How to figure Mean High Water on chart?

[seabreez]

Originally Posted by SV__Grace

Sigh... no, that is a potentially dangerous bit of misinformation, but I suppose it depends upon the chart or reference being used or the area of the world you are in.

Most of us in the U.S. use charts created and updated by NOAA and the digital charts like Navionics (which I use) use the NOAA data.

According to NOAA, as stated on Page 1 in the Coast Pilot -

Vertical clearances of bridges and overhead cables
are in feet above mean high water unless otherwise stated

Clearance for bridges are measured from Mean High Water (MHW). That is the minimum clearance you will ever have. You will always have more clearance than posted on the chart. No one organization would take the responsibility to be sued for wrongly posted the error of less clearance.

[Stu Jackson]

Originally Posted by seabreez

Clearance for bridges are measured from Mean High Water (MHW). That is the minimum clearance you will ever have. You will always have more clearance than posted on the chart. No one organization would take the responsibility to be sued for wrongly posted the error of less clearance.

The definition of MHW is:


Mean High Water (MHW) is a Tidal Datum representing the average of all the daily tidal high water heights observed over a period of several years. In the United States this period spans 19 years and is referred to as the National Tidal Datum Epoch.[1] The most current MHW values are found in the North American Vertical Datum of 1988 (NAVD 88).



Therefore, since it is an AVERAGE, the statement "...minimum clearance you will ever have..." is incorrect.

[requiem]

Originally Posted by seabreez

Clearance for bridges are measured from Mean High Water (MHW). That is the minimum clearance you will ever have. You will always have more clearance than posted on the chart. No one organization would take the responsibility to be sued for wrongly posted the error of less clearance.

I believe in the UK the charts use HAT (Highest Astronomical Tide), which matches your statement better. Where I am we have mixed tides, and MHHW is above MHW. The lesson? If it's close, simply avoid high tide.

But the general concept is good; as you and others like boatman61 point out, it works as an effective minimum in many cases.

[flyingfin]

MHW = a typical high, high tide during each year. So if you go under the bridge at low tide, you have more vertical clearance. Not too difficult. Find out tide using this predictor:

WWW Tide/Current Predictor - Site Selection

[StuM]

Originally Posted by requiem

The units must only be consistent, and it comes down to the definition of a radian. Consider a wide slice of apple pie where the length of the outside crust is the same as the other sides of the slice; the angle of the wedge is one radian (~57 degrees).

This property is why radians, and not degrees, are so useful for this; they are tied by definition to both the distance and the height.

Thus, the equation is really "X units (range) * Y radians = Z units (height)"

Note that the above equation is an approximation. It equates the arc of a circle with the subtension of the angle(essential the tan of a right angle triangle with an adjacent side of x units and an opposite side of z units)


It is a good approximation out to about 10° or 200 mils, much beyond that and the error becomes significant.

[Benz]

I have yet to encounter a tide table that doesn't tell me the height of each particular high tide in relation to MHW. It's in one of the columns, marked with a + or -. It is dead simple to calculate what the clearance of a bridge at that day's high tide should be by applying that figure.
There are other factors that can alter the height of tide, though: an onshore wind keeping water piled up inside the ICW, heavy rain runoff, etc.
I'm slightly amused by the poster who dissed the accuracy of chart data, then suggested using rangefinding binoculars by knowing exactly where you are on the chart....

[Firehoser75]

Grace,
First, if both depth under the bridge (water depth) and bridge clearance are concerns for your boat, maybe avoid that bridge? I would find going under a bridge with only a couple of feet to spare to be a risk I would try to avoid. Go another way, go at low tide, etc. etc.

Another option is get a "golf or hunting" style range finder. Motor up slowly to the bridge, stop if possible and sight up to the bridge for a clearance reading, having already determined the height of your mast from your viewing position (eye level from the fore deck). Depending where this waterway is (river, ocean, etc.) there are a number of factors that may influence the actual clearance, so calculating may be off a bit. If it is that close (a few feet), I would want to actually measure at the time and also take the other precautions I mentioned like going at a fairly low tide. As stated by Stu, the US system does not give the minimum clearance for all (or even most high tides). As an aside, in Canada our charts do (there will be a very few highs each year where the clearance would be below the one listed).
From your charts, you know the chart datum depth (charted depth). Using tide tables ensure that there is enough water (at the time you want to go through) for your draft, and go through before high water at a tide that is well below a "high high" and measure using the range finder giving time to reverse or turn if needed.
I would just try to avoid this situation.

[PILOT P]

Hi- all navigation charts have a small tide level reference table which refers to tidal heights at Spring Tides and Neap tides and their heights above Chart Datum
Depths on a chart are depths below Chart Datum
Heights on a chart are drying heights above Chart Datum

All other heights etc clearance height of bridges are from Mean High Water SPRINGS

Once you have the correct reference data - which should all be on the chart it’s easy math
Have fun
Cheers

[PILOT P]

See attached table and notes from local chart

[arsenelupiga]

Originally Posted by Benz

I have yet to encounter a tide table that doesn't tell me the height of each particular high tide in relation to MHW. It's in one of the columns, marked with a + or -. It is dead simple to calculate what the clearance of a bridge at that day's high tide should be by applying that figure.
There are other factors that can alter the height of tide, though: an onshore wind keeping water piled up inside the ICW, heavy rain runoff, etc.
I'm slightly amused by the poster who dissed the accuracy of chart data, then suggested using rangefinding binoculars by knowing exactly where you are on the chart....

it is not on chart but sattellite photo. One can assume position on chart/photo GPS is within 10m. As I have nothing else to do, will go thru example i used to cross from gold coast to moreton bay. Should be handy as we plan this route shortly.

if i position boat where is marked on the pic and plan to cross close to left tower (on the right side) then i know the distance is 1000m.

Look with binocular and can see that from water surface to the OH line is 3.5 bars.

I have formulae that i normalised /tested before with measurement error = 20% as binocular moves and gps error:

Height(cm) = Distance (m)*number of bars

= 1000m * 3.5 = 3500 cm = 35 m

As we could make 20% error, substract 20 % to be on safe side so we get

0.8*35m = 28m

So, one can go below if top of mast is less than 28m from water where marked. Mine is 21, so am good to go.

Lets not forget waters in river get swelled by floods, or industrial water discharge or weather phenomena that can increase water levels unexpectedly so i would no use chart depths to calculate.

Hope this clears

[Benz]

Originally Posted by arsenelupiga

it is not on chart but sattellite photo. One can assume position on chart/photo GPS is within 10m. As I have nothing else to do, will go thru example i used to cross from gold coast to moreton bay. Should be handy as we plan this route shortly.

if i position boat where is marked on the pic and plan to cross close to left tower (on the right side) then i know the distance is 1000m.

Look with binocular and can see that from water surface to the OH line is 3.5 bars.

I have formulae that i normalised /tested before with measurement error = 20% as binocular moves and gps error:

Height(cm) = Distance (m)*number of bars

= 1000m * 3.5 = 3500 cm = 35 m

As we could make 20% error, substract 20 % to be on safe side so we get

0.8*35m = 28m

So, one can go below if top of mast is less than 28m from water where marked. Mine is 21, so am good to go.

Lets not forget waters in river get swelled by floods, or industrial water discharge or weather phenomena that can increase water levels unexpectedly so i would no use chart depths to calculate.

Hope this clears

How far away from the bridge must you be for this to work with any accuracy? Often a bridge will be at a bend where getting far off is not possible. You come around and there it is.

[SV__Grace]

Thanks for all your responses and ideas. Some learnings and things to consider-

1. Aiming for low tide in a close situation makes sense, but let's not forget currents.

The situation I was in is a narrow pass with fast currents, and the best combination of tide and current to transit was at night time, not viable for us at that time. If we had no choice we could wait for more favorable conditions (such as waiting for a weather window) or do an overnight passage, which I've done in the past. Hey, we're full time cruisers and not in a hurry. Schedules and deadlines can be dangerous and we're glad we don't have them anymore.

2. Going around or avoiding the bridge is sometimes possible, and in this case it was, but it's a long haul and if we can make it under the bridge that would be first choice. The reason for my post was the difficulty I was having figuring out if I had enough clearance.

Thanks to this forum and some users smarter than I, now I am clear about clearance.

3. In today's world of the internet and digital charts and chart plotters, there is no need for most of recreational boaters in familiar cruising grounds to use paper charts and printed tide tables. And, some may have MHW info and some will not. Because yours does is not an indication that mine does.

You would think that using the latest B&G chartplotter (we have a Zeus3) and high priced navigational software (we have Navionics platinum) that this info would be at our fingertips, but its not included, a big omission.

Thanks to one post in this thread I now have bookmarked a page on the NOAA website that lists MHW for different locations. Had I known about this website I would have had the information I needed.

4. I'm a big believer in having the right tool for the job, and I had a good tool (binoculars with range finding feature) but didn't know how to use it. Thanks to the support of the smart folks in this community I now know how to use it and look forward to testing it in the future.

5. Being math-impaired the calculations for range finding was confusing, and finding an online calculator made all the difference for me.

Thanks for all your input!

[Adelie]

Originally Posted by PILOT P

Hi- all navigation charts have a small tide level reference table which refers to tidal heights at Spring Tides and Neap tides and their heights above Chart Datum

Depths on a chart are depths below Chart Datum

Heights on a chart are drying heights above Chart Datum



All other heights etc clearance height of bridges are from Mean High Water SPRINGS



Once you have the correct reference data - which should all be on the chart it’s easy math

Have fun

Cheers

Alas, not all charts have this table.

For instance there is no table on the chart for Mission Bay, San Diego, CA.
The info is on the Chart for Approaches to San Diego Bay and since they are usually printed together it’s usually ok provided you know to look there. But they are not always printed together.
Irony here is the MB has 3 bridges in the 31-38’ range and SDB has one at 195’ which is not listed on the approaches chart.

https://www.charts.noaa.gov/BookletC...okletChart.pdf

https://www.charts.noaa.gov/OnLineViewer/18773.shtml

[requiem]

Originally Posted by Adelie

For instance there is no table on the chart for Mission Bay, San Diego, CA.
The info is on the Chart for Approaches to San Diego Bay and since they are usually printed together it’s usually ok provided you know to look there. But they are not always printed together.
Irony here is the MB has 3 bridges in the 31-38’ range and SDB has one at 195’ which is not listed on the approaches chart.

Are those the three bridges on the inset of 18773 around "Coronado Cay" and "East Island", southern part of the bay? I do see the tide benchmarks on the far left side of the chart.

[Adelie]

Coronado Cays and East Island are in San Diego bay and are so low they don’t bother to list clearance.

The 3 bridges I posted about are in Mission Bay (about 5mi North of San Diego Bay) which shows on pg 9 of PDF of chart booklet 18765. On pg 21 is the table of tidal info. This table is generally not included when the inset for Mission bay is reproduced in other maps.