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Old 29-07-2008, 13:54   #1
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Paper Chart Question

Hello All:

I learned costal navigation pre Gps when Loran was jsut getting started. I plan to bring our boat down to Mexico this year and have heard that the GPS and the paper charts don't necessarily work well together. My question is can you trust the paper charts? If I triangulate a position with a hand bearing compass am I more or less accurate in the boats locaton as opposed to the Lat/Long?
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Old 29-07-2008, 14:05   #2
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Hi Charlie,
So long as your GPS is using WGS-84 as its datum, the plotted position on the electronic plotter and the plotted position on the paper chart should be the same.

Keep in mind that some charts are made previous to WGS-84 and survey grade GPS systems. But for US waters using NOAA charts that is not a concern. I don't know about the Mexican charts. I would find out what datum they used, their age and how the surveys were done. If the Mexican chart datum is different, you will need to change your GPS datum to the charts datum and then hope for the best.

Depending on how close you are to the shore, a hand bearing compass can give you a decent fix, but not down to a few meters like a GPS can do for you. I would do both methods of getting a fix along with radar range fixes to see what works best for you. Cross reference the three methods on one chart at close to the same time and advance your LOP's for each method to common time. Eventually you will get a feel for what methods are giving you good info and when they give you good info as well as what gives you not so good info.

I especially like taking radar ranges because they give you something that is pretty certain. Questionable chart datum, old surveys etc can't really screw you up. You either plotted the right distance or you didn't.
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Old 29-07-2008, 14:06   #3
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I would not expect your GPS location to be less accurate than the triangulated hand bearing compass location. The accuracy issues with GPS is often the GPS is more accurate then the map. Depends on the exact map and the exact location on the map.

All maps have some error in some places and the error comes from the fact maps are made from many sources of different accuracy. In order to register all the data to one map base you need to fudge everything a little bit.

Good news! Navigation didn't change when they invented GPS. Don't forget anything you already know and Loran has had a stay of execution for a period of time.

The issue of working with GPS and a paper map comes mostly how the units are displayed on the map and the chart. Not all GPS devices allow you to configure the values. You may have to work with decimal, fractions of 1 minute. It can get confusing and of course confusion is not anything you want. Unlike carpentry this would a think twice and measure once operation.
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Old 29-07-2008, 14:09   #4
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Should be the same, but not always! Here in the eastern Caribbean, some paper charts and the electronic chartplotter charts that I use frequently are in conflict with reality. I've learned to trust my eyeball, and view charts as "planning tools" only.

Of course, the charts in Mexico may be perfectly fine, but I'd still be very cautious until I knew that for sure.
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Old 29-07-2008, 14:21   #5
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GPS will give you a very accurate position on the globe, which will not be reflected on most paper charts. The paper charts, mostly plotted centuries ago, are seldom that accurate.
I wouldn't worry about it though, the prudent navigator works to a scale of miles, and then "eyeballs" to within feet.
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Old 29-07-2008, 14:37   #6
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GPS will give you a very accurate position on the globe, which will not be reflected on most paper charts. The paper charts, mostly plotted centuries ago, are seldom that accurate.
I wouldn't worry about it though, the prudent navigator works to a scale of miles, and then "eyeballs" to within feet.
Exactly.

Also, part of that "prudence" is not expecting anything to be accurate in between large sets of rocks, or very narrow areas. Just stay in known deep water until it's time to anchor and then pick your way in carefully and via the most depth, not the shortest route.

It's important to err on the side of caution and watch all the inputs you have (chart, gps, depth sounder, eyeballs, markers, etc...) The charts and GPS will be just as accurate (if they are WGS-84 datum) as any electronic chart plotter would be.

If they are not WGS-84, they are a rough guide and you have to take that into consideration when navigating.
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Old 29-07-2008, 14:37   #7
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Charlie,

We have been in west coast Mexican waters now for over 3 years.

If I plot my GPS position on a chart, it MAY or MAY NOT be true to the land. That is to say that in some areas, the charts are very accurate, in others, well, let's just say no so!

We had friends watching our “Where are we” plot (GPS) on our WEB site that is generated when we send an email. When we got to Cabo, we had an email from a friend asking us why we went aground on Cedros Island! On our chartplotter, (and the paper chart if I plotted it) showed us about one half mile on shore!

We have found that most of the harbor charts for Mexico (like Cabo, La Paz and others) are very accurate, other charts are NOT. In the Sea of Cortez, we measured one chart to be off by 0.8 miles.

We helped in a rescue a couple of years back. We were on the radio and relayed the GPS location of the boat in trouble. It went to the Coast Guard. They had to contact the Mexican Navy to decode the true position.

Keep you eyes open and use the charts to get you within a mile or 2 from you destination, then switch from geo-nav to tactical-nav. Using the radar helps.

As was said by Pblais, “he GPS is more accurate then the map”.

Greg
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Old 29-07-2008, 15:04   #8
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...The charts and GPS will be just as accurate (if they are WGS-84 datum) as any electronic chart plotter would be...
Sully,

I've found that to be true (almost) all of the time in the U.S., but not here in the eastern Caribbean.

My GPS and my chartplotter software are both set to WGS-84. But when I motored past the red daymarker at the entrance to the channel into Jolly Harbour, Antigua, a month ago the chartplotter showed the marker to be about 1,000' away from where it really was. The charts, paper or electronic, are only as good as the surveys upon which they were based.
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Old 29-07-2008, 15:15   #9
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Sully,

I've found that to be true (almost) all of the time in the U.S., but not here in the eastern Caribbean.

My GPS and my chartplotter software are both set to WGS-84. But when I motored past the red daymarker at the entrance to the channel into Jolly Harbour, Antigua, a month ago the chartplotter showed the marker to be about 1,000' away from where it really was. The charts, paper or electronic, are only as good as the surveys upon which they were based.
Yup... that is what the quote you snipped said (or was supposed to say)...

The gps and paper chart are just as accurate as the chartplotter by itself. Both may be highly inaccurate at times, due to charting errors, but they are not to be treated as different entities if both run WGS-84 datum.

And yes... I've had plenty of times (even in the States) where the charts were wrong. Certainly working megayachts in the Caribbean, I saw even more cases where the chart was wrong.

The worst was one day in NY Harbor when some kind of thing was interfering with my GPS. It was only interfering with the GPS linked to my navigation software, so it came back to navigation by eye and hand bearing compass. It was like that all the way from the Verazano Bridge to Riker's Island... then the GPS came back. That was a GPS failure though, and not a charting error.

The point of my little snippet there was that the GPS & paper charts together are 100% equivalent to a chartplotter, which is just the GPS&Charts in one package.

Sorry for any confusion.
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Old 29-07-2008, 16:48   #10
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Charlie,


We have been in west coast Mexican waters now for over 3 years.



If I plot my GPS position on a chart, it MAY or MAY NOT be true to the land. That is to say that in some areas, the charts are very accurate, in others, well, let's just say no so!



We had friends watching our “Where are we” plot (GPS) on our WEB site that is generated when we send an email. When we got to Cabo, we had an email from a friend asking us why we went aground on Cedros Island! On our chartplotter, (and the paper chart if I plotted it) showed us about one half mile on shore!



We have found that most of the harbor charts for Mexico (like Cabo, La Paz and others) are very accurate, other charts are NOT. In the Sea of Cortez, we measured one chart to be off by 0.8 miles.



We helped in a rescue a couple of years back. We were on the radio and relayed the GPS location of the boat in trouble. It went to the Coast Guard. They had to contact the Mexican Navy to decode the true position.



Keep you eyes open and use the charts to get you within a mile or 2 from you destination, then switch from geo-nav to tactical-nav. Using the radar helps.



As was said by Pblais, “he GPS is more accurate then the map”.



Greg
Thanks Greg. That is an interesting tidbit about having to contact the Mexican Navy to find out where you were.



I agree with what people say about the GPS being accurate but the question was more site specific to the West Coast of Mexico. I couldn't get my mind around how accurate the charts would be in Mexico. Some of them are from 1800's surveys. I wouldn't trust the GPS and the 1800's charts to play well together. Knowing where you are foe Lat and long is good but only if the land mass on your chart is 1) using the same datum, 2) accurate. I have used the chartplotter and had my boat on dry land with the plotter but mid channel in the real world.

I was just wondering how an 1800's chart was inaccurate. I can understand where the lat/long don't line up but what about the range and bearings.I don't have a problem using the depth sounder and the range bearing and am OK with the radar but it would be nice to quantify where the problem with the chart lies.
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Old 29-07-2008, 17:38   #11
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...
I wouldn't trust the GPS and the 1800's charts to play well together.
...
The point is that using a GPS with 1800's charts is exactly the same as using a sextant with 1800's charts. Except the GPS will give you a more accurate position on the globe. But the GPS will not fix the errors on the chart. The errors are on the charts, not the GPS or sextant.

As suggested above, use all available methods and after a while you'll get a feel for what works best for you.

-dan
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Old 29-07-2008, 17:40   #12
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So long as your GPS is using WGS-84 as its datum, the plotted position on the electronic plotter and the plotted position on the paper chart should be the same.
That is not quite true. Many charts based on older surveys are not referenced to WGS-84 datum. For example, North American Datum 1927 (NAD-27) used to be common in the Caribbean. You need to check the chart legend for the correct datum, and then set your GPS accordingly. Most units have the capability to display coordinates in many different systems. If you use NOAA or NGA charts, then WGS-84 is the correct datum.
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Old 29-07-2008, 21:21   #13
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Newer printings of charts using datums other than WGS 84 or NAD 83, will ofter have a correction listed that must be applied to the GPS (WGS84) lat/lon before plotting it on the chart.

Not relying solely on GPS goes without saying - using visual or radar fixes when they're available is just good navigational practice.

To Hud3's comment on the daymark - not necessarily a problem with the survey; sometimes they move nav-marks. How do you apply chart corrections on a chartplotter?

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Old 30-07-2008, 00:47   #14
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Peter Prowant covers the discrepancies between charted positions (paper or electronic) and “actual” positions (as determined by GPS) for Bluewater Charts at:
GPS is Great, But...
https://www.bluewaterweb.com/Newsletters/9-01gps.asp
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Old 30-07-2008, 02:33   #15
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In my learning reading I noted that GPS charts are scanned in from the 'best' available paper charts. They might not be the ones you've got and your GPS doesn't update with chart and navigation changes / corrections.
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