ERRORS ON MARINE NAVIGATIONAL CHARTS

Updated 06/20/2009


A discussion about chart errors. Paper vs. electronic. Raster vs. vector.



STANDARD DISCLAIMER - This is not a comprehensive guide. They may be errors or omissions. It is only my personal take on what some of the dangers are when using charts. Use your own common sense when making decisions.




Paper charts can contain three main types of errors:
1. Positional errors
2. Relational errors
3. Time errors

1. Positional errors
A chart may well have been made before GPS. Some even over 100 years ago. The position it says you are at may be off even a number of miles.

2. Relational errors
The objects on the chart may not be correctly positioned in proper relation to each other.

3. Time errors
This is the real gotcha. Things have changed since that chart was made. Shoals have built up, rivers moved, etc.

Two types of charts:
1. Paper
2. Electronic

1. Paper Charts
With paper charts you will not automatically know where you are. You have to use various means to determine that. Like taking compass bearings from shore objects. This may seem like a disadvantage, and to some extent it is. But if the chart is off, then let's say you use a GPS to get your exact position. You plot that on the chart. Since the chart is off, you have now plotted your self in the wrong position in relation to the chart. That's the trick. You don't care exactly where you are, you only care where you are in relation to the shoals and rocks. That's how you use paper charts. If there are any relational errors, they probably are not a problem if you are sticking to main channels. These charts were most likely done by mariners who made sure the important stuff was right. So, relational errors are usually only a factor if you are approaching a shore that a ship would not normally approach. Then you use a little more caution. But you always have to keep in mind that the bottom may have changed. Someone may have put in a new jetty. A new light may have been installed. This is why it is critical to keep up-to-date charts. The positional accuracy may not be improved in the latest chart, but the new light should be there.

2. Electronic Charts
Electronic charts are mostly made from the paper charts. Even the pretty vector charts. So they have all the same possible errors as the paper ones! But they have more. As stated above, if the chart is off positionally, the GPS will faithfully plot you within 10 meters of the exact place on Earth where you are. But since the chart is off, you know exactly where you are by the numbers, but you don't know where the underwater rock is by the numbers. You only know where it is in relation to the point and the island.
See more errors in the raster/vector discussion below.

If you are going to use electronic charts, you MUST know what the possible errors are and how to deal with them.

The only time you can totally trust a GPS is:
You have been that way before and saved the track.
While saving the track, you kept watch on what the GPS is reporting as its accuracy.
You also have a 3d type depth finder so you know that not only did you not run into anything, but it was clear for a reasonable distance on either side (or you know the waters well enough).
While you follow the track back out, you keep an eye on the GPS reported accuracy to make sure it is capable of keeping you close enough to your track to be safe.

In other words, the only time you can totally trust a GPS is when you are using it independantly from the charts. You can trust a GPS when comparing it to data from a GPS (or to itself), but not in relation to a chart.

Note for the following section: There ARE some charts made by modern methods using GPS positions that are very accurate. These are mainly for the main shipping channels.

Electronic Chart Types:
1. Raster
2. Vector

Raster Charts
These are made by scanning a paper chart. It is actually just a picture of a paper chart. It has all the errors that were on that paper chart. Also the position encoding can be in error or a programming bug can cause the plotter to display it in the wrong position. This is not as uncommon as you might think. I have seen charts plotted at 45 degrees to what it should have been. What's dangerous is when it's plotted just far enough off to be a danger, but not far off enough to be obvious.

Vector charts
These are also made from paper charts. The have all the errors of the paper charts plus all the errors induced by electronics. They are basically had traced on a computer. That gives it the added possibility of human error. The error that occurs the most is that an object is simply missed and doesn't appear on the vector chart. Another problem with vector charts is that when you zoom in, the lins still look sharp and clear. On a raster chart, it gets grainy and blury so you know you are zoomed in and detail is questionable at best. On a vector chart you must be very aware of your zoom level. This can be further masked when you zoom back and it jusps to a lower resolution chart and you may not notice.



Conclusions:
From the above you may think I don't like electronic charts. Far from it. I have Nobeltec Admiral, SeaClear and OpenCPN to name a few. You probably got to this page from my page that has links to where you can find free electronic charts (here)

My point is that electronic charts can only be used safely if you have a good working knowledge of how to use paper charts, and you know what the additional problems are that creep in when adding electronics into the mix. I STRONGLY believe this!

I won't go into the possibilities of electronics failure, etc. We'll leave that for another discussion.

Please email me with any comments.