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Old 05-01-2009, 14:41   #1
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Paper Charts and GPS

I am wondering about using paper charts with a GPS. I learned to Navigate with costal navigation and like paper charts. I have a plotter and like that as well. Problem is on some paper charts there is no datum reference. So lets say that you get within five miles of a location, using a radar range. You know that there is a rock that you must avoid at 3 miles. There is a lighthouse and an obvious point that you can get a bearing on with a hand bearing compass(HBC). Your chart plotter shows you at three miles. Then you plot your position on the chart using the GPS and it shows you in the same place as the plotter. Then you take out the HBC and find your position. It shows you at 5 miles. Now you have two places where you might be. First thing that I would do is stop and head back on a reciprocal course.

My question:

Are the paper charts going to give you a more accurate position with the HBC?

My guess is that the paper charts are going to give you a better idea of your location b/c you are using matching technologies. The HBC and the paper charts.

Using the lat and long of the GPS will give you an accurate location of where you are in the world but not where you are in relation to the chart. Am I right? Has anyone had real world experience with this?
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Old 05-01-2009, 15:15   #2
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My thoughts have changed from "Let those that want to use paper charts use them"
Now it has changed to no one should use paper charts as they are not accurate or easy to use. Paper charts and their use are less safe than a plotter.

Remember, thats just my personal opinion.

Do you think papaer and gps will be more accurate? Safer etc? than chartplotter with electronic charts?




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PS I am not trying to be devisive etc here, jus askin
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Old 05-01-2009, 15:16   #3
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GPS is accurate within 50 meters and usually much lower. You can be almost assured its not going to be over a hundred meters. You have to be down to three or four satellites to get a really bad fix. Right now there are at least 30 GPS satellites in orbit that I know of. So if you take roughly 40% of that, you are left with plenty of satellites in the visible sky, not too high and not to low for your GPS unit to choose from. Gee, this works out to about 12 satellites which coincidentally is what the most satellites that your typical GPS receiver can receive. Yes, GPS does indeed give some really wacked positions on rare occasion which puts your boat 3.2 miles inland from a few seconds to a few minutes. I call them system hiccups. But its usually real obvious when this is happening. Ok, that's resolved.

As far as charts go, their accuracy can be anywhere from extremely accurate to all over the map...pardon the pun. The two greatest variables are the chart geodetic systems and the accuracy of the survey team that created the data for the chart.

Most GPS units can be set to whatever geodetic system you want. Whether it is WGS84, NAD83, NAD27, ED50 or any of the other dozens of geodetic systems out there. Of course the problem here is you must know what datum the chart is done in. Without this information, the chart is pretty much worthless if you dont do an offset. It could still be a very inaccurate chart or a very good chart even if you do know the geodetic system it was done in.

The other factor is the survey team that did the chart. The older the survey date and the less accurate the survey team, the less accurate the chart is going to be. Most first world countries have pretty accurate surveys. Places like Mexico, not as good.

Its a real art figuring out how much weight you should place on the accuracy of a chart. Probably the best thing to do when you get a questionable chart is to compare the lat and long of a clearly defined object on that chart to the lat and long of what your GPS says that position is. This is your offset. An offset is measured in bearing and distance or it can be measured in latitude difference and longitude difference. I find lat and long difference easier. Imagine laying one chart over the other with their land masses lined up exactly one over the other...the difference in the lat and long between each chart is the offset. Make an adjustment for the offset but also keep in mind that if the survey team may not have been very accurate and other points on that same chart could be different with respect to the distances and bearings to each other.

Navigation will always be both an art and a science. The art is in how much you weigh your information. You weigh heavier navigational data that is more likely to be accurate and less so for those things that are likely to be less accurate.

In response to what MarkJ has to say. Your plotter is only as accurate as the survey that was done to create the electronic chart. The same goes for paper charts. Its also true that most electronic charts are done with pretty accurate surveys since we had GPS before we had electronic charts made by GPS surveys. So yeah, electronic charts are more likely to be accurate, but if they were created from old pre-GPS charts done by pre-GPS surveys then the accuracy of a new electronic chart made from an old survey is not necessarily accurate..or at least not good enough to stay off the rocks. So again, place the weight on what you think is the most accurate navigational information.

I think its always a good idea when offshore to plot your position to paper occasionally in case your electronics crap out. At least you can start a somewhat accurate DR. It beats guestimating where you were when Mr Plotter decided to quit.

Hows it going out there Charlie? PM me.
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Old 05-01-2009, 16:08   #4
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Repeat after me... "We are all a battery failure away from the Age of Sail". Take a lightning hit and the boat goes dark, those paper charts are going to look very, very attractive. (True story: ) Walking up Thames St. in Newport, RI one summer evening, a couple asked us where Armchair Sailor (nautical bookstore) was and if it was still open. It seems their laptop and printer had died, they had no paper charts, and had to get back to New Jersey ASAP. Armchair Sailor was closed. Point made, I trust.

Understand we have two GPS receivers running on the boat and either can drive a latptop with a chart plotter program (Coastal Explorer). Still, we have paper charts on board and once out of home waters, we at least mark our location on the paper chart, updated about every 30 minutes (and often leave nav notes, courses, etc., too).

Finally, don't get trapped by "if it's electronic, it must be really right". "Garbage in, garbage out" still obtains as much as ever.
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Old 05-01-2009, 16:09   #5
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Quote:
using paper charts with a GPS
Second, I usually do the same thing - have not discovered
any supernatural preferences of the plotter
spend a lot for good BA charts, though

RBEmerson, I hear what you say

last summer had a plotter behaving so strange (had an error of +/-
1 NM), that turned it off for the whole voyage
Managed very well with a small TomTom GPS and aforesaid charts.
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Old 05-01-2009, 16:31   #6
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Electronic are great for planning, but there is just something about a paper chart that gives me a better perpective of the whole situation.... and in a tight situation I can have it out at the helm, sensing the wind, looking for water boiling off a rock or reef.....
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Old 05-01-2009, 16:43   #7
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agreed....I use all my sensors... If I take a fix using a HBC from two charted points on land with a reasonable angle, the next thing I do is check my depth sounder to see if it corresponds to my reading. No depth finder?...break out the lead line..I guess.
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Old 05-01-2009, 17:00   #8
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Originally Posted by Tempest245 View Post
agreed....I use all my sensors... If I take a fix using a HBC from two charted points on land with a reasonable angle, the next thing I do is check my depth sounder to see if it corresponds to my reading. No depth finder?...break out the lead line..I guess.
I agree. I am getting slack where I take out the paper chart and just do a rough idea if things look right I say OK. If they don't I stop the boat and look at it more closely.
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Old 05-01-2009, 19:09   #9
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A perfectly accurate GPS is worthless when compared to a very inaccurate chart. for the most part you really don't care where you are in terms of lat. long. . What you care about is where you are on the chart. When the chart has a high degree of error your compass and GPS are not a reliable frame of reference.

At that point you need to use piloting skills that are based on verifying what you see in the real world against the chart. You might assume that the relative accuracy of the chart is at least close but it may be positionally off by several miles. Resolving issues with map projections is very critical since the projection in error is worse than the chart error in many cases. If you are using US charts in the US this won't really be an issue. US Charts are all WGS 1984 projection and are generally very accurate. The exception is NAV Aid locations.They get moved around just a little bit and new locations are reflected in the notice to mariners. Other parts of the world may be very different.
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Old 05-01-2009, 19:59   #10
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i use paper charts, hand held gps, ships compass.

and a handbearing compass where needed.

i think the navy teaches it's officers that a good navigator uses all possible sources of information to ascertain his position. of course the navy has the unlimited resources of the taxpayer to equip their ships with all those 'sources'.

i can't see where electronic charts are any better than paper charts - i think they're both derived from the same source material. traveling on the icw a chart and binoculars are pretty much all you need. coastal cruising i use paper charts, compass, handbearing compass, and the depth sounder.

offshore and in the bahamas i use a battery powered handheld gps, paper charts, ships compass, and occasionally a hand bearing compass. i also have the depth sounder turned on. the paper charts, by the way, are the 'explorer' charts by monty and sara lewis - in my humble opinion the best charts of the bahamas that money can buy.

my only incursion into electronic charting is laptop based - the trial version of 'coastal explorer' and the free charts from noaa. i've occasionally printed out some charts from this setup and used them in the cockpit.
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Old 06-01-2009, 03:48   #11
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I don't get it. the gps gives you lat long. The chart is marked with lat long.

I have never seen lat long be wrong on a gps or a chart. You should be able to pinpoint your position on the paper chart using the gps.

The GPS datum is irrelevant until you are using the GPS chart. Then the datum must match.

Being able to shoot landmarks with a compass and triangulating a position is pretty easy and is a good skill to have for coastal cruising.

Being able to shoot the stars to determine lat long is a difficult skill that many think is still required in order to get a fix when at sea. I don't.
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Old 06-01-2009, 05:10   #12
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I don't get it. the gps gives you lat long. The chart is marked with lat long.

I have never seen lat long be wrong on a gps or a chart. You should be able to pinpoint your position on the paper chart using the gps...
You're right - you don't get it.

In the Bahamas and Caribbean, some of the paper charts are extremely old and contain errors of 500 feet or more.
In other areas of the world, the errors may be several miles.
Sometimes charts are inconsistent with each other, and the same latitude-longitude plot may locate your boat in two different locations on two overlapping paper charts.

See “Using Nautical Charts with Global Positioning System”
Using Nautical Charts with Global Positioning System

See also pages 137 - 139, in "GPS for Mariners" ~ by Robert J. Sweet
GPS for Mariners - Google Book Search

And
Navigating With GPS, Charts, and Eyeballs
Navigating With GPS, Charts, and Eyeballs
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Old 06-01-2009, 05:17   #13
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ive experienced a bad signal from the satalites,i estimated that i was about 80 meters closer to my objective than the gps/plotter indicated.My buddy who was on the helm was an electronics technician,chose to believe the plotter despite those tell tale signs of turbulent water indicating shallow depth,i honestly think that he would have driven us onto the rocks.

I occasionally check the indicated plotter position with a hand held compass,99% of the time it is deadly accurate

I use a combination of paper charts(UK Admiralty) and a Gps/plotter (Navaman),i also have a handheld GPS which i use as back up

Another very usefull tool for Navigation is Google Earth,i find it usefull if you are entering a harbour for the first time,i combine it with Nautical almanacs to give me an as clear as possible picture in my head of the approach,once again i preferr to have more than one type of system
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Old 06-01-2009, 06:58   #14
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Quote:
Repeat after me... "We are all a battery failure away from the Age of Sail". Take a lightning hit and the boat goes dark, those paper charts are going to look very, very attractive.
I keep my charts in a sealed W/P bag safe in a locker . . . "Just in case"!

I'm told there is a 1,000,000:1 chance or less in being struck by lightning just once in your lifetime!

I'm a gadget freek . . . . I'd rather have two (or 3) GPS's than a stack of charts.
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Old 06-01-2009, 09:12   #15
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+1 on charting error. Although it depends on where you are. But, yes, some charts may have the shape of things right but have the basic position off by literally miles. In general, the further one gets from commercial shipping, and the further one gets into (for lack of a better term) the third world, the greater the chance that the charts may be wrong. OTOH, in the parts of the Caribbean I've visited or chartered in, the charts have proved to be pretty close to right.

There is, however, another source of error. For anyone who sails in the Chesapeake, for example, it will prove instructional to look at the little insert showing when areas on the chart were last surveyed. Some soundings are now literally twenty to sixty years old and parts of the Chesapeake's soundings date back to the 19th century (1800's). Meanwhile, in living memory, large islands have "sunk' (notably, Sharps Island at the mount of the Choptank, Coaches, Poplar and other areas at the entrance to Eastern Bay) due to storms and erosion. Even small islands (e.g., Grog Island, north of the Rappahannock) have vanished in only a span of five to ten years, and the spit south of Milford Haven and Gwyn Island is disappearing. The frequency of surveys has simply been overtaken by the course of natural change.

The point is, trusting a GPS alone, or any one source of navigation information, is, at the best, unwise. But saying "them voodoo boxes is no damn good" or "paper is so outdated it can't be trusted" is unwiser still. The smart navigator uses all the information available and decides, from that information, how best to proceed.
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