basic chart question

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newsailor72

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Jul 20, 2011
175
1974 Macgregor 22 SoCal - dry storage
I just started learning to read charts using whatever I could find online. I learned some of the basic symbols/abbr, but I don't know what the circles (see short arrow) signify in the attached pic (it's not a compass rose). also the numbers throughout the water, do they represent soundings (see long arrow)?

thx
 

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Don S/V ILLusion

Don S/V ILLusion

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Sep 25, 2008
7,435
Alden 50 Sarasota, Florida
!. mooring areas and 2. depth at MLW (mean low water). Usually in feet that close to shore but the chart legend will indicate
 
W

Winkfish

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Jul 28, 2010
914
Boston Whaler Montauk New Orleans
The numbers are soundings, and somewhere on the chart it will tell you if they're in feet, meters, fathoms. etc. As for the circles, I can't make out enough of the writings and notes inside them to see what they are. There should be some kind of reference to look somewhere for what they mean. They look too perfect to be natural, maybe they're some kind of limited access area. What chart is it?
 
J

jimg

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Jun 5, 2004
175
catalina 27 dana point
Chart

Also, on that particular chart, those large circles are mooring areas for Very Big ships, not you or me!
 
Bill Roosa

Bill Roosa

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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
MLW (mean low water) is the most commonly encountered water level at the lowest low tide for that spot. It does not include the effects of wind, abienormal weather etc just the mathematical lowest water level you would expect to encounter over that spot.
Now bridges are another issue. they are measured at mean high water. So when you go under the bridge with 45' clearance that means that at the most common high high (not a mistake, two highs) tide you are assured to have 45' of clearance. Kinda makes sense. Can't really screw it up unless you actually try!!
Then there is that moon thing, it goes around about once a month and makes the water move around more or less depending on how much light comes off it. When new or full you get high high and low low tides, when it is waxing quarter or waning quarter you get "normal" high and low tides.
If you really want to blow your mind ask someone about vertical and horizontal angles and how lighthouses are described.
Welcome aboard
 
dlandersson

dlandersson

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Feb 16, 2011
227
Macgregor 26X Michigan City, IN
This is a very important chart: :D

http://www.ukoln.ac.uk/services/treasure/

newsailor72 said:
I just started learning to read charts using whatever I could find online. I learned some of the basic symbols/abbr, but I don't know what the circles (see short arrow) signify in the attached pic (it's not a compass rose). also the numbers throughout the water, do they represent soundings (see long arrow)?

thx
Click to expand
 
Stu Jackson

Stu Jackson

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Feb 26, 2004
23,047
Catalina 34 224 Maple Bay, BC, Canada
Priceless - what a great reference source! :)

I'm agonna print sum of 'em out and hide 'em all over the boat...er, I mean ship!

Jack, me boy, Jack, where are ye? Arrgh...
 
J

JohnS

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Sep 25, 2008
177
Islander (Wayfarer/McGlasson) 32 St Georges Harbor
Bill Roosa said:
MLW (mean low water) is the most commonly encountered water level at the lowest low tide for that spot.
Click to expand
This part of the definition used to confuse me (and maybe I'm still confused?). If I understand it correctly:

In most areas there are two low tides per lunar day (~25 hours), one lower than the other. By "lowest low tide", they mean the lower of those two tides on any given day. (The other low tide I guess would be the "highest low tide".) So in calculating MLW, the "highest low tides" are ignored, and the "lowest low tides" are averaged.

And as Bill stated, the actual "lowest low tide" on any given day can be a fair amount lower than MLW due to new or full moon and/or strong offshore winds.

I assume the same must be considered with bridge heights and MHW.
 
JoyRide28

JoyRide28

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Nov 2, 2010
114
Precision 28 Ashland, Oregon
Could the circles represent specific, numbered mooring/anchoring locations??
 
Stu Jackson

Stu Jackson

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Feb 26, 2004
23,047
Catalina 34 224 Maple Bay, BC, Canada
JohnS said:
This part of the definition used to confuse me (and maybe I'm still confused?). If I understand it correctly:

In most areas there are two low tides per lunar day (~25 hours), one lower than the other. By "lowest low tide", they mean the lower of those two tides on any given day. (The other low tide I guess would be the "highest low tide".) So in calculating MLW, the "highest low tides" are ignored, and the "lowest low tides" are averaged.

And as Bill stated, the actual "lowest low tide" on any given day can be a fair amount lower than MLW due to new or full moon and/or strong offshore winds.

I assume the same must be considered with bridge heights and MHW.
Click to expand
John, not quite. Per Dutton's, the depth of the water is the datum plus the height of the tide for any given day. For a negative tide for any given day, the water will be less deep than shown on the chart.

The datum is for epochs, as described in the link below.

Think of it this way: the bottom is fixed, the tide tables tell you what changes every given day. At a zero tide height, the bottom will be what's shown on the chart. + tides or negative tides make it deeper or shallower than shown on the chart. Check to assure that your chart datum is the same as the tide tables.

West Coast has been using MLLW for years, the Atlantic Coast used to use MLW.

This should help: http://tidesandcurrents.noaa.gov/datum_options.html

Same thing for bridges, only opposite as you note.

Bill Roosa said:
MLW (mean low water) is the most commonly encountered water level at the lowest low tide for that spot. ...just the mathematical lowest water level you would expect to encounter over that spot.
Click to expand
Also, not quite.
 
Ross

Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
Mean = the statistically average result of all of the numbers applied to a given variable.
 
Charlie Jones s/v Tehani

Charlie Jones s/v Tehani

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Mar 1, 2012
2,182
1961 Rhodes Meridian 25 Texas coast
Interesting that three people have told him about Chart # 1. In one instance, back to back posts. Does no one read the previous answers?
 
weinie

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
I've noticed that more than one person mentioned to the OP to consult chart #1. Dont people read before they post?
 
ggrizzard

ggrizzard

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May 27, 2004
2,054
Hunter 30_74-83 Ponce Inlet FL
Sometime the OP doesen't understand what "Chart 1" means or that it is available from the NOAA web site or free from many chandlers.
Just sayin!.:dance:
 
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