Help reading tide tables and charts

[Don]

This is a copy of todays and part of tomorrows tides in Olympia, Washington2003-01-06 8:41 AM PST High Tide 15.39 feet 2003-01-06 2:39 PM PST Low Tide 6.82 feet 2003-01-06 7:18 PM PST High Tide 11.75 feet 2003-01-07 2:15 AM PST Low Tide -0.36 feet 2003-01-07 9:17 AM PST High Tide 14.99 feet 2003-01-07 3:30 PM PST Low Tide 6.33 feet In reading tide levels.....are the given tides taken from a low tide reading and every reading is so many feet from that level? If thats correct then who determines low tide levels and are all charts 'reading out depths using the same 'low level reading'? If thats the case then am I correct in saying that all the depth reading on the chart will be correct except for a minus tide?Example: If at 8:41 AM High tide is 15.39 feet ...then reading the chart it reads 2 fathoms or 12 feet then I,m assuming I add 15.39 feet to 12 feet...total 27.39 feet. Then at Low tide follow the same process? And if its a minus tide just take the minus reading of say .036 and subtract it from the chart reading? We've been having some real high tides and some minus tide of 3 to 4 feet. I want to feel confident that I'm understanding the tide tables and that I'm using the correct method of calculations in reading the charts.......would appreciate any comments.......

 

[Gary Wyngarden]

TideCharts

Hi Don,Yeah we've been having some extreme tides up here as well. There's a complex answer and a simple answer to your question. The tide tables publish predicted tide levels above and below a level called "chart datum". This is a concept arriving at a level I believe called mean lower, low water which is an average of averages that one crusing guide writer up here, Al Cummings, has simplified into "as low as it usually gets."The simple answer, as you have already suggested, is that you take the published tide table figures and add or subtract them from the depth on your chart. In your example if the chart says 2 fathoms and we have a 12 foot tide, the water depth should be 24 feet. Or conversely you might have to subtract a few feet on the minus tides.Remember these are only predicted tides. Storms like the ones we've had recently can push the tides well above or below the predicted amounts and it pays to be watchful.Hope this helps.Gary WyngardenS/V Shibumi H335

 

[Les]

Tide (not the detergent)

Don,Everything you would ever want to know about tides and how to determine their effects is at this link. (http://www.irbs.com/bowditch/pdf/chapt09.pdf)Essentially you apply the - tide delta to the depth on your chart. The datum for the chart is listed somewhere in the notes on each chart but are generally either the mean low water (MLW) or the mean lower low water (MLLW) levels. Meaning, on the average, you can expect to find at least that amount of water at low tide. But, as MLW is an average of all low water readings at the average low tide and MLLW is the average of all the lowest water readings based on the average lowest tide, you could still find on a particular day with a very negative low tide, ie a big minus tide, that the water over a particular point on a chart could be lower that charted.In your case, if there was a point on a chart of Olympia that showed 10 feet. And if the -.36 feet fell inside the average low tide range used to figure the MLW or MLLW you would still have 10 feet of water over the point. However, if the -.36 feet was such an aberation that it wasn't used in the equation that figured the 10 foot depth then it actually would be less than a 10 foot depth.Also as the numbers refer to a particular area ie Olympia and if you are somewhere else you have to interpolate the tidal difference. And it will not be consistant from one day to another. I would subtract the minus delta from the chart reading and make sure I still had a margin for error.Les Andersens/v Mutual Fun

 

[Don]

Appreciate the info

Thanks Gary and Les. Thats what I wanted, was a confirmation. Appreciate you both taking the time to explain it more accurately.