Charts
A difficulty was encountered when the early cartographers attempted to
represent the earth’s spherical surface on a flat sheet. It can not be
done, of course, without distortion being introduced in some manner.
There are various methods of taking care of this error and one is
adopted for one certain purpose while another scheme is used in some
other work. These methods of caring for the error or distortion are
known as projections, the principal being the Mercator, the gnomonic
and polyconic. The Mercator projection is almost universally used for
navigational purposes; the gnomonic projection facilitates the use of
great circle sailing, and the polyconic is used for surveying sheets.
The Mercator chart represents the earth as though it were a cylinder
instead of a sphere.
If we take the skin of one-half an orange, and assume it to represent
the northern hemisphere of the earth, an attempt to forcibly bring it
flat upon a table will result in the tearing or stretching of the skin.
It can, however, be brought flat to the table in a regular uniform way
by cutting it in a saw tooth fashion from the stem (pole) to the edge
(equator), as shown in the diagram.
[Illustration: FIG. 13.]
The shaded portions represent actual earth’s surface and the blank
parts show the error introduced by using this method. In this form it
is useless as a chart, so the real parts are stretched or extended each
way to the dotted lines making a complete chart. It is now, however,
without a vestige of accuracy in representing the bodies of land and
water as they really exist. The result would be that if a round island
should be in the latitude of the top of the chart it would be stretched
into an elongated island lying east and west giving a very erroneous
inaccurate idea of it, as shown by the east and west shading. If,
however, the island had been on the equator where no east and west
stretching would have occurred the island would appear in its natural
shape, but the farther north or south it lies just in proportion to
the latitude will it be stretched in an east and west direction. Such
a condition will not serve the purposes of navigation and it becomes
necessary to extend the degrees of latitude, making them appear
longer and longer as the equator is departed from. This stretches the
elongated east and west island in a north and south direction and
brings it back approximately to its actual shape of a round island. If
there was a round island in 10° N. and a similar-sized similar-shaped
island in 50° N. the Mercator chart would show the northern one to be
almost twice as large owing to this artificial distortion. But its
relative shape would remain practically correct. It will be seen that
the latitude scale on the sides of the chart carries an increasing
value towards the north--on a chart where a degree is about ¼´´ long at
the equator it would be about ½´´ long in 60° N. or S.
A minute of latitude is equal to a mile on this scale, but it becomes
necessary to use it in the latitude in which the measurement is taken.
If a course runs N. 60° E. from latitude 30° N. to 40° N. and the
distance is desired, take at the middle latitude at the side of the
chart a convenient multiple of distance, say 30´, on the dividers and
step off the distance. Or the whole course can be taken off at once and
with the points of the dividers at equal distances north and south of
the middle latitude read off the number of minutes of latitude lying
between them.
In very high latitudes the Mercator chart is not reliable. The
distortion becomes excessive and bearings taken will not plot correctly.
All the meridians on a Mercator chart are parallel and cut the equator
at right angles. They all lie in a true north and south direction. The
parallels of latitude all lie east and west and are parallel to each
other and at right angles to the meridians. The degrees of longitude
on the globe grow smaller and smaller as the pole is approached due
to the actual convergence of the meridians, but as all meridians are
parallel on the Mercator chart the length of a degree must be shown
the same length at the top as well as at the bottom of the chart. In
just the proportion that the degrees of longitude have been lengthened
artificially beyond their true length must the degrees of latitude be
lengthened in each latitude. This amount is shown in Table 3, Bowditch,
reckoned as the distance in miles each parallel is from the equator by
the Mercator projection. Thus in latitude 40° N. the distance is 2400´
or miles, the table shows that in the construction of a Mercator chart
this parallel should be increased artificially to 2607.6. These are
called the meridional parts.
On a Mercator chart the ship’s course is represented by a straight
line and cuts each meridian at the same angle and is called a rhumb
line. For all practical purposes on short runs this rhumb line is the
best to use, but it is not the shortest distance between two points.
Should you be able in a course a thousand miles long, to see your port
of destination your rhumb line course at the outset would not head
your ship for it, but (in northern latitude) to the southward of it.
However, as you proceeded the ship’s head would gradually draw towards
the port and you would eventually arrive. What appears to be a straight
line on this chart is really a curve on the sphere of the earth. Your
line of actual vision is a great circle, and in order to follow such a
bee line you must constantly change your compass course (on a long run)
and describe a curve on the Mercator chart unless the ship is headed
north or south or east or west along the equator, in which cases she
is sailing on a great circle. The well-known Hydrographic Office Pilot
Charts are on the Mercator projection and show all steamship tracks as
curves, for they are great circles.
The gnomonic chart is based on a projection of the earth’s surface
upon a plane tangent to any chosen point which is to be the center of
the chart. The eye is assumed to be at the center of the earth looking
outward to the point of tangency. It will be seen that the surface of
the earth adjacent to the point of tangency will be very accurately
shown on the chart, but becomes distorted gradually from the center,
the sides of which show the land in such an unnatural shape that it is
hardly recognizable.
With a chart on this projection great circle sailing is much
simplified. The straight line between two points indicates the great
circle to follow, and the course and distance is obtained by following
the directions and illustrated example given on each chart. They
are constructed for the different oceans and are for sale by the
Hydrographic Office.
The course can be transferred to a Mercator chart by taking successive
positions from the gnomonic chart and plotting them according to
latitude and longitude, and joining by straight or curved lines.
In setting out on a voyage the port of destination could it be seen
ahead would indicate the great circle course, and in order to continue
to head directly for it, the course must be continually changed. While
in the North Atlantic bound for Europe the course must be changed
constantly to the east (right) in order to remain on the great
circle--the straight and shortest distance.
The course and distance can be computed by the form given in Bowditch,
in which two sides and an included angle are given to find the other
side and the (course) angle at the point of departure. The co-latitudes
of the points of departure and destination and the angle between
them at the pole, are respectively the sides and the included angle.
However, the gnomonic chart gives the course and distance graphically.
* * * * *
When a chart is purchased or received from the Government offices the
date of issue stamped upon it should be carefully noted. It can safely
be taken for granted that the chart has been corrected up to that date
and it is incumbent upon the navigator or master to seek in all Notices
to Mariners subsequent to this date for any that affect the chart. If
a Notice contains information requiring a correction the number of the
chart appears in boldface type. The alterations should be made neatly
with India waterproof ink, and if by the nature of the information
it is impracticable to make the changes a note should be made in a
conspicuous place.
Charts are printed from copper, zinc or aluminum plates and small
changes easily made by hand are not changed on the plate until an
accumulation of errata make it necessary, or sweeping changes of a more
extensive nature takes place such as a new survey, dredged channels,
etc. A chart under extensive correction is brought up to date in every
particular, including the latest geographic spelling, new docks and
public works. The date is noted on the right of the center margin and
the dates of smaller hand corrections are indicated at the lower left
corner; the figures denote the number of the weekly Notice to Mariners,
in which the information is found, and the year.
The different scales of charts range from those of the world to a
harbor plan. There are charts of oceans; general coasts, such as from
the St. Lawrence to below New York; intermediate coasts, as from
Eastport, Maine to Cape Ann; and approaches to ports, say from Cape Ann
to Cape Cod for the port of Boston; and lastly there are harbor plans.
Those covering large areas are known as small-scale charts while harbor
charts are called large-scale charts.
A chart depends on the surveys that furnished its data, and its
accuracy and reliability rests upon that survey. Even with the most
careful surveys, where the lead is used to ascertain depths, there are
many instances where pinnacle rocks have escaped detection by coming
between the casts of the lead taken by the surveying party. These
isolated rocks become points of great danger to vessels of deep draft,
and it becomes a measure of safety to avoid rocky coasts and offshore
patches by giving them a wide berth. Spaces devoid of soundings may
well be viewed with suspicion if in reasonably shallow water, for it
would appear to indicate a lack of thoroughness in the survey, at least
a lack of soundings. The wire drag, a device used to sweep important
areas to a certain depth, is the only sure way of discovering all the
dangers of the bottom.
The aids to navigation shown on the charts are described by symbols and
abbreviations as fully as possible with the limited space. All symbols
are placed in the location of the aid, but in some cases the actual
position may be in doubt by the nature of the symbol, for instance a
buoy’s location is denoted by the ring that accompanies the symbol and
not the triangle; a light vessel by the position of the dot of the
light, or between them if there are two dots (lights). Buoys and light
vessels often drag their moorings or go adrift entirely, especially in
the winter season. It is therefore the part of wisdom to check a ship’s
position by shore marks when possible and be prepared to find buoys out
of position. The mechanism of a light buoy is often disarranged through
various causes.
The characteristics of all lights are briefly given with the visibility
and height above the sea. The charted visibility is the distance they
should be seen from a vessel’s deck on which the height of the eye is
fifteen feet above the sea, so, from the deck of an ordinary power boat
a light will not be seen until well within the range of visibility
as published, while from the deck of a large steamer the light will
be seen outside its charted visibility. This refers to high-powered
lights where the curvature of the earth has to be given consideration.
A flashing light is one in which the flash is of less duration than
the eclipse, while an occulting light has an eclipse equal to or less
than the period of light. Flashes and eclipses are often grouped and
receive the name of group flashing or group occulting. An alternating
light is one in which two colors are shown each for an equal interval
with no intervening eclipse, but if an eclipse separates the color
flash from a white flash, for instance, it becomes a flashing white
light varied by a red flash. It is a very common practice to insert
sectors of different colors into the arc of visibility of a light in
order to cover a dangerous shoal or to indicate a channel. Bearings
defining these sectors are taken from seaward and not from the light.
The term luminous range will be met with, and indicates the distance
the power of the light can carry the visibility irrespective of an
intervening horizon. A light may have a luminous range much in excess
of its visibility which is limited by the horizon but in a haze or fog
its penetrating power will greatly exceed that of a light of similar
visibility but less luminous range. The power of a light is more
commonly shown by units of a thousand candle power, thus, 5.6 indicates
a power of fifty-six hundred candles. The catoptric (C.) light employs
the reflecting, and the dioptric (D.) the refracting principle.
There is a large amount of useful information given on every chart that
the average mariner allows to escape his notice. This failure on his
part is mostly due to familiarity, or reliance on pilots with local
knowledge.
The first important feature of a chart to be considered is the
shoreline, which is shown as a continuous line representing the
high-water mark. This, it must be borne in mind, is much changed at
low water, and where the range of tide is large the shoreline is
proportionately in error. Again, where the water is shallow the change
is more marked than where the shores are steep-to. If account is not
taken of the stage of the tide it is easy to be very much deceived.
In approaching a strange harbor the chart should be scrutinized for
prominent marks, and these identified as soon as possible, then the
lesser objects can be picked up by their relative positions with the
already identified landmarks. Among the lesser marks may be found
cliffs, boulders, sandy beaches, vegetation, buildings (particularly
church spires and houses with cupolas). Prominent elevations of land
always serve to identify a locality. The chart shows these elevations
clearly by contour lines.
A twenty-foot contour line, for instance, shows the line of the cut,
should the hill be sawed off twenty feet above the sea. When contour
lines are wide apart the land has a gradual slope, and as the grade
becomes steeper the contour lines come closer proportionately.
The chart, wherever possible, represents the earth as seen from
overhead, but in the case of vertical objects, they are of necessity
shown horizontally. One of the notable cases of this is in the
representation of cliffs, which in order to show their height are drawn
with the side view as seen from the water.
Numbers seen on the land show the height above the high water.
From the topographical features of a chart we turn to those of
hydrography. All the depths indicated on a chart are those existing at
mean low water, on the Atlantic Coast, and mean lower low water on the
Pacific Coast.
All the British Admiralty and most of the Hydrographic Office charts
are reduced to the level of low water at ordinary spring tides. While
there is usually more water to be expected than shown, it must be
remembered that when the plane of mean low water is used, the low
waters that, roughly speaking, come between the moon’s first and last
quarters, will fall below the soundings on the charts.
[Illustration]
[Illustration]
The effect of an abnormal barometer and high winds must at times be
borne in mind, for a continuous northwest wind will make a vital
difference in the depths along the Atlantic seaboard, especially in
the Delaware and Chesapeake Bays.
Very often there is information that can not be symbolized on the chart
and is placed in italics in the form of a _note_. These are always
important and should be read carefully.
Sailing directions are written to supplement the charts and preserve
for the mariner a mass of information which otherwise would not reach
him.
It is always well to pay attention to the current arrows, as they are
a means by which the strength and direction of the tidal stream may be
ascertained. The symbol of a tide rip should not be ignored by one in a
small boat, as the conditions might be right to make them dangerous.
On the water areas of the chart we find contours of depths as on the
land are contours of height. It is a good scheme, if one wanted to take
the trouble to run a contour line indicating a depth a few feet greater
than the draft of his boat, and tint the shallow water with a brush.
Large vessels of deep draft upon approaching the coast are guided by
the ten-fathom curve unless the shore is very steep-to and the water
very deep. The masters of such vessels would remain outside that curve
until their position was well established. Lighter draft vessels are
guided by the five-fathom curve in a similar manner.
When approaching the land and the landmarks are not available, the
character of the bottom further assists the mariner as he sounds slowly
towards the land. The kind of bottom is indicated by abbreviations
which are obvious on almost every portion of the charts.
The three-fathom curve is the most important to the greatest number
of navigators and for this reason is made the most pronounced. In the
majority of charts it is shown by a “sanded” area within it, but in
many new charts it is heavily tinted.
In changing from one chart to another while working in an unfamiliar
locality, take especial note whether the soundings are in feet or
fathoms.
It is an excellent practice when a vessel is brought to anchor and
cross bearings taken, to estimate the radius of her swinging circle by
adding the amount of chain out to the length of the vessel and with
this describe a circle on the chart and note if there is any danger of
tailing into shoal water at any quarter.
* * * * *
This has been a long voyage and I am glad to tie up and let the
printers take charge. If any of my readers see places where I have
stood into the shallow water of inaccuracy, I will be grateful for a
passing hail that I may shift helm and get out with as little damage as
possible.
INDEX
A
Acceleration for longitude, 91
Amplitude, 12, 97
Aphelion, 8
Apogee, 139
Astronomical triangle, 102
Azimuth, 12, 96
altitude, 98
Weir’s diagram of, 98
Azimuths and amplitudes, 93
C
Calendar, 64
Celestial latitude, 14
longitude, 14
Chart, contours, 155
corrections, 151
scales, 152
Charts, 146
datum, 155
diagrams of symbols, 156
fathom curves, 158
information on, 154
symbols, 152
Chronometer time, necessity of accuracy in, 87
Circles of equal altitudes, 110
Circum-meridian altitude, 82
Compass, 93
deviation of, 94
error of, 94
naming of, 99
Coordinates, 9
Corrections for observed altitudes, 68
D
Day lost and day gained, 37
Dead reckoning, 1
Declination, 15
daily change of, 24
Deviation, naming of, 100
Dip, 72
Double altitude problem, 120
E
Earth’s orbit, 8
revolution around the sun, 20
Eclipses, 141
Ecliptic, 14
Equal altitude circle, 109
Equal altitude method, 104
Equation of Time, 56
diagram of, 62
error due to obliquity, 60
Equinoctial colure, 26
Ex-meridian altitude, 82
bodies below the pole, 86
planets, 86
stars, 86
F
First point of Aries, 14, 26
G
Gnomonic chart, 150
Great circles, 10, 17
Great-circle sailing, 150
Greenwich mean time on navigator’s watch, 54
H
Horizon, rational, 11, 75
sensible, 12, 73
visible, 11, 73
Hour circles, 13
I
Index correction, 68
J
Johnson’s method, 123
diagrams, 126
K
Kepler’s Law, 56
L
Latitude, 78
by Polaris, 88
factor, 129
formula for finding, 80
Laying a course, 96
Lights, characteristics, etc., 153
Longitude, 101
and time, 102
by equal altitudes, 104
factor, 125
prime vertical, 103
stars and planets, 105
Lunar cycle, 143
M
Mean time into sidereal, 106
Mercator Projection, 146
diagram, 147
Meridian altitude, 78
time to observe a star, 81
to report at noon, 81
Meridional parts, 149
Middle latitude sailing, 4
Mizar, 89
Moon, 139
full, 140
harvest, 143
new, 140
Moon’s declination, 143
diagram, 144
nodes, 143
retardation, 142
revolution, 140
right ascension, 142
N
Nadir, 11
Napier’s diagram, 96
Nautical astronomy, 5
New navigation, 102, 129
advantages of, 133
diagrams of, 135, 137
finding position without plotting on chart, 138
illustrated examples, 134
Notices to mariners, 151
Nutation, 32
P
Parallax, 75
horizontal, 76
moon, 77
Parallel sailing, 3
Perigee, 139
Perihelion, 8
Plane sailing, 2
Planets, 6-7
Polaris, 88
diagram, 90
time to take sight, 92
Precession of the equinoxes, 27
causes of, 31
Prime vertical, 12
observations on, 103
Q
Quadrature, 140
R
Refraction, 71
Rhumb line, 149
Right ascension, 26, 50
S
Sailings, 3
Seasons, the, 20
Semi-diameter, 69
Ship’s time, 40
Sidereal time, 43
(local) how to obtain, 52
longitude from, 53
year, 66
Solar system, 5
Summer, length of northern and southern, 25
lines, as chords and tangents, 112
when under the sun, 121
Sumner, Capt., his experience, 107
line, displacement of, 118
tangent method, 114
value of, 115
method, 107
Sun’s change in azimuth, 120
eastward movement, 16
T
Time diagram, 51
Tropical year, 66
V
Vega as a pole star, 31
Vernal equinox, 20
dates of, 67
Vertical circles, 12
W
Weir’s azimuth diagram, 98
Z
Zenith, 10
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Transcriber’s Note:
Words may have multiple spelling variations or inconsistent
hyphenation in the text. These have been left unchanged unless
indicated below. Obsolete and alternative spellings were left
unchanged.
Words and phrases in italics are surrounded by underscores, _like
this_. Those in bold are surrounded by equal signs, =like this=.
Obvious printing errors, such as duplicate words, backwards, upside
down, or partially printed letters and punctuation, were corrected.
Final stops missing at the end of sentences and abbreviations were
added.
The following items were changed:
“access” to “excess”
“Manoeuvring” to “manoeuvering”
“effect” to “affect”