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Go Back   SailNet Community > On Board > Gear & Maintenance > gps vs chartplotter
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Thread: gps vs chartplotter Reply to Thread
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Topic Review (Newest First)
05-18-2007 07:09 AM
PBzeer I find that my primary use of GPS w/software is when navigating shoal areas. To this point, it has pinpointed me very accurately for remaining within a channel or cut. It also gives me a look ahead at my course, so that when the eye says you're on a good line, it backs that up, or, shows your ranges were a bit off.

I also like that it keeps a track of where I've been, and of the mileage I've gone.

Yes, electronics can go down, but as long as I am near enough to land to see the shore, I have no difficulty, figuring out where I am. Another part of that, is understanding markers and their numbers, and even the orientation of the markers as you pass them.

Even if electronics were bulletproof, I would still carry paper charts.

Currently at Dinner Key Marina, Florida
05-18-2007 07:08 AM
RickBowman
Quote:
Originally Posted by hellosailor
I think I see what tubsmacker is getting at with stick-on compass roses, though. Instead of fumbling around and trying to plot lat/lon intersections on charts, all you need to do is find the compass rose, and measure off the same distance/bearing that your GPS is showing to it. One motion with a ruler, or one motion with a good eye, and you've got the distance and direction quickly. The downside is sticking on compass roses, but isn't there always a downside? I'm not about to run out and buy (or print up) a pack, but I can see the point.
HS,
I also can see no useful purpose for a stick on "rosa dos ventos". The course should be ploted along with the navigational labeling written on the course line; heading, speed, distance before shoving off. If a GPS/Plotter is aboard then they also should be set up with the course information which has caught many of my charting errors before the trip began.
I leave the cavalier attitude towards accepted navigational standards or piloting to another skipper to practice with.
05-18-2007 06:48 AM
RickBowman
Quote:
Originally Posted by Tubsmacker
Rick, It was the time taken to plot a lat long whilst on a night sail in a busy shipping area. We were using pilotage not DR as we were in the Solent UK and some people were using a lat long to check position.
Tubs,
Sounds like there were different levels of navigational experience of the students aboard the school vessel? It sounds like a great oppurtunity for sailors to get real world experience. I have read about one fantastic sailing school in England that has an inside pool capable of making storm conditions for students to practice in life rafts. I found the info about it in Practical Boat Owner magazine, one of my favorite.
05-18-2007 06:30 AM
RickBowman
Quote:
Originally Posted by sailingdog
Rick-

BTW, it is WAAS...not WASS...

One thing that seems to be missing is that all chartplotters are GPS units, with some additional graphical features. Not all GPS units are capable of displaying graphical chart or map data.

Also, I would emphasize that the GPS has to be setup to match the Chart Datum, or it will be wrong—and often wrong enough that you could lose your boat to the errors involved.
Certainly you are right on with your observations about the GPS and WAAS. Thanks for adding to the list of features.

On a lighter note, hav you herd of the Venicean Festival n Charlevoix? Do u sale in those watters?
05-18-2007 06:14 AM
RickBowman
Quote:
Originally Posted by Cruisingdad
Sailing offshore without paper backups and 30 minute PAPER plots verges on poor seamanship to me.

- CD
CD,
Would you agree that this also applies to nearshore or coastal cruising as well? It's comforting to know exactly where the heck our boat is, and by keeping an accurate log will help at least show where we have been. Aren't most Catalina owners nearshore sailors?
05-18-2007 02:44 AM
sailaway21 Tubsmacker brings up a good point in his story of a Solent passage. This passage is considered "piloting". Piloting involves the use, primarily, of visual aids to navigation, on land and a sea. The most commonly derived LOP is the visual bearing. The most important tool is the eye. Radar and soundings are also useful, if equipped. The use of GPS, Loran, celestial, or other offshore navigational devices in these situations is a dubious proposition. "Piloting" implies that there is a visual observation of position and the navigation of the vessel is conducted based upon it. The position may not even be plotted on the chart due to the exigencies of the situation. Radar is of such great use in such situations as it, more than any other navigational aid, mimicks the eyeball. As Tubsmacker intimates, a quick bearing and distance can be put down on the chart in a flash; plotting Lat/Long takes longer. Visual bearings, especially the use of ranges, are inherently better than any electronic aid. We're not as concerned with the fact that the lighthouse at Steppingstone is 0.1nm off in lat/long from it's charted position as we are with the fact that a million seamen have safely navigated it's waters by lining up on it and sailing a course leaving it a mile to port. If one wishes to glance at the GPS, in a piloting situation, for fun that's, of course, fine but there are usually more important navigational chores to be performed. A good fathometer will do far more to keep you off the hard stuff than a GPS in such situations. Tubsmacker's story illustrates two good points. A lot of sailors are not too proficient with the dividers, parallel rules, or triangles and their use with a paper chart. And secondly, they're in the chartroom when all the action is taking place between and beyond the anchors.(g)

After watching "The Hunt for Red October" perhaps one too many times, the novice navigator may be inclined to think that it is perfectly appropriate to navigate utilizing computer derived navigational information. As with all computer based systems-with a pencil and paper you can make an error but, to make a really big error you need a computer. The reason that piloting is done visually is that the human brain intuitively distills what the eye is observing without conscious thought. The GPS navigator pops out of the chartroom to report the vessel is setting down to port and the pilot replies, "yeah, I changed course for it a few minutes ago".

NOAA's assertion that electronic charts are the greatest advance since radar is rather bold, and probably hyperbole. Radio updates of Notice to Mariners is the primary advantage and it's not too big a chore to update those on a paper chart. As one who has had responsibility for maintaining a world wide chart catalog I can say that most corrections to the chart are not overly significant. Of course, one must go through all of the corrections, each and every time, to ascertain this fact. But, the point remains, it ain't like somebody up and moved Bishop Rock. For those reasons, I view radar and the fathometer as the two greatest electronic navigational tools yet invented. One could reasonably, if a bit haphazardly, circle the globe and various ports without charts if equipped with those two items. I am not advocating that, but merely illustrating how those two devices produce the most valuable information the pilot needs.

The weakness to NOAA's claim, as well as the use of chartplotters, to the small boat sailor is that it removes the human element from the navigational process. If you employ the humble paper chart, and religiously correct it via the NTM, you are subconsciously memorizing the chart. Every correction that is made to the chart will have been processed by the navigator's mind. As example, you receive a new edition of your old harbor chart. Can you espy each of the changes between it and the old chart? Of course not. And you need to sit down and examine it to ensure that nothing of great importance has changed since you last transitted it's waters. The automatic update of an electronic chart makes it more likely that you will not notice that buoy #10 has been repositioned or eliminated, until you go to make your normal course alteration off of what you thought was buoy #10. The potential for losing power, regardless of size of vessel, ensures that paper charts will be with the prudent navigator for many, many years to come. It is facile to say that computers do not make mistakes, only humans do. Where the computer and the human interface is the possible source of error and all computer derived solutions henceforth will reflect that error, often without the knowledge of the human. "Radar assisted collisions" are not necessarily a thing of the past. Computer assisted groundings are a thing of the present. In both cases, were the vessels not equipped with either device the navigator would have operated the vessel in a much different manner, the manner erring on the side of safety.

All navigational devices have their place, the great danger is in uneducated reliance upon them. The cruel irony is that the user does not know he or she is undeucated until disaster strikes. If you've got good water under the keel, and are a decent distance offshore, you may be lost, but you're pretty well safe. If you're motoring up a rock strewn channel in fog on GPS alone, you may know where you are, but you're probably not very safe.

For my fellow Luddites who utilize certain paper charts, and the same DR tracks on those charts, I have a suggestion. Scotch makes a tape that can be placed over your course plotted, usually two strips to either side works well. The tape is not removable but it can be written on nicely with a number 2 pencil and the pencil marks are erasible. It allows you to extend the life of the paper chart on that 1% of it that you are normally using. Most mates lay out their DR track in pencil, have it double checked, and then ink it in before applying the tape. The inexperienced mate decides that red ink would be the perfect color for his permanent DR track, only to discover that he can not see it under his red chart table lamp! And he has put the tape down before the realization.(g) Use black ink.
05-18-2007 12:06 AM
hellosailor "In some parts of the world, neither paper nor electronic charts are sufficient. "
Heck, that even applies in reasonably well-known and populated parts of the world like NYC. The official NYC cartography goes back to pre-colonial Dutch maps of New Amsterdam, and I have no idea what that datum is called. But it is significantly off (1/4 mile and more) from the NAD and WGS datums with the sometimes infuriating result that land navigation packages are almost all off by 1/4 mile in NYC when they try to shift from pure GPS to street addressing (which comes form different tables) or vice versa. Google Maps and most car nav systems license from NavTeq, who feel the error is not worth correcting. (Other maps that use the free US Census "TIGER" map base suffer form the same problem, actually worse because the USC has updated only one of the four counties in NYC, leaving a split in their datum, which was never intended for navigational use anyhow.)

Even the USGS topo maps, which in theory would match the marine charts, mqanage to mislocate large structures on Jones Beach (Fire Island, LI, NY) by 1/3 mile if you miss the datum shifts--which sometimes do change. Maptech's online map server gives you no indication that the maps it is seamlessly shifting among, use different datums. Don't indicate datums on them at all.

So...cheap maps, GPSes and chartplotters, still don't make John Doe a naviguesser. Anywhere.
05-17-2007 08:28 PM
EscapadeCaliber40LRC I agree, redundency in depth, all technology, but eyeballs for the last quarter mile. I like a chartplotter. I don't trust those "blue screens of death" Microsoft devices though but that's a personal quirk of mine.

With a chartplotter, besides the chart you get way point and route planning, tidal charts for lots of places, a great anchor drag monitor, and a good night light. And it is great for backtracking once into a place like Luperon.

I use a chartplotter at the helm, I have a spare unit, in case the first one craps out and I keep a good set of paper charts close at hand, plus a few other GPS tools including some nav software on my laptop just to have complete redundancy and immediate alternatives. I have a box of clear plastic garbage bags to protect the helm mounted chart plotter and I remove that plotter at the end of the sailing day and relocate it to a parallel hookup at the nav station for anchor watch/nightlight duty at night. The handheld gps is in my back pack which also serves as a ditch bag.
So for me it is eyeballs first, chartplotter second and paper last although I use all these tools interchangably throughout the trip.
05-17-2007 07:47 PM
camaraderie EC40...had similar experience there...but to be fair the mapping there is way out of date and cannot be relied on...unlike the rest of the Caribe.
I imagine someday I will get rid of paper (when I can have 3 battery operated chartplotters that cost the same as my GPS today!)... but for now...eyeballs first...paper next and chartplotter last.
05-17-2007 07:11 PM
EscapadeCaliber40LRC I agree, just don't trust them. They are wrong. At the Eastern end of Hispaniola they are over a mile off, back at Luperon, you just end up sailing across a fair bit of land on the way in. Eyeball navigation is the only reliable tool. There's a good range guide in Van Zant's book, you just need to figure out where the turn is otherwise you'll find the mud on one side or hit coral on the other. The 84 Datum is off whether it is in electronic or paper form.
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