<ost navigational charts reflect water depth at low mean tide. Does that also apply to bridge height? The chart indicates vertical clearance at 35 ft and horizontal clearnace at 70 ft. The 70 ft reflects the distance between the span. The vertical distance is measured from the bottom of the brige to the surface of the water, but at what level, low mean tide, hihh mean tide or some other tidal action. I need 34'5" clearance to avoid making a mess of my windex, mast head light. and possibly the mast itself. I can't find a reference on any chart for measurement other than water depth. So, can anyone tell me the water level from which bridge clearance is measured? Thanks Hayden
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coastal chart datum
Assuming we're talking about NOS charts here, although soundings (depth) are shown from mean lower low water on all coastal charts, height, including vertical clearance, is shown from mean higher high tide. I understand that the datum may be different in areas such as the great lakes where tidal influence is not a factor.
Assuming we're talking about NOS charts here, although soundings (depth) are shown from mean lower low water on all coastal charts, height, including vertical clearance, is shown from mean higher high tide. I understand that the datum may be different in areas such as the great lakes where tidal influence is not a factor.
Might I suggest you pick up. . .
a copy of Chapman's Piloting, and small boat handling. You will find this an invaluable reference. Chris s/v Tidesong
a copy of Chapman's Piloting, and small boat handling. You will find this an invaluable reference. Chris s/v Tidesong
minimum distance
The verical clearance on nautical charts list what is supposed to be the minimum clearance, which would be the clearance at high tide. Keep in mind that most bridges have a measurement board on the end of the fenders on either side of the span you should pass through. This will tell you what the clearance is when you approach the bridge.
The verical clearance on nautical charts list what is supposed to be the minimum clearance, which would be the clearance at high tide. Keep in mind that most bridges have a measurement board on the end of the fenders on either side of the span you should pass through. This will tell you what the clearance is when you approach the bridge.
Bridge Height references
It can depend on where you are & what chart you are using. NOAA Chart #1 says: "Heights -- Heights of lights, landmarks, structures, etc. are referred to the shoreline plane of reference. ..." "Shoreline -- Shoreline shown on charts represents the line of contact between land and a selected water elevation. In areas affected by tidal fluctuation, this line of contact is usually the mean high-water line. In confined coastal areas of diminished tidal influence, a mean water level line may be used. The shoreline of interior waters (rivers, lakes) is usually a line representing a specified elevation above a selected datum. ..." Always check the information on the chart regarding the datums used. --Ron
It can depend on where you are & what chart you are using. NOAA Chart #1 says: "Heights -- Heights of lights, landmarks, structures, etc. are referred to the shoreline plane of reference. ..." "Shoreline -- Shoreline shown on charts represents the line of contact between land and a selected water elevation. In areas affected by tidal fluctuation, this line of contact is usually the mean high-water line. In confined coastal areas of diminished tidal influence, a mean water level line may be used. The shoreline of interior waters (rivers, lakes) is usually a line representing a specified elevation above a selected datum. ..." Always check the information on the chart regarding the datums used. --Ron
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