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Old 18-09-2006, 16:34   #16
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I note that the Australian "radar" under discussion here is called LADS (LASER Airborne Depth Sounder). It uses infrared and green light to make soundings from an airplane flying at a height of 500 meters. There is no radar involved.

The satellite system mentioned on that web page is a regular weather satellite that takes pictures of clouds. It is used for:

Quote:
The laser is unable to penetrate cloud covered areas or turbid water, and therefore LADS use must coincide with relatively non-cloudy/non-turbid conditions.

This limitation is a result of the basic physics of the measuring device. In the northern Chesapeake Bay, I can rarely see anything more than about 1 foot (30 cm) below the surface. Consequently, the LADS system can't replace the sonar array here, because it can't see the bottom. There is just too much suspended material (i.e. dirt) in the water.

In the absence of more detail, I'm skeptical of claims that you can measure the ocean depth from orbit, especially with radar. I did a web search, and the closest I found was the TOPEX/Poseidon satellite. Some media reports incorrectly reported that it measured ocean depths, but in fact it measured the height of the SURFACE of the ocean. I suspect this was a typical case of a reporter not understanding the description, and writing what he thought he heard. (This is a depressingly common occurance, especially when advanced science/technology is involved.)

Still, somebody has figured out how to make some contours of the depth of the ocean from that data. On
http://www.ngdc.noaa.gov/mgg/bathyme...TML#depth_pred you can see
Quote:
We are using these dense satellite altimeter measurements in combination with sparse measurements of seafloor depth to construct a uniform resolution map of the seafloor topography. These maps do not have sufficient accuracy and resolution to be used to assess navigational hazards but they are useful for such diverse applications as locating the obstructions/constrictions to the major ocean currents and locating shallow seamounts where fish and lobster are abundant.
and
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The satellite-derived gravity grids reveal all of the major structures of the ocean floor having widths greater than 10-15 km (6-9 mi). This resolution matches the total swath width of the much higher multibeam mapping system on a ship (100 m resolution) so the gravity maps are the perfect reconnaissance tool for planning the more detailed shipboard surveys.
This is very interesting, but hardly usable for a nautical chart.
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Old 18-09-2006, 18:20   #17
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Swath Bathymetry systems use fan-shaped acoustic beams, of swept frequency ( to use correlation and filtering techniques to reduce noise interference) to produce topographic seafloor maps. These systems all use sound (sonar). They are either mounted on a surface platform (boat) or in a towed body to get them below wave action and surface noise and closer to the bottom. Major sources of error include platform pitch, roll, yaw and heave, and what sound velocity to use to correct the data. The sound velocity differs a fair bit in the ocean, depending upon temperature, depth, and salinity/density. Its possible to profile this information at a specific spot with an expendable probe, but when you are getting data from far out to the sides of the Swath, the weighted average of each sound profile path is very difficult to know. There are a number of other errors due to navigational errors, heading errors, and in the case of a towed array, the incremental errors inherent in gettng an accurate position on the towfish itself and tying that into ship's navigation.

Blue Green lasers work in shallow, clear water. Nowhere else. Its light, after all.

sonar, in the form of Swath Bathymetry, is the best anybody has developed yet, for mapping the seafloor.

As for the ocean surface, of course sealevel is different almost everywhere. The low pressure in the middle of a cyclone raises the sealevel, hence a storm surge, for example. The Panama and Suez canals are pretty good examples of different surface heights in close locations.
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Old 20-09-2006, 15:24   #18
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Measuring depth with radar is really false. It comes up in a lot of "junk" science. Ground penetrating radar is another one that comes up too. Projecting radar into something and expecting a signal back is just a basic limitation of radar - it won't.

Sometimes even your boat depth meter can act funny too. Churn up a lot of mud and the transducer gets a real treat as it thinks the transducer is laying on the bottom. It tends to overwhelm the circutry and can take a minute or two to come back to life. Sort of scares you a bit sometimes.
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Old 20-09-2006, 15:44   #19
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you're right about that. I have seen the GPR working, though. But its shallow, very limited.

As for acoustics, sound waves travel through transfer mediums at a velocity determined by the physics of the medium. nominal 1000 ft./second in air, roughly 4800 ft./sec in seawater, 5000 ft/sec in the seabottom, more in solid rock etc.

Every time an acoustic wave passes through an interface between two mediums with different velocities, some energy is reflected. Best reflection comes from going to a suddenly slower velocity ( water to an air interface) but some reflects going the other way, too.

If your depth sounder is set sensitive enough, the acoustic backscatter from turbidity can certainly cause it to detect at the face of the transducer.

uh, what I meant to say was, yer right.
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Old 20-01-2007, 11:36   #20
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Originally Posted by Lodesman
Talbot,

The Canadian Hydrographic Service still produces charts with surveys done by Captains Cook and Vancouver. Seems almost wrong to plot a GPS position on them.

Kevin
No, you need to use a sextant and a chronometer and splice the mainbrace at noon. (And don't forget buggery and the lash -- the other two great traditions of the Brjitish Navy.)


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Old 20-01-2007, 18:30   #21
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Some 3 or 4 years ago, when I was selling marine electronics for a living, I had heard that the shuttle had done a radar mapping of the planet for the department of defense and that the results were extremely accurate. The stories that I heard were that the DOD had been lobbied to release the the data (sans "sensitive areas") in the interest of public safety & well being ... and that finally they had capitulated. Of course .. we haven't seen anything ... yet. I have also heard that NOAA has been using aircraft to measure water depths in harbors, and that they have been updating charts as fast as possible ... good news ... but once again, I haven't verified that what I have heard is true. Now for what I have seen ..... has anybody seen NOAA's ENC charts (the free stuff) of Cuba? WHOA ... big improvements over the old British Admiralty charts .... uh ... George, George ... I SWEAR to you, I'm not going there!
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Old 20-01-2007, 19:40   #22
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Dunno, Bob.<G> I suppose if the USCG and others go to Cuba, accurate charts for the trip would be nice. But would YOU trust those charts, considering the same government is telling you not to go there? It would be too easy for them to slip in a datum shift on the qt and not tell the rest of us about it. Got complaints? Oh, and how did you get there to have them?<G>
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Old 21-01-2007, 05:42   #23
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I had heard that the shuttle had done a radar mapping of the planet for the department of defense and that the results were extremely accurate.
A lot of what you hear is inaccurate. The shuttle could not do such a project. What is true is all the Garmin data was created from Soviet satellite data. Garmin bought all the data for a good price so they could make the data they now sell.

Quote:
I have also heard that NOAA has been using aircraft to measure water depths in harbors, and that they have been updating charts as fast as possible ... good news
Chart making includes a lot of different data especially when making harbor charts (1 : 10,000) these are the really detailed level charts. Aerial Photogrametry is cheaper and easier to use because it captures all the land stuff as well as shoreline features with good depth information. When used in stereo plotters the level of detail is quite accurate. On a limited basis is can be used for depth too.

Currently they have a 52 ft Cat called "Self". It tows side scanning sonar. It scans for about 2 miles per scan and produces the best bathymetric data. Waters that are deep or murky don't scan well and can not be scanned from aircraft. The use of "radar" for depth sounding is BS.

Yes, the DOD has highly accurate satellite imagery, but for chart making it would be as if using a high powered microscope to survey the state of Rhode Island. Since you have all seen Blue Chart data you get a feel for how accurate old school level satellite data can be and for the most part it it's pretty good. After that it gets harder as in exceptionally expensive. Once you got the imagery then begins the expensive part. NOAA could produce more charts and more detailed charts - if they had the money and the mission to do so using all the technology they already have.

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has anybody seen NOAA's ENC charts (the free stuff) of Cuba?
That would be USECOM1 there is also US2ECOM2 that covers most of the southern Bahamas. These are 1 : 120,000 scale charts. I have them. They don't cover the detail that the charts for Puerto Rico do where you get the nice 1 : 10,000 scale port charts. The Cuba charts is just coastline and major navigation aids. The Bahamas are similar. They both connect seamlessly to the US Offshore charts. They also have a chart that connects the the US mainland to Hawaii USPO02M that is a 1 : 1 million scale chart that covers 2,000 nm by 2,000 nm. The SW corner is Johnson Atoll and the NE corner is Vancouver. This is continued north around all the Alaska peninsula to make a nice western Pacific chart. None of this data is what you would call "new". It would be a nice chart to have were you headed in that direction. The Hawaii charts and the US coast charts are as you would expect nice and detailed.
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