I built a prototype of a device like Curtis is describing. I guess it is "obvious" that such a device is useful, so I don't need to try to patent the idea.
I used a GPS module, a fish
finder, and an embedded computer (a Rabbit RCM2200 for anybody who knows what that means). The fish finder was actually cheaper than a device that reads depth only. It would be cool to get the fish/bottom data, but it only emits a regular NMEA
depth finder sentence once per second.
I view the device as possibly useful for surveying anchorages
. I have not used it for any real application, but I have surveyed the area around a marina I used to live in and found useful information. When we moved to the current
marina, our keel
found the shoal near the entrance, and I plan to go out and survey
that area also.
Paul has some high standards. While you might not meet those standards with this sort of device, you can still collect useful data. A big question in engineering is "how good is 'good enough'?"
The accuracy of the location of any particular sounding is reduced by GPS error, but GPS error does not appear to change that quickly over time. That is, if your you travel in a straight line for 5 minutes, the plot of your successive positions will be a straight line. If you complete your survey in a reasonably short time, you can ignore the GPS error.
In practice, I find that the point where I start the survey still has close to the same coordinates at the end of a survey. That is, I start my dinghy
in my slip, motor
around collecting data until the computer memory is full (128k or something like that), then return to my slip. The sounding from the beginning of the survey and the sounding from the end of the survey are so close together that I can only plot one of the numbers on the resulting chart.
Another way to view this is that the size of a digit on my chart is substantially larger than the cumulative GPS error over the time I make a survey.
There is a more significant source of error: The GPS and fish finder are reporting data only once per second, and they are not synchronized at all. When you get a depth from the fish finder, you only know that the depth is somewhere between the most recently reported positon and the next report from the GPS. (Strictly speaking, you might have turned the boat, so you don't even know that for certain.)
In practice, I have to discard most of the depth readings because I can only cram so many digits into the plot area. Choosing which readings to keep requires a clever algorithm, and I'm not entirely happy with the performance of the one I am using.
Wave height is not a significant problem. If the waves are big enough to be a problem, I am not out there in my dinghy. Either the depth finder will average it out, or the plotting algorithm will select the shallower (more conservative) reading. Either way, I know that I will add my own margin of safety
. If I do not have confidence in the precision of the data, I must compensate in the safety
My device does not address tides at all. The answer is to collect the data and recognize that you do not necessarily know the reference datum for the water level. You can look up the tides, but only for places so far away that you can't use it to correct your readings. You also don't know what the recent weather
did to the water depth. (Baltimore has a typical tide range of 1 to 1.5 feet, but some days the depth can be down to -3 or -4 feet because of strong north winds.)
But does it matter? Suppose you know the typical tide range is 2 feet, it is an hour before high tide at the reference point 10 miles away, and your draft
is 6 feet. Should you anchor
in a place that says "7" on your survey? How about "8"?
I don't expect my data to reveal a submerged piling or a small rock, but then I would not be able to detect them by any other means either. I could detect a shoal across the mouth of a creek or decide if the deep reigon is wide enough to anchor
I also don't expect to overlay my data on the navigation software
, then click on a point to go there. The data should reveal whether there is a usable channel, though. When you enter a creek, you may see your depth finder go from 10 feet to 8 feet in the course of a second or two. I think it would be nice to know whether that means you have to turn sharply to avoid a grounding, or whether you can proceed with confidence that the depth remains at 8 feet for a substantial distance.
If there is any interest, I can post some pictures of the device and some sample data.