Looking at the NOAA tide tables, it is Mean Low Low Water. What I dont understand is: I noticed I could not launch my boat at a dead low tide which was MLLW 0.80 for that time that day. Man, the tide couldn’t have been any lower that day. So now I plan a trip to make sure its not THAT low when I go out again and I notice 0.80 is not even low tide for that day (its inbetween). So does the MLLW not have anything to do with an elevation? Can anyone explain this?
Look above on the menu for Sciences… Lunar/Tides. You will see the key missing data: The actual vs predicted tides. Wind can make up to 2 ft difference in the amount of water inshore. Today, for example, is running + 1 ft over predicted.
2000 SeaPro 180CC w/ Yammy 115 2 stroke
1966 13’ Boston Whaler w/ Merc 25 4 stroke “Flatty”
www.sswaonline.com
not quite sure I understand your question, but yes, MLLW is an elevation. Essentially it’s just an imaginary line at the average of the lowest water level each day (using only the lowest of the two low tides a day) over a fixed time period (most NOAA stations use a 19 year average.) Tide predictions use this as a baseline, so a tide of +.80 is predicted water level of .8 ft above MLLW.
like Optiker said, the “prediction” here is not perfect and can vary with a lot of things. Another thing to keep in mind is that tide prediction is only good for that one location at that one time. If you’re a few miles away the time and height of the predicted tide may be about the same, or significantly different (also depends on a lot of factors) so use the prediction closest to your point of interest that you can find. compare what you see to the observed level to get a feel for what MLLW looks like at a given spot.
I usually just use the NOAA tides and currents site, but I’m sure there are some other more user friendly ones out there.
Angler 204 FX
Yamaha 150
I see, so if MLLW elevations are determined DAILY - then a MLLW of 0.80 on tuesday can be a good bit different than a MLLW of 0.80 water on Friday? (being that MLLW water is determined daily, unlike MLW).
Right?
sorry, I re-read the explanation above and it is a bit confusing…
no, “MLLW” (Mean Lower Low Water) does not change (at least not often) - it is a fixed value that is only adjusted every 20 yrs. it’s an average of the lowest observed level for each day over 19 years. Imagine you have a dock. Every day for 20 years, you go out and mark the lowest water level of the day on one of the pilings. Take the average location of all those marks, paint a line, and call it MLLW. You can then measure the actual water level against a fixed height, so if the tide is +.80 you know that the water is .8 ft deeper than it is at your MLLW “datum”. After another 20 yrs of marking heights, you repaint your line at the new average.
I don’t think “MLW” (mean low water) is a defined datum on NOAA charts/predictions, but as far as I know is pretty much interchangeable in meaning. The “lower” in MLLW refers to the fact that only the lowest of the two low tides each day is used in the average.
MLLW +.80 should always be the same. If it’s predicted to be +.80 one day but the actual water level is different than another day that it was predicted to be +.80 it’s because there was some other factor at work, like wind or maybe how much is being discharged from a reservoir (in a tidal river), offshore wave heights/directions/periods, even high/low pressure weather systems.
kind of made my head spin when I started trying to understand it. there are a lot of other “datums” or baselines used on charts too, but MLLW is the most common.
Angler 204 FX
Yamaha 150
dang cletus, you musta done gone ta o-shee-ain-o-gaffic schoolin’ ain’t ye?
amphidromic nodes still twist my brain into a knot every time i try to wrap it around them…
quote:
Originally posted by barbawangdang cletus, you musta done gone ta o-shee-ain-o-gaffic schoolin’ ain’t ye?
amphidromic nodes still twist my brain into a knot every time i try to wrap it around them…
shoooot, no i’ms just a injuneer folk by schoolin’!
no, I learned more that I ever wanted to about chart datums from my commanding officer during my qualifications as a submarine officer. as part of our qualifications we had to prep a piloting chart and brief the CO. well, i spent a lot of time drawing pretty lines and colors and danger bearings, soundings, knew every turn bearing, course, etc. Had it locked. Started rolling through the piloting brief, having seen it a dozen times, throw out the standard “and all charts are WGS-84 approved…”
“So, ENS Wolf, what does WGS-84 approved mean?”
umm… ensign salute shrug… deer in the headlights…
So I got to spend some extra time studying and try again!
Turns out it’s pretty important to understand exactly how deep the water is when you when you’re driving a 565 ft, 18,000+ ton ship in a channel that’s only a few hundred yards wide and might only give you a few feet between the keel and totally f’d…
Angler 204 FX
Yamaha 150
MLW water is on datums navd29 or what have ya.u seem pretty knowledgable but according to wikipedia MLLW is determined daily, and the MLW is actual lowest waterline observed per 20years… MLW and MHW are on datums and have a published elevation. I already know some of this because of my profession.
good on ya for checking it out yourself - everybody’s an expert on the internet, and you never know if what those “experts” are telling you is BS!
however… I try to keep my mouth shut unless I’m pretty darn sure I know what I’m talking about, and I’m pretty darn sure the MLLW referenced by NOAA is not determined daily.
I haven’t read the wikipedia article on this yet, but keep in mind that wikipedia is written by Joe Schmoe internet “experts” - they do have pretty good quality control, but not always enough. Not to mention they may be in another country with different standards. Go to the source whenever you can!
http://tidesandcurrents.noaa.gov/datum_options.html
not really sure how else to explain it.
Angler 204 FX
Yamaha 150