| my account | login-logout | resources | classroom help | support | catalog | home | get webcard |

Online Classroom


Post New Topic  Post A Reply
search | help desk | commons
  next oldest topic   next newest topic
» Online Classroom   »   » Public Discussion of Inland and Coastal Navigation   » Problem #7-11

   
Author Topic: Problem #7-11
Ted


 - posted February 08, 2008 02:57 PM      Profile for Ted           Edit/Delete Post 
I cannot find the tide and current stations because I cannot find the notation or symbol that apparently represents these two items respectfully. Where will I find this information? Can you describe what they look like? Thank you.
From: Yukon
David Burch


 - posted February 08, 2008 04:26 PM      Profile for David Burch           Edit/Delete Post 
There are no specific symbols for these secondary or primary stations. they are not shown on charts at all. the goal of the exercise is to look them up in the current tables by lat and lon and then plot them yourself. this is good practice in general as that is the only way to get a picture of where there is current data you might care about.

At one point, there was a company near seattle that made custom charts with these data on them and they were quite a nice aid. But they are no longer in business.

From: Starpath, Seattle, WA
Ted


 - posted February 09, 2008 01:54 PM      Profile for Ted           Edit/Delete Post 
Are these current and tide stations actually represented in the water by a device of some type? If so what form do they take? For example are they buoys? That of course doesn't make sense as then they should be located on the charts.
Thanks.

From: Yukon
David Burch


 - posted February 09, 2008 03:30 PM      Profile for David Burch           Edit/Delete Post 
At some point in time actual measurements were made at the reference points call secondary and primary stations. But i do not believe there are any permanent meters there to give live data in the region of our chart (eastern end of Str. of Juan De fuca), if so, it is not made available to the public.

There are areas with live current data, San Francisco Bay is one place. They have meters throughout the bay. A current meter can be attached to a charted buoy for example.

Generally these are devices that are anchored to the bottom by large concrete blocks with a sonic connector. they measure current, temp, and salinity at several depths recording data every 15 minutes (?) or so in the device, which is left in place for some number of days or months, then they (NOAA) come back fire it off and pick it up. take it to a lab, extract the data and study it. Note this data has to be deeper than the typical draft of vessels in the neighborhood.

Our current tables in Puget sound for example are based on very old data to my knowledge. In fact, it is my understanding that they do have better data available but have just not updated the tables with it. May even be such a matter as budget. But there are scientists at UW and PMEL that know much more about currents in Puget Sound than is available in public documents.

a Google search should find some pictures of these meters. here is one example
http://www.pmel.noaa.gov/tao/proj_over/mooring.shtml

it looks like they use a different type of meter than i am used to when i worked on this type of thing for a short time. the older style was a cylinder about 10 inches in diameter and about 2 feet long.

Note that the currents way below the surface are typically quite different than on the surface (sometimes even reversed) so eventually they do need some data from the surface or just below it to use in the current tables.

From: Starpath, Seattle, WA


All times are Pacific  
Post New Topic  Post A Reply Close Topic    Move Topic    Delete Topic next oldest topic   next newest topic
Hop To:

Starpath School of Navigation

Copyright, 2003-2018, Starpath Corporation

Powered by Infopop Corporation
UBB.classicTM 6.3.1.1