NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
MHHW
From: Lu Abel
Date: 2016 Feb 17, 12:20 -0800
From: Lu Abel
Date: 2016 Feb 17, 12:20 -0800
While most NavList posts involve the "where am I?" part
of a navigator's tasks, "what's above and below me?" is certainly
one of them, too.
With the help of charts and a knowledge of the state of the tide, if we know where we are we can also ascertain how much water we have below us. To make it likely that we will have at least the charted depth below us, chart depths are referenced to Mean Lower Low Water.
Almost all areas of the United States have diurnal tides -- two highs and two lows per 24h 50m tidal day. On the east coast of the United states, those two highs are nearly the same as are the two lows. But on the west coast, there is a significant diurnal inequality -- one of the two lows is much lower than the other and one of the two highs is much higher than the other. (For example, a tidal reference station I use shows today's low/high peaks (in feet) as 2.54, 6.79, 0.06 (that's 2-1/2 feet lower than the previous low!), and 5.41 (1-1/3 ft lower than the previous high)
If charted depths were linked to the average low tide, there would be a significant amount of time when the water depth would be less than the charted depth. By linking to the average of only the lower of the two low tides -- Mean LOWER Low Water (MLLW for short) -- we have a lower datum and therefore fewer times when the actual water depth is less than the charted depth.
But in figuring out "what's above me, do I have clearance?," charted clearances are referenced to Mean High Water (MHW) -- the average of all high tides.
My question (after this lengthy preamble) is simple: is there a way to determine either from NOAA's tide tables or some other source, what the Mean HIGHER High Water is for a specific tide station?
For example, I live in the San Francisco Bay area. I suspect that most of the people here know how to determine the tides at any tidal substation by using the reference station (in our case, the Golden Gate) and offsets of time and depth for a particular substation. If I look at the Golden Gate tides in a commercial tide book, I notice that tides 5.8 ft and lower are printed in ordinary type and those of 6.0 ft and higher are in bold type, leading me to believe that MHW at the Golden Gate is 5.9 ft (can't find a 5.9 to confirm or any confirmatory info saying "MHW is X" Regardless, that's MHW not MHHW.
How can I determine MHHW???
With the help of charts and a knowledge of the state of the tide, if we know where we are we can also ascertain how much water we have below us. To make it likely that we will have at least the charted depth below us, chart depths are referenced to Mean Lower Low Water.
Almost all areas of the United States have diurnal tides -- two highs and two lows per 24h 50m tidal day. On the east coast of the United states, those two highs are nearly the same as are the two lows. But on the west coast, there is a significant diurnal inequality -- one of the two lows is much lower than the other and one of the two highs is much higher than the other. (For example, a tidal reference station I use shows today's low/high peaks (in feet) as 2.54, 6.79, 0.06 (that's 2-1/2 feet lower than the previous low!), and 5.41 (1-1/3 ft lower than the previous high)
If charted depths were linked to the average low tide, there would be a significant amount of time when the water depth would be less than the charted depth. By linking to the average of only the lower of the two low tides -- Mean LOWER Low Water (MLLW for short) -- we have a lower datum and therefore fewer times when the actual water depth is less than the charted depth.
But in figuring out "what's above me, do I have clearance?," charted clearances are referenced to Mean High Water (MHW) -- the average of all high tides.
My question (after this lengthy preamble) is simple: is there a way to determine either from NOAA's tide tables or some other source, what the Mean HIGHER High Water is for a specific tide station?
For example, I live in the San Francisco Bay area. I suspect that most of the people here know how to determine the tides at any tidal substation by using the reference station (in our case, the Golden Gate) and offsets of time and depth for a particular substation. If I look at the Golden Gate tides in a commercial tide book, I notice that tides 5.8 ft and lower are printed in ordinary type and those of 6.0 ft and higher are in bold type, leading me to believe that MHW at the Golden Gate is 5.9 ft (can't find a 5.9 to confirm or any confirmatory info saying "MHW is X" Regardless, that's MHW not MHHW.
How can I determine MHHW???
