On Sunday 03 July 2005 00:43, john mercer wrote:
> Hello I was talking to a friend of my wife and i he did a lot of boating in
> Washington state until recently. I asked him if American Charts had meters
> on them for depths at all. He told me that the new Charts have feet fathoms
> and meters. I don't know what is listed first the imperial or metric. Is
> there anyone on this list who has access to marine charts or knows where i
> could go on the net to find out. I wonder about marine charts in the UK? my
> friend told me that as far as he knew it has been recently that US marine
> charts have given depths in meters. Take care John Mercer.
In my submarining days, we used "HO" (U.S. Hydrographic Office) and
"BA" (British Admiralty) charts; my experience with them dates from 1971. The
BA charts were in meters. HO charts were migrating to meters as charts were
updated and reissued. Recall, the mid '70s was a time when we had a major
push at the government level in the U.S. toward metrication.
I think that charts in the U.S. vary on the units used. I have in hand
at the
moment NOAA 11527 Ed. 16 which indicates depths in feet at mean lower low
water. This chart (1:20,000 at 33°01') has three distance scales marked on
it: in nautical miles, yards, and meters.
This chart was produced by the National Imagery and Mapping Agency
(NIMA). A
very tiny annotation indicates that the size of the navigational area of the
chart is "1099.6 × 707.3 mm".
I went to the website for the Office of Coastal Survey at
http://chartmaker.ncd.noaa.gov/
The Office of Coastal Survey is under the National Ocean Survey (NOS), which
is under NOAA in the U.S. Dept. of Commerce. At that link I found "NOS
Specifications and Deliverables" (2003 plus updates) which can be seen by
going to
http://nauticalcharts.noaa.gov/hsd/specs/specs.htm
This is a fairly intersting pdf formatted, 150 page document.
The third chapter ("Hydrographic Position Control") includes this
description
of the horizontal position accuracy requirements:
The NOS specification for hydrographic positioning is that the total
error in position of soundings, dangers, and all other significant
features,
at the 95 percent confidence level, will not exceed 5 meters + 5 percent
of the depth. This accuracy requirement is independent of survey scale.
The fourth chapter ("Tides and Water Levels Requirements") includes
this
description of the tidal information accuracy requirements:
The allowable contribution of the error for tides and water levels to
the total survey error budget falls between 0.20 m and 0.45 m (at
the 95% confidence level) depending on the complexity of the tides.
Similar specifications for other tidal parameters are also given in meters.
The use of the meter goes all the way back to the specifications of the water
level sensors:
For tidal range less than or equal to 5 m, the required water level
sensor resolution shall be 1 mm or better; for tidal range between
5 m and 10 m, the required water level sensor resolution shall be
3 mm or better; and for tidal range greater than 10 m, the required
water level sensor resolution shall be 5 mm or better.
I did find one dimension for a legacy sensor, described as having a 2" orifice
for a bubble stream intended "to minimize systematic measurement errors due
to wave effects and current effects". Figure 4.3 shows a page of the tidal
record for a station at San Francisco during July 1998 wherein the tidal
heights are given in meters to three decimal places, a resolution to the
nearest 1 mm. Survey benchmark descriptions such as this are shown:
The primary bench mark is set in the top of a 0.9-m (3') high
concrete seawall at the NW end of Crissy Field on the Coast Guard
property, 15 m (49') east of the NE corner of the crews quarters
building, 6 m (20') south of the south side of the garage building,
and 1.1 m (3.5') north of an angle in the seawall.
Some of the older benchmark elevation information (e.g., for an epoch ending
in 1978) is given in feet. A sample station recovery sheet for a benchmark in
Oregon gives driving directions from a stated intersection of two highways in
terms of kilometers and meters. The data blanks on the form allow either
meters or feet to be used and on this sample meters are selected.
Assured by this and much subsequent material that NOS works in terms of
the
metric system, I proceded on to chapter 8 ("Deliverables"). There I found:
All soundings on the preliminary smooth sheet shall be plotted in units
specified in the Hydrographic Survey Letter Instructions or Statement
of Work. The following conversion factors shall be used:
To convert soundings from meters to feet, 3.28084 ft/m.
To convert soundings from meters to fathoms, 0.546807 fm/m.
So, it sounds like our government works metrically but provides our citizens
with feet and fathoms on **at least some** charts. Ironically, the charts
themselves are specified metrically:
Approximate dimensions for the title block are a height of 15 cm and
a width of 20 cm.
Appendix 8 provides the color specifications for the depth contours for charts
with soundings in fathoms, feet, and meters.
The final answer is not here. Obviously, at least some coastal charts
are
done with depths in feet or fathoms.
Jim
--
James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE
http://www.cofc.edu/~frysingj
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