cannot guard against a danger you are not aware of nor
can
you anticipate something you have never been told about.
The recent precision navigation thread was brought to mind when I read an
article in the Dec/Jan issue of Professional Mariner magazine titled "NOAA
surveyors working to correct errors ion Columbia River charts". The article
began:
"The first rule in navigation is to know where you are at all
times, and for that you need a chart. With GPS, computer
plotters and radar giving a position within a few yards,
charts - paper or electronic - take on new significance.
However, all charts are based on latitude and longitude
calculations taken by sextant not satellite, so it's quite
possible that a ship's actual position by GPS will not agree
with the chart."
The article then goes on to describe that the lat/long positions for objects on
the charts are frequently different from the lat/long of that object taken with
a GPS receiver! Therefore, if you navigate with GPS on those charts, while the
chart plotter may show you in the middle of the river, you could actually in a
very different position relative to the geography.
The article said the biggest discrepancies between charted lat/long and GPS
lat/long were on the section of the Columbia between St. Helens, Ore. and
Longview Wash.
Kevin
Kevin wrote:
There is an article in the Dec/Jan issue of Professional Mariner magazine titled
"NOAA
surveyors working to correct errors ion Columbia River charts".
snip<<
The article then goes on to describe that the lat/long positions for objects on
the charts are frequently different from the lat/long of that object taken with
a GPS receiver!
REPLY
The datum shift is only one of many cartography problems.
A recent trend to "clean up" the appearance by removing clutter has resulted
in yet another bit of confusion.
Very old charts often depicted the soundings as a line of numbers showing
exactly where the survey launch travelled.
Those parts which had not been sounded were blank.
Areas which were thoroughly surveyed had so many numbers printed that the
end result was visual clutter.
It was therefore decided to use depth contours instead of actual numbers to
depict water depths.
contour lines are derived by connecting same or similar soundings with a line.
In classic bowl configuration with the deepest part in the middle and
gradually shoaling towards shore this works fine.
However it does not work as well in area of steep rocky ledges and sudden
drop offs or narrow canyons.
Perhaps most deceptive is the fact that these contour lines assumes a gradual
and even gradient change from contour line to contour line.
To my mind what is worse is the fact that this process was often applied to
older charts which shoed individual survey boat tracks as a line of numbers.
The contour lines obliterated these accurate sounding and instead showed the
blank white areas as being included in the depth contours.
This falsely implies that all parts of the displayed body of water was equally
surveyed and thus of equal accuracy and value.
About ten years ago a navy survey vessel toured up through the Great Lakes basin
doing a very detailed survey using a host of modern equipment including
magnetometers and gravity meters.
To everyone's surprise they discovered all sort of unsuspected features on the
bottom.
their findings included submerged forests ( fossilized ) river valleys, and
tall cliffs not to mention water falls, caves and plateaus and canyons.
None of this had been suspected form previous traditional surveys.
Vector charts like c-Map and Navionics are the kind that connect the dots or
point depth soundings to generate shore lines and depth contours.
the mathematical algorithm is based on an assumption that the gradient from
point to point is even and consistent.
As you change scale, the selected points used to recalculate the drawn
display changes.
This is why fine details only emerge under maximum magnification.
C-Map relies on the validity of the data they bought from the hydrographic
departments of each country.
Unfortunately some area have not been charted in recent times. The data was
accurate and relevant at the time of the last survey.
That data is the best the government can provide but it may not be accurate
data!
I am not saying you cannot trust charts at all but I am saying that with GPS
radar and even forward scanning sounders many recreational skippers tend to
go boldly forth where no boat has sailed before.
To do so safely requires a fair bit of discipline and a great deal of safety
procedure - . . . either that or a fat wallet to pay for bent props and keel
dings. < VBG>
Cheers
Arild
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