NOAA chart updates under way

[Pblais]

I was out on a short 10 day cruise on the Chesapeake Bay recently and over the radio listened to transmission from the research ship "Self".

It's a 55 ft CAT towing advanced sonar. It scans ahead about 2 miles. The CAT is towing this all around the lower Chesapeake to collect new data for updating the depth information. They appear to be running N/S transacts with over lapping coverage.

They were on the radio quite a bit warning off other botas to pass behind and to avoid a prallel course as the sonar is sensitive to wake as well as prop noise.

For those that always wondered how they really do it this is how. They also match the data with surveyed points and air photo's to accurately compute the peths for all the charts. It's nice to know they are doing updates as most of the depth info we have now in this region is more than 20 years old.

[Talbot]

20 years old, and you reckon that is old. Have a look at some of the charts for more out of the way spots, and you will find them based on surveys done by sailing ships!

[hellosailor]

It is a budget thing, Paul. NOAA, NMA, DHMA, whatever the acronym and the administrator is this week...They have something like two [sic] survey ships that have to cover the entire US, and no budget for anything more. Same old story, Congress knows there aren't enough sailors (or sailing lobbyists) and the marine trade....I guess manages with what they've got.

More important to spend money on the "BUY MILK! EAT BEEF!" campaigns. sigh.

[Pblais]

Original surveys from the early 1800's were commonly used during WW II in the Pacific. Many were exceptionally accurate as far as the surveys went. Worldwide surveys were not a common thing until very recent times.

Given the Chesapeake is mostly shallow with two primary shipping channels Baltimore and Norfolk and the US Atlantic fleet it would seem they would have been more up to date than just getting to it now. I know my home chart is 1978 for depth data. With the Yorktown Weapons Station and USCG Trainning facility across the river from me I would have expected a more up to date chart. The river is actually very accurate as far as I would trust. The center is very deep (90 ft) but it is the shallows that concern me more.

This is one place you want a depth meter that works well and you keep a sharp eye on it. You can easily run aground 3 miles from shore. It's a long walk back!

[hellosailor]

"With the Yorktown Weapons Station and USCG Trainning facility across the river from me I would have expected a more up to date chart. " Not necessarily. The USN, and for that matter the Defense Hydrographic folks, used to have all sorts of good data that would never be shared with anyone outside the military. They may have their own better data for what they need, and for the rest, as you say there are marked channels. Don't the depths in the many parts of the Chesapeake change so rapidly (from year to year) that they would obsolete a lot of surveying before it could get into print?

[Pblais]

Don't the depths in the many parts of the Chesapeake change so rapidly (from year to year) that they would obsolete a lot of surveying before it could get into print?

How could I tell

I don't really like to test the shallows more than I have to. Generally the places that are bad are always bad. It's just the places that are sort of bad that change.

[adjuvantjfoster]

Originally Posted by hellosailor

"SNIP.... The USN, and for that matter the Defense Hydrographic folks, used to have all sorts of good data that would never be shared with anyone outside the military. They may have their own better data for what they need, and for the rest, as you say there are marked channels.SNIP....

Just a few minutes ago I opened a thread further down on our forums that addresses this very point.....(gin).... check it out at

<http://www.cruisersforum.com/forums/showthread.php?t=5098>

To keep with my perception of forum etiquette, I don't wish to hijack this thread, so please post any replies to the thread I started, in that thread.

John

[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

[Jentine]

That system strikes me as a waste of time and resources. It seems that I read that the U.S. Government had mapped the depths of the oceans from space using RADAR. If that is not enough, the Australians do it from aircraft at a cost effective rate. http://www.aims.gov.au/pages/facilit...g/rs-lads.html
Captain Cook's soundings are not much more archaic than towing a sonar unit.

[ssullivan]

This thread is right on. It's the main reason I cringe as I watch weekend warriors cut channel markers, plot courses close to shoals and generally take courses that skirt the land.

It's prudent to treat the depth data with caution, using heavily traveled routes. I ususally take the same route as commecial shipping traffic does (keeping a good eye out), because on those routes you KNOW they are of a reasonable depth.

The only time I even think about depth is when I'm tossing my boat into a small, shallow spot for anchoring. In that case, I proceed at a knot or so into the area, no matter what the chart says.

A little tip about charts outside the US and large scale charts in international waters is:

If you are worried about hitting an obstacle, reef, island, etc... you should "run down the latitude" to reach a destination rather than just proceed on a direct great circle route toward it. It's a safer way to approach a hazzard that was charted many years ago. The reason for this, is that in the older days, latitude was much more accurate than longitude. Sailors relied on marking hazzards at a certain latitude. If you passed through an area of ambiguous longitude, you would be told to steer clear of a certain latitude in order to clear of the hazzard. Because many of the charts for open ocean and hazzards in poorly charted areas were created in these times, it's the safest way to plot a course through these areas, using these old navigation tools.

[hellosailor]

Jentine?
"That system strikes me as a waste of time and resources. It seems that I read that the U.S. Government had mapped the depths of the oceans from space using RADAR. "
Which "that" is that?

The earth mapping projects from the shuttle are not there so provide maps for boaters. Since the shuttle is in an orbit that can be calculated very precisely, it allows them to produce results that show tiny changes in land and sea level and, among other things, map gravitation changes in the surface of the earth. Yes, "sea level" is not one level, it varies with local gravity and other factors. Fly over it in an airplane and you don't really know what height you are at, you can only guess it roughly. Fly over it in an orbiter...and you can start to find the inner workings of the earth. Something we still know very little about.
So the shuttle flights aren't just "mapping", they are very much the same basic science that was being done years ago by Newton, Darwin and others. The maps are just a fringe benefit.<G>

[GordMay]

Further to Sean’s advice to “run the Latitude”: Latitude can be deduced from the height above the horizon of the celestial pole star, or of the angle of the Sun above the horizon during its noontime passage south of the observer - while finding one's local longitude [*1] requires an accurate clock (chronometer) to compare local noon with universal time. John Harrison is credited with the invention of the chronometer in 1761 [*2].
More info':
*1 http://www.nmm.ac.uk/server/show/con...355/viewPage/1
*2 http://www.solarnavigator.net/history/john_harrison.htm

[ssullivan]

Gord's post is the exact reason why latitudes were more accurately measured in those days than longitudes were.

[Jentine]

The towed sonar. I am sure the Australian RADAR can differentiate between the surface of the water and the depth of the water for an accurate measurement. Additionally, the information from the space orbiter will become available to NOAA eventually. Dragging a SONAR scope around on a cable is only a couple of generations better than a lead line. Same thing but faster (hopefully).

[hellosailor]

Jentine, you've got me confused here. "The towed sonar. I am sure the Australian RADAR can" Wait a minute, are you talking about the Aussie's making charts using towed SONAR, or using some time of RADAR? And radar to measure WHAT?

No conventional radar or sonar system, deployed by ship or aircraft, can accurately measure distance from the center of the earth (or the center of mass of the earth) in the same way that orbital systems can. With sonar and radar all you can measure are local surfaces--and that doesn't tell you how those surfaces vary from the spheroid, or whatever shape you prefer to call the shape of the earth. You can measure how deep water is, sure. You can measure tidal levels, too. But that doesn't tell you how local sea level deviates from global levels--which it does. You need orbital mechanics to get that last level of precision measurement. (You can't quite do it with GPS either, because GPS suffers atmospheric distortions. You can compensate and get close--but not to the same degree.)