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Latitude and Longitude
Introduction to Chart Navigation for
“THE CHARLESTON HARBOR CIVIL WAR
BATTLE MAP”
There is general acceptance that the
available navigational information and data used to develop this graphical
chart, to position shipwrecks and general battle areas is far more accurately
presented than the navigational means available to the subject matter of the
char
t.
All
locations on Planet Earth can be described by two numbers--its latitude and its
longitude. If a ship's captain wants to specify a position on a map, these are
the "coordinates" they would use. Knowledge of the harbor and it’s history with
its unchanging landmarks and recent verifications of facts assisted by the
availability of information on the internet made this map possible.
The technical aspects go back to the basics. There are two
angles,
measured in degrees, "minutes of arc" and "seconds of arc." These are denoted by
the symbols (°,',") e.g. 32° 43' 9"N
" means an angle of 32 degrees, 43 minutes and 9 seconds (do not confuse this
with the notation (', ") for feet and inches!). A degree contains 60 minutes of
arc and a minute contains 60 seconds of arc.
DEFINITIONS:
Latitude: Measurement of distance in degrees north or
south of the Equator; from the Latin latus, meaning "wide".
Longitude : Measurement of distance in degrees east or
west of the prime meridian; from the Latin longus, meaning "length".
How
To Convert a Decimal to Sexagesimal
You'll often find degrees given in decimal degrees
(121.135°) instead of the more common degrees, minutes, and seconds (121°8'6").
However, it's easy to convert from a decimal to the sexagesimal system.
Here's How:
The whole units of degrees will remain the same (i.e.
in 121.135° longitude, start with 121°).
Multiply the decimal by 60 (i.e. .135 * 60 = 8.1).
The whole number becomes the minutes (8').
Take the remaining decimal and multiply by 60. (i.e.
.1 * 60 = 6).
The resulting number becomes the seconds (6").
Seconds can remain as a decimal.
Take your three sets of numbers and put them
together, using the symbols for degrees (°), minutes (‘), and seconds (")
(i.e. 121°8'6" longitude)
longitude and
latitudes have been divided into degrees, minutes (') and seconds (").
There are 60 minutes in each degree. Each minute is divided into 60 seconds.
Seconds can be further divided into tenths, hundredths, or even thousandths.
Latitude Longitude Conversion
a. The number is positive so its a NORTH latitude.
So 32.71909722° is 32.71909722° N.
e. Multiply the remaining 0.145832' by 60 to get the
number of seconds: 0.145832 x 60 = 8.74992 seconds (8.74992").
b. Break up the 32.71909722° into 32° + 0.
71909722°. So the number of whole degrees
is 32°.
f. Round off: 8.74992 seconds becomes 08.75 seconds.
c. Multiply the remaining 0. 71909722° by 60 to get
the number of minutes: 0. 71909722 x 60 = 43.145832 minutes (43.145832 ').
g. So, the latitude is 32° 43' 08.75"
NORTH latitude.
d. Break up the 43.145832 ' into 43' + 0.145832 '.
So the number of whole minutes is 43'.
h. So the final answer for the latitude
is 32° 43' 08.75"N."
Symbols:°Degrees
'Minutes
"Seconds
Degrees, Minutes and
Seconds
DDD° MM' SS.S" 32° 43' 08.75"NThis is the most common format used to mark maps. It's also the most
cumbersome to work with. It's a lot like telling time… There are sixty seconds in a minute (60" = 1') and
There are sixty minutes in a degree (60' = 1°).
The
Charleston Harbor Civil War Battle map is taken from the format used
by Navigational charts. The minutes are divided into tenths of a minute giving
each mark a value of 6 seconds.
Keeping in mind a few easy conversions between seconds and decimal minutes will
help when working with maps that use degrees, minutes and seconds.
15 seconds is one quarter of a minute or 0.25 minutes
30 seconds is one half of a minute or 0.5 minutes
45 seconds is three quarters of a minute or 0.75 minutes
Degrees and Decimal
Minutes
DDD° MM.MMM'
32° 18.385' N 122° 36.875' W
This is the format most commonly used when working with
electronic navigation equipment.
Decimal Degrees
the format most computer based mapping systems display
DDD.DDDDD°
32.30642° N 122.61458° W
or +32.30642, -122.61458
The basic unit of latitude and longitude is the degree and
subdivisions of a degree. NOAA will also use a decimal point, such as 32.789°N
referred to as decimal degrees on their Chart of Obstructions. Decimal degrees
are an option on Global Position Systems (GPS) or with online topographic maps,
but decimal degrees are not used on printed maps. On these topographic maps the
latitude and longitude units are expressed in degrees, minutes,
and seconds. Each degree is subdivided into 60
minutes('). Each minute is divided into 60 seconds(''). The Seconds are tick
marked and divide the minutes into 10 parts giving each mark a value of 6
seconds (“)
The numbers on the left and right side of Charleston Harbor Civil War Battle
mapare latitude. (north is to the top of the
screen).
The numbers across the top and bottom of the map are longitude.
This section of the map shows the Longitude in minutes
at the top of the map, the Degrees run from 79°43’ 30” to 80°
(not shown)
on the right of the map is 32°, latitude (not shown) only the minutes are
showing. This map, which covers the area that is 32° 42’ to 32° 47’.
Note: that latitude and longitude is only shown on the
full version of the map. Along the edges of the map excerpt only the
minutes are written.
The map reader must realize that 43' latitude on this map is actually
32° 43', because 43' lies in between 32° and 33°.
Latitude and longitude is used to give the location of points on the map.
POINT
Latitude
Longitude
Explanation for Plotting Locations
Housatonic
32°
43' 08.75"N
79°
46' 34.74"W
Housatonic is actually shown further south
for clarity and was actually much closer to the Hunley. Some
reports state that she was well within 650 feet of the Housatonic. Map
Excerpt is the upper right corner of the Charleston Harbor Civil War
Battle Map so its coordinates are shown only in minutes. You can
determine where you are approximately by which direction the numbers for
latitude and longitude increase. Latitude increase going north on this
map so we are in the northern hemisphere. Longitude increases going to
the west, so this map is located west of the Prime Meridian.
Constance
32°
46' 31.80"N
79°
45' 41.29"W
To determine the location of Constance we
need to read across to the side of the map (to determine latitude) and
up to the top of the map to determine longitude. Constance lines up with
a fraction over 5 tick marks above 46’' and before 47', Each tick mark
between the minutes is valued at 6” (six seconds). The marks divide the
minutes by 10
Barge
Loaded with Stone from the Stone Fleet
32°
45' 48.03"N
79°
44' 53.29"W
This wreck is part of the second stone fleet and is
scattered over a large area.
Blockade
Runner “Rattlesnake”
32°
46' 25.63"N
79°
48' 12.29"W
Latitude does not line up directly with the 4th
tick mark ( 6” x 4 = 24”) but is slightly North. Instead we had to
estimate the additional 1.63” to get its location at 32° 46' 25.63"N.
Longitude is half a notch higher than the 2nd tick making it
12.29” to get 79° 48' 12.29"W
Remember we are reading longitude from East to West and each tick
between 48’ (minutes) and 49’ (minutes) is equal to one-tenth of a
minute or 6 seconds. one half a minute is 30 seconds or half way between
Blockade
Runner “Stonewall Jackson”
32°
46' 40.74"N
79°
48' 02.00"W
Solved the same as above. The only difference is
it is just seconds over the 48’ and appears very close to the shore of
Isle of Palms where the sand has always accreted and shifted. The
actual location of this wreck is in 18’ of beach sand. The plot line
should have actually been a little lower but got in the way of text.
The Confederate States Submarine H L Hunley
