I saw your forum from internet. I am surveying assistant from Hong Kong. I am trying understand how to get MLLW from tide tables. Tide tables have values of tide and corresponding time at certain location.
Is it correct that if tide table say for example :
Time Tide Time Tide
12:00 PM 0.34 3:00PM -0.21
Does it mean that if I have level or total station & I measure sea level at 12:00 PM I can use the value of 0.34 as my MLLW datum?
How accurate is this process compared to actual tidal observations?
No experience in bathymetry survey.
> I saw your forum from internet. I am surveying assistant from Hong Kong. I am trying understand how to get MLLW from tide tables. Tide tables have values of tide and corresponding time at certain location.
>
> Is it correct that if tide table say for example :
>
> Time Tide Time Tide
> 12:00 PM 0.34 3:00PM -0.21
>
> Does it mean that if I have level or total station & I measure sea level at 12:00 PM I can use the value of 0.34 as my MLLW datum?
>
> How accurate is this process compared to actual tidal observations?
>
> No experience in bathymetry survey.
Probably not. Tide tables are predictions of daily heights of tides relative to some datum (usually NOT MLLW). MLLW, or Mean Lower Low Water is the aritmetic mean of the height of the tide at some specific point, measured over 19 years. I wouldn't base ANY kind of survey on a daily tide prediction table, and certainly not on a single measurement of sea level!(Others here may have other experience with this).
If the table is referencing MLLW, what it's telling you is that the tide is predicted to be at "x" feet above or below the MLLW datum. But it would be highly erroneous to attempt to "calculate" what MLLW is, based on one or two observations.
Also, tides at any specific point can vary widely from the control station used in the table, although it is possible to interpolate between two nearby stations. The MLLW figures for Hong Kong (or wherever you're doing your work),(if they exist), should be published. Have you looked to see if they are?
Welcome to this board. You should find it helpful, educational, funny and frustrating at times. You have one good response already. There will be more as the day goes on.
I would highly recommend that you post your question to the National Ocean Service (NOS), Center for Operational Oceanographic Products and Services (CO-OPS) at [email protected]. CO-OPS is the U.S. federal government agency responsible for the installation and maintenance of the nation's network of tide stations. They produce of number of different products and services related to the observations of tides. They have an excellent staff of scientist and oceanographic engineers that I am sure will be happy to respond to your questions. You can review their various activities at -- http://tidesandcurrents.noaa.gov/
:good:
Just looking at the Sea Level Trends on the Co Ops site, I see 3 tide stations around Hong Kong.
A bathymetric survey is easy enough, just like topo but you usually cannot see the surface... From there you can modify your TIN / Surface Model to the datum desired.
Have fun.
I suggest you contact Dr. King at Hong Kong Polytech. He can help you.
Thank you all for reply. I lookup more details from other surveyor groups. Difficult to approach professor at university. He might say enroll first class of survey. I only have little units survey courses & rest from company work.
> ... He might say enroll first class of survey.....
Then again, he might not. But I understand that the rules of etiquette are different there. So it goes.
I do not think that you can accurately determine water levels from tide charts alone. I was recently asked a similar question for a place in the Columbia River shipping channel. With a little googling we were able to find a NOAA web page that gave water levels at a guage relative to a datum for specified times. I'm sure such data exists for your location, especially if it is near a shipping channel.