From Inland Waters Resources - Chart Errors
- Posted with permission from the author - me.
Some people point out that the information on electronic charts
came from the paper charts
originally, and others say the paper charts are now all produced from digital information. Each statement trying to prove some point about chart accuracy.
So, let's demystify some of this, mystify some other stuff, and basically just veer off into some wild speculation.
I don't know what the different information flows are, so mainly I'm just speculating on things, but I think overall it will show how complex the situation is and how, no matter what, you can't blindly trust any of it. Errors can creep into one format that doesn't creep into the other. Neither format avoids all the errors that can occur in the other. And there are many errors that can occur in both.
I'm starting this in the sextant era.
Captains got positions by sextant
and kept notebooks. Charts were drawn using these notes. Sometimes a mapmaker came along. They may have taken these positions and plotted them right there. In some cases, they went ashore. There they could get better fixes and do averaging, sometimes getting quite accurate. Then, using standard surveying techniques, they could very accurately plot the relation between various land features. They might have a dinghy
hold a position over and underwater feature and plot its position from land. In general, the relation between points in an area would be more accurate than the positioning of the overall chart on the earth. Local magnetic changes could have them skew the chart. Inaccurate sextant readings could have them plot the location off as much as a coupla miles.
Along the way, governments started collecting these notebooks and charts and created their own database. Then some mapmakers might go through this to create a more uniformly drawn and notated set of charts. When they found multiple sources of information that disagreed with each other, how they resolved those differences is something I'm not even going to try to guess at.
Various governments might form alliances and share information.
In all this combining of information you'd hope it resulted in an increase of accuracy, but I'm sure errors crept in and politics played a part.
Then along came computers.
The first digital information surely came from someone manually plotting the points and typing them in. Then came digitizers. Along the way, finally, you'd have a database of points. But these would first be for the most used charts. So, charts would have been digitized at different time periods and using different methods.
New areas being charted might have been dealt with in different ways. Early on, some may still have been done as paper first. As time went by, they shifted to going on a computer first, then hand plotted onto paper charts. Eventually we get to the point where it's totally digital. GPS
positions fed directly to a computer and travels through various processes and end up on charts.
Now, in what order did the following happen?
- Paper charts are created from the digital data.
- Paper charts are scanned for use on computers.
- Vector charts are used.
- Paper charts are created directly by the computer
I don't know, but none of it was where a switch was flipped and everything started happening the same way. It was an organic process with multiple methods being used simultaneously.
So, what do we have now?
We have charts in various formats. Some of the features on those charts came from modern times. Some came from the 1800's notebooks. Some came from GPS
Some had errors introduced when they were plotted by hand. Some had errors introduced when some computer conversion routine mishandled an overflow condition when sending it to the plotter. Some errors are introduced by the software
that displays it.
Some of these errors are common to both formats (raster/vector). Some only happened when the paper chart was produced, and others only when the vector data was compiled.
So, what is this diatribe is meant to communicate?
Saying all the data came from paper charts is way over simplifying the situation. And saying paper charts now all come from computers also way over simplifies it. They are basically meaningless statements and in general, don't prove what the person was trying to prove.
It also shows that the data has made convoluted, error prone routes to your screen
, no matter what the format they end up. And that some errors may be different between the formats.
Once again, it boils down to not trusting one single
method of navigation
. In electronic navigation
, I want multiple sets of charts, the more the merrier. I want multiple sets (and types) of hardware
. I want multiple sets of software
and operating systems. Now, this may simply mean a computer and a chartplotter
, both with their own sets of charts. For myself, I also want a set of paper charts. But I can totally respect someone who is fine relying on a good electronic setup.
When it comes to navigation, many many things cannot be said as an absolute. And this includes many things that on first glance you may think can be said as an absolute.