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Old 03-05-2006, 14:07   #1
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An instrument I wish they made...

I've run down the feature list in my head several times...thought I'd commit it to "paper" to see if anything comes close.

I would like a handheld, waterproof GPS with integrated recording depth sounder and tide prediction tables. I would install this device in my dinghy so that, as my kids rip around all over creation, they are, in fact, collecting depth data. Once back aboard the mother ship, I could transfer the data from the handheld to the nav computer. Because the software would know the tide level at the time of the reading, it could adjust for tidal variances and update my charts with a fairly up-to-date picture of the bottom.

Where we cruise on the West Coast of Florida, a foot or two of depth error makes the difference.

Just dreaming but it seems like all the pieces are here...just need someone to put it all together.

Curtis
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Old 03-05-2006, 14:44   #2
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Not a bad idea but

I worked on a project that was doing similar things on land from a moving vehicle. There is a small problem with collecting depth data as you buzz around with your GPS. You can't sample all that accuratly from a moving, pitching, and rolling platform with a GPS. We used a ring laser gyro integrated into the GPS plus we used stationary base station GPS corrections to make the GPS more accurate. The ring laser gyro can compute out all the rolling around quite accurately for hours at a time. That part is really amazing science. That would make the lat / long location reading plus or minus about 10 inches depending on how fast you were moved. I'm not sure exactly what the elevation readings would be but regular GPS is pretty bad at elevation in real time while moving again we need that ring laser gyro. My own GPS is about + or - 20 feet vertically. Bad news though, the ring laser gyro costs about $100,000 and it's about the size and weight of a large car battery and needs at least that much power too.

You would need that level of accuracy plus you need to post process all the data to correct for the waves, tide levels corrected for local and weather induced distortions different from the base tide prediction levels extrpolated from the nearest tide station(s). The data generated by your depth sounder is already greatly averaged so the display you see isn't bouncing around all the time with different numbers so that part is a problem too.

A foot or two of depth really is no problem if you have enough water on top of it . The reality is our maps are very imperfect. Storms and conditions effect water levels on a regular basis greatly. There will always be some limits to how accurate they are and how much you can blindly depend on them. I sure don't depend on them.
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Old 04-05-2006, 06:14   #3
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Paul,
I don't know that it would have to be that accurate.

I'd be happy with plain old depth sounder readings (adjusted for tides) and standard WAAS GPS readings.

That way, I could send the boys out in the dink and say "see if we can get in that channel"...they go off and wakeboard and play for a couple of hours in the target zone, come back for lunch and I have a reasonably decent depth chart.

The way we do it now is to send them out with a weighted string to take soundings and bearings. Good skills for boys that are midshipmen's ages to know, but I'd hesitate to rely on their findings if the bottom was rocky!
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Old 04-05-2006, 06:57   #4
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Quote:
The way we do it now is to send them out with a weighted string to take soundings and bearings. Good skills for boys that are midshipmen's ages to know, but I'd hesitate to rely on their findings if the bottom was rocky!
I'm not clear how it would be any better than that. Sampling random data inaccurately would in the end give you a general answer of the depth, but it still would not tell where the rocks are and exactly where the channel was. Random motion of them playing might in fact cover a for piece of ground but not always to any uniform density.

Sending them out ahead in the dink with a handheld depth sounder might be the higher technology solution to what you do now and that would actually work. The new garmin handheld Rihno radios with the GPS built in can display current position of the person you are talking to so at least that is getting better, but again unless they are on a specific path you really can't rely on it enough to pilot the boat. Using that tyhey could radio depths and you could plot positions. That would as accurate as you could get now.

That is the key - is the data accurate enough to pilot the boat through the rocks?
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Old 04-05-2006, 10:58   #5
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Not sure about the totality of your system, but I am sure it would be easy for a plotter to take notice of local tide height and display depths as is rather than relevant to chart datum. It would make it much easier to check against echo sounder.
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Old 04-05-2006, 19:03   #6
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I have a neighbor that has rigged up pretty much what you have described. He has a Lowrance color chart plotter/ fish finder mounted on a piece of wood with the GPS antenna mounted there too. The transducer is mounted on a board that clamps to the transom of his dink, power source is now via cigarette lighter plug. The Lowrance unit allows for recording the water depth, then displaying it (along with GPS coordinates) at a later date. While some may doubt the accuracy of this arrangement ... it is better than the charts of this area! By having made the whole affair portable, he can easily return it to the helm of his Gulfstar.

Bob & Lynn
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Old 05-05-2006, 07:26   #7
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You just have to be a bit careful with these types of solutions. When you record a GPS reading and then use another GPS to find the same point you have to double the GPS error as the two readings are cummulative. With WAAS (only in North America BTW) that gives you about a boat length of error without WASS it's about 3 or more boat lengths. All this is is still only about 95% accurate.

NOAA Chart bathymetric data is mostly 20 years old for most charts in most places and not just around here. Then adding your home grown GPS coordinates on top of a chart that isn't all that accurate adds the other complexity.

Now finally how close can you pilot the boat while watching the GPS display in a tight situation constrained by depth?

It means sailing by wire through a very narrow channel is a crap shoot under ideal conditions. In a tight situation I would watch the depth sounder even closer than normal. Using the fishfinder real time without the survey would proabably give as good results if not better. A degree of doubt is healthy when using such data. If you need a GPS coordinate to get to a spot within less than 100 yards you can have a problem.
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Old 06-05-2006, 00:18   #8
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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 margin.

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 in.

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.
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Old 06-05-2006, 00:18   #9
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Old 06-05-2006, 05:06   #10
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Mark,
Your setup sounds very close to what I had in mind. The only enhancement I can think of would be the ability to use the collected data as an overlay to the nav charts on the plotter.

Wouldn't it be nice to be able to leave this thing in data collection mode all of the time and continually refine a bottom chart of your regular cruising grounds?

Anyone have any pull at Garmin ?
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Old 07-05-2006, 22:59   #11
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Originally Posted by Curtis
Mark,
Your setup sounds very close to what I had in mind. The only enhancement I can think of would be the ability to use the collected data as an overlay to the nav charts on the plotter.
The collected data could be trivially converted into an ordinary raster image. I would think any reasonable nav software would have no trouble overlaying an image on a chart, but I don't actually use any nav software so I wouldn't know.

Quote:
Wouldn't it be nice to be able to leave this thing in data collection mode all of the time and continually refine a bottom chart of your regular cruising grounds?
Yes, I thought of that too, though I haven't bothered doing anything with it.

I've also thought that you could gather data from hundreds of boats in an area. You wouldn't get fantastic coverage because people tend to stay in the channel, but just variation in course from boat to boat would give you a lot of data. The problem is how to prevent people from contributing incorrect or even malicious data.

Quote:
Anyone have any pull at Garmin ?
Not me.

I thought it could be interesting to develop this idea into a product. I can run a project like this (I have more than once) but I am missing one key capability: Sales. First you need to sell the idea to an investor who can provide the initial product development costs, then you need to sell the product to distributors and customers. I don't have the money to fund product development, so unless I meet up with somebody who does (or who has the sales aptitude to get it), it's just going to remain one of my little projects.
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Old 11-05-2006, 02:47   #12
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By accident, I stumbled across this site. Can this be of any help?
www.drdepth.se/
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Old 11-05-2006, 07:18   #13
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It's an interesting program. It seems to do a nice job correlating all the readings. Looks perfect for documenting fishing holes.
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