Paper vs. Electronic Charts - what's really the difference?

[LJH]

Originally Posted by goboatingnow

I don't understand this, you can plot on paper and I can input a waypoint faster. I plot on paper too but I don't see what you mean specifically here.

Dave

Originally Posted by nigel1

I think this is referring to a position plotted on a paper chart using bearings and or ranges, establishes a position relative to the land mass or other object, rather than to a chart co-ordinate, and that its quicker to do this on a paper chart than on an electronic one.
Although, with some practice, it is just as quick to perform the same function on an ENC, I'll go as far to say, even quicker if you make your self familiar with the equipment

I was referring to the case that your GPS position shows up as on land on a chart. ie an error with the chart. I agree with Nigel that the bearings can be done quickly electronically he as shown, but I keep my laptop at the nav station and when short handed I will use paper in the cockpit before running inside to the computer.

[jeremiason]

I think electronic charts have there place in life... I use Navionics on a Raymarine E-120.

I have found that electronic charts are generally very accurate in places there is commercial traffic, ports or lots of recrteational traffic. Where they lose their accuracy is in remote areas or areas that have no commerical traffic or ports.

Sailing in the of the Sea of Cortez, Mexico, Pacific Central America and now in the the Caribbean islands of San Blas, Panama, I have found many of the small islands and reefs are off by as much as 1/4 mile.

One thing I do when using electronic charts in remote areas is overlay the radar on top of the chart. This gives you an idea of how accurate the chart is in relation to the real world.

I still carry large scale paper charts or Yatchsman Chart books for the areas I sail. I also usually have a couple of cruising guide books to help.

In the event I have a power failure, I can still plot witht the paper charts or more likely switch to one of the two hand held GPS units I have.

The bottom line is nothing is perfect... But in my humble opinion, used with caution, electronic charts are a great tool and I wouldn't sail without them.

[Greggegner]

Originally Posted by jeremiason

Sailing in the of the Sea of Cortez, Mexico, Pacific Central America and now in the the Caribbean islands of San Blas, Panama, I have found many of the small islands and reefs are off by as much as 1/4 mile.

One thing I do when using electronic charts in remote areas is overlay the radar on top of the chart. This gives you an idea of how accurate the chart is in relation to the real world.

I agree, a paper chart that is inaccurate will get you in more trouble than that same chart digitized where you can add radar and make quick measurements from the chart plotter. Inaccurate digital charts are made from the paper charts. Paper which I like only gives a back up to electronic issues and not a more accurate way to figure out where you are or where land is.

[Kettlewell]

There are differences in charts of the same area, even in populated and well-charted regions. I recently looked at several different charts of an area I know extremely well and some of the non-NOAA charts had errors on them. In one case, a fairly major underwater obstruction didn't show up and in a place where I have seen several boats hit the darn thing. I found these errors in just a few moments of casual observation--I wasn't really looking for problems. Panbo also documented some of the problems with various electronic charts in Camden harbor (Maine). That's not to say that NOAA paper charts or the raster versions of them are error free--far from it. In fact, I have and use some old and very out of date charts in certain areas that are far better than the current versions in terms of showing the physical characteristics even though the buoyage and such is wrong. The bottom line is you have to use all charts in a safe manner--don't assume what you see is what you'll get.

[Dockhead]

I wrote this above: "Details -- including dangerous obstacles -- disappear at larger scales in electronic charts."

And someone wrote to me a PM, pointing out that I have confused large scale and small scale. "Large scale", apparently is the opposite of what I thought -- it means a more detailed chart, the "large" meaning the large denominator in the scale, e.g. 1/50000.

I had not known that! Thanks, David.

[jackdale]

Originally Posted by Dockhead

I wrote this above: "Details -- including dangerous obstacles -- disappear at larger scales in electronic charts."

And someone wrote to me a PM, pointing out that I have confused large scale and small scale. "Large scale", apparently is the opposite of what I thought -- it means a more detailed chart, the "large" meaning the large denominator in the scale, e.g. 1/50000.

I had not known that! Thanks, David.

A memory aid "Large scale = lots of detail."

1/10,000 is a larger number than 1/500,000.

Charts are also classified on the basis of scale

There are several classifications of nautical charts, done at different scales, appropriate for different types of travel. Harbor charts (1:50,000 scale or larger) are intended for navigation and anchorage in harbors and small waterways. Coastal charts (1:50,000 to 1:150,000 in scale) are for navigation inside offshore reefs and shoals, for entering larger bays and harbors, and for certain inland waterways. General nautical charts (1:150,000 to 1:600,000 in scale) are for use when the course is well offshore and positions are being determined by landmarks, lights, buoys, and characteristic surroundings. Sailing charts (smaller scale than 1:600,000, ie less detailed) are plotting charts for offshore sailing and approaching the coast from the open ocean.

[cal40john]

Originally Posted by Dockhead

I wrote this above: "Details -- including dangerous obstacles -- disappear at larger scales in electronic charts."

And someone wrote to me a PM, pointing out that I have confused large scale and small scale. "Large scale", apparently is the opposite of what I thought -- it means a more detailed chart, the "large" meaning the large denominator in the scale, e.g. 1/50000.

I had not known that! Thanks, David.

At least for my Garmin Bluecharts I do have some information disappear at larger scale (smaller area, zoomed in). I haven't figured out rhyme or reason and I have played with the detail level menu as well, that has some interesting effects that don't always make sense to me either.

[Thonord]

Originally Posted by noelex 77

Charting errors are very common, but if the GPS is "getting lost" something is wrong.

I beg to differ on the first part. Charting errors occur, but are not common.

On the second I agree, but its usually not the GPS' fault.

GPS error happen all the time, you just don't notice them, all the time.

Errors due to geometry and atmospheric - eh whatstheword, can be more than 50 meter. Its not the receivers fault its a system "fault". These are exasperated by poor installation practices.

GPS seems to be a miracle, and it is - however your GPS is a radio-receiver relying upon very week signals. Radiosignals are easily blocked - everybody knows that, but how many frequently check the Satellite constellation page, and think: Hmm, Why doesn't it use nr. 16? It should be in a perfect løocation.
It may be your installation! Or the environment around the antenna. Maybe you have a (shudder) built-in antenna? Maybe its tucked away in a cupboard? Maybe its a GPS mouse, stripped to the windshield wiper motor!
Do you even know where your windshield wiper motors are? Are your windows electrically heated?

A decent amateur GPS, the kind used in pleasure craft, should have an accuracy of 10 meters (30 feet), but that is maybe 90% of the time. You need to plot a significant amount of time to appreciate the magnitude of error during the remaining 10% of the time.
You may have the biggest and bestest GPS known to mankind and your dynamic accuracy may be better than 10 metes, but again - 90% of the time.

No one would provide the biggest and bestest with a shoddy antenna installation, but remember - its really your, dime a dozen" GPS that needs the better installation.

Those of you who have a fixed error, proven by being alonside, have two options. A chart error or wrong datum. Prolly the latter.

My suggested rule of thumb is: If greater accuracy than +/- 100 meters is required - go visual, keep linear relationship to visibility and speed (0-visibility, 0,speed)!

My advice: Check out VisualGPS.

[settingsun]

I left sailing when it was a paperchart world and came back to go cruising in an integrated electronic chart/gps/autopilot world.

What a difference!! I was hooked on our first 70 mile passage, and 8,000 miles later I'm still hooked.

No more squinting at the land holding the chart up and saying, "do you think that's a point or an island?"

We cruise Pacific Mexico where the data on the electronic charts is often off by a mile or so. But the coastline has the right shape, more or less.

Whenever we come into an anchorage we turn on the radar to see how far off the chart data is.

As for rocks and islands, it's anyone's guess as to whether the stuff on the electronic chart is in the right place or even exists.

But I wouldn't give up my push-button "set the autopilot to the cursor" navigation any day.

Paper charts are on board, but thank goodness we haven't had to use them.

[noelex 77]

Originally Posted by Thonord

I beg to differ on the first part. Charting errors occur, but are not common.

My advice: Check out VisualGPS.

A lot depends on where you sail. If navigating popular areas serviced by commercial shipping the maps are always great. Once you get to out of the way places errors are common.

Originally Posted by Thonord

GPS error happen all the time, you just don't notice them, all the time.

Errors due to geometry and atmospheric - eh whatstheword, can be more than 50 meter.

With a good instalation I just don't see this. My GPS is on and plotting virtually 24/7 and has been this way for the last almost 6 years. I have never seen errors remotely approaching 50m. There are times sailing where I would not be aware, but the GPS is plotting continuously at anchor. A 50m error would be very obvious.
This is the government graph of errors plotted over 3 months.

[Thonord]

Originally Posted by noelex 77

A lot depends on where you sail. If navigating popular areas serviced by commercial shipping the maps are always great. Once you get to out of the way places errors are common.

True, but entering the out of way places - using WGS84 - is more frequent.

Originally Posted by noelex

With a good instalation I just don't see this. I have never seen errors remotely approaching 50m

Thats why I reccomend VisualGPS. Its simple to use, free and may surprise you.

Originally Posted by noelex

This is the government graph of errors plotted over 3 months.

But has no relevance to this discussion. It is recorded under optimal conditions.

This is not!

The upper part is relevant to sailing in costal conditions in Norway.
Disregard the lower, red, jumps. They are the the result of me going "Bananas" and really making it difficult for my GPS.

Tom
Edited
For some reason the image became a thumbnail. This is larger.

It is a stationary plot.
Edited, again:
Trying again

[Thonord]

The image is very small here. It should be much larger. Maybe it is in my album.
I should also add, the plot is taken under stationary conditions.

[noelex 77]

Originally Posted by Thonord

True, but entering the out of way places - using WGS84 - is more frequent.

Setting the appropriate datum does not fix the errors. You can manually adjust the map to coincide better, but then it will be wrong for the bay.

[QUOTE=Thonord;1129688]

Originally Posted by Thonord

True, but entering the out of way places - using WGS84 - is more frequent.
Thats why I reccomend VisualGPS. Its simple to use, free and may surprise you.

My GPS is already plotting my position 24/7. There are no plots that drift remotely 50m out of position. It will have some short term dampening like all instruments, but this is how the position data is presented.


This was over 12 hours swinging at anchor in Europe with EGNOS you can see there is not a single plot outside a few meter range in the E-W direction. ( the N-S direction change is due to the wind gradually changing). EGNOS has only been working for a short time in Europe. The results without EGNOS are a bit worse, but nothing like 50m.

[Thonord]

Originally Posted by noelex 77

My GPS is already plotting my position 24/7. There are no plots that drift remotely 50m out of position. It will have some short term dampening like all instruments, but this is how the position data is presented.

I speak no evil about Furuno. On the contrary - I like Furuno.

However your plot is filtered, dampened and you use EGNOS. You have a proper antenna installation, and I guess there are few horizont obstructions. You also know what you are doing.
The people I target with my warning are those who believe in the "accuracy" of their automotive GPS . The vehicle on screen is always right smack in the middle of the road. They assume it comparable in a maritime environment.

That said, it would be interesting to see an actual plot of your "raw" GPS data.

Tom

[Dockhead]

Originally Posted by Thonord

The people I target with my warning are those who believe in the "accuracy" of their automotive GPS . The vehicle on screen is always right smack in the middle of the road. They assume it comparable in a maritime environment.

Right -- I bet some people are not aware that automobile GPS works differently -- your vehicle icon is "snapped" to the nearest road, unless you switch off that function (as you can in some SUV nav systems, like on my Range Rover, for going off road). In short, the system assumes that you must be on a road, so if your position plots out 50 meters to the side, it moves you over to be on a road. It gives a false impression of greater accuracy than it has.

Obviously this system doesn't work for marine navigation.

But my plotters have all shown a circle of uncertainly of the fix. Very useful at the larger scales.