Paul, if someone reported the DGPS position of a rock (for instance) that wasn't charted or was charted in a "wrong" position, wouldn't that be valid information?
Anything could be valid but perhaps not accurate. Uncorrected GPS is not accurate enough for chart quality. Real charts are actually that good in terms of the base they are plotted on. True DGPS is corrected from a local base station. The idea is you measure the GPS from a static location and then use the GPS time stamps to add the correction to the recorded data point. The GPS signal has a highly accurate time signal and it is actually why GPS can be so accurate. Consider GPS signal to be a very very accurate clock. You really can take the data saved from a base station and match it with the data point you collected in the field and compute the error at the base station and apply it to the field data point. Through yet another process you can record
data from a static location for a very long period of time and eventually compute the location to some amazing accuracies. It's how they measure the movement of the continental plates.
US charts are actually more accurate than you might think. They start with low elevation raitioed and rectified air photos then apply them to a control grid.
So if I give you a GPS point of one particular spot there is the error of the raw data I collected combined with the error of your chart, combined with the error of your own GPS when you try to find it. So the rock you located might be wrong because the base is worng or the location is wrong but you can't know which.
That is just one point on the ground. As you add points you add the issue of relative error. If you measure two spots then compute the distance between them the margin of error doubles relative to each other. That is why all charts and all land maps have a control grid that is developed from a highly precise process. It prevents error "creep" and allows you to tie back to a control point to restrict the error. As you get into large area maps like sea charts the whole shape of the earth gets to make this process even more complicated. Above I ignore all that and for local charts you pretty much assume a flat earth. For larger scale charts you can't or you get charts that look like those old maps from the 16th century.
Coastal Charts are made pretty much the same way they make USGS topo maps and are about equal quality. Converting them to electronic makes them no more nor no less accurate. In fact all the NOAA charts are made electronically and have been for many years. The format they use is a Maptech
format not used for consumer charts. It's designed as an editor format.
The problem with "reported information" is mostly one of accuracy. Say you have a channel into a port. Of course they silt and shift with the storms. We all know of locations like that. Just reporting simple GPS/Depth won't be accurate enough to correct the chart to the same level of accuracy it was before. The relative GPS error won't locate accurately enough and the depth data has so much variation and problems I wouldn't guess how inaccurate that data "could be". You always have to take data with a pessimistic outlook. Now it gets even worse. We have our shifting cahnnel out there and we get new data all the time. So just when is the data to be thrown out because it is too old? How would you know? Now what about 10,000 ports
Watching local channels you navigate and observing them over time is what amounts to "Local Knowlwedge".
To rechart a small area you need to collect the data all at one time. You need to collect it such that it defines all the boundaries not just the centerline. You need the depths of where you can't go as well as where you can go so you can locate the "edge" as accurately as you located the raw depths. When you are navigating you want to know where the edges are as much as just a single
centerline. How much sea room is there? You want enough points to describe a 3 dimensional space.
Also, if a number of reports of depth info through say a passage came in and there was a program to sift through this information looking for congruence(?), wouldn't that be valid information?
Not unless it is based in magic. I don't see how it could do that what so ever unless it was done in real time. Depth is a serious issue as a moving boat is actually pretty bad for depth accuracy. The display is "dumbed" down so the numbers don't "jitter" as you look at it. The GPS has limits as well. A couple boat lengths is as good as you could bet money
on. It might not be that good after we compute the NMEA
time errors and the averaging that goes with it. When I did this in a vehicle we were using millitary data bus speeds like they use on aircraft instrumentation to capture data direct to disk. We couldn't do it real time since we had to post proces it all with the base station data. We also had a ring laser gyro to compute the roll, pitch
, yaw errors too.
The bigger problem is every boat goes down the center. You would need a lot of boats going aground to find the edges of the channel. You only know about what gets reported. That one single
problem you can't overcome even if you could build a real time trasnmitter and shore receiver (you could). 100 boats come in / out of the channel all down the middle. What new information did you collect? You measure day after day and slowly the channel is shifting. When do you know it moved? How do you distribute the information? What if it moves back?
For your first job. Fine me a rock that is located wrong and then compute how wrong it is. And fianlly, how did you know it was wrong in the first place? For myself, I consider not hitting the rock good enough.