INTRODUCTION TO MAPS AND
CHARTS
GEOL 1033,
General Oceanography
Review Lesson 3 in the Study Guide
Eratosthene's Map of the World (3rd Century BC)
•
The world, according to a chart from the third century B.C.
Eratosthenes drew latitude and longitude-like lines through important
places rather than spacing them at regular intervals as we do today.
The Alexandrian perception of the world is reflected in the size of
the continents and the central position of Alexandria.
6th Century AD Roman Map of the World
•
The world, according to a crude Roman chart from the 6th century A.D
(by Cosmas Indicopleustes). Eratosthenes’ map, 9 centuries earlier,
shows how much Greek knowledge had been lost by much of the western
world during that time period.
World Map
~700 AD
•
World map ca. 700
A.D. attributed to
St. Isidore.
Demonstrates Greek
geographic knowledge was retained
in some areas.
LATITUDE
•
•
•
•
•
Need a standardized grid reference system
Parallels = lines of latitude
0° = equator
90° N & 90° South = rotational poles
Each degree:
– Divided into 60 minutes
– Represents ~60 nautical miles
• Each minute:
– ~1 nautical mile
– ~6 076'
– ~1 853 m
• Deviations (little):
– Equator to poles
– Earth is not a
perfect sphere
(oblate spheroid)
side view
LONGITUDE
• Meridians = lines of longitude
– 0° = prime meridian = Greenwich, England
– 180° East & 180° West to the International Date Line
in the Western Pacific Ocean
– Meridians converge toward the poles
– Therefore, distance represented by a degree decreases to zero at poles
– 1° longitude only equals 60 nautical miles at the equator
• Time zones:
180° E + 180°W = 360°
360°/24 h = 15°/h =
= a time zone
MAP PROJECTIONS
• There are many kinds of map projections
– “Project” the curved surface of Earth onto a flat paper
– No one flat projection does everything you want from a
spherical surface.
– Each kind has its advantages and disadvantages
– Compromise or use more than one kind of map.
• Some common types:
–
–
–
–
Mercator projection
Conic projection
Polyconic projection
Gnomonic projection (= “Great circle chart”)
MERCATOR PROJECTION
• Mercator was a European map maker
• In 1569 he invented the map projection that bears his name
• On it, he used lines of latitude & longitude as reference lines,
producing a grid system
MERCATOR PROJECTION
• High latitudes are distorted, so there is no simple distance scale
EARLY OCEANOGRAPHIC CHARTS
• Early maps, such as Mediterranean Sea charts:
– Had no latitude
– Had no longitude
– But, had:
• Distance scale
• Compass "rose"
– Ignored sphericity of Earth, but OK for small map areas
• Ptolemy (~200 A.D.) is credited with introducing
– Latitude
– Longitude
– But not widely accepted for ~1 800 y
MERCATOR PROJECTION
• Advantage of this map projection:
– Any straight line (a rhumb line) is a line of constant compass direction.
– Therefore, it has become a standard map projection for navigation.
• Disadvantage:
– Straight lines do not necessarily represent the shortest distances between
two locations.
– For a line to represent the shortest distance beween two points, it must lie
on a "great circle" of a sphere.
• A great circle is a circle whose plane passes through the center of a sphere.
• For example, equator and all meridians are great circles on Earth's surface.
CONIC PROJECTION
• Shows less high
latitude distortion than
Mercator projection.
POLYCONIC PROJECTION
• Conic projection still distorts high latitudes and distorts low
latitudes.
• Least distortion where cone is tangent to globe.
• Polyconic projection has many cones of different apical angles
joined to reduce these distortions
• Only “tangent strips"
are used
• Tangent strips joined
to make map
• Errors are cut into
unimportant regions
GNOMONIC PROJECTION
• Any straight line is on a great circle
• Also called a "Great Circle Chart"
• All straight lines between any two
points represent the shortest
distance between these two points
• Projected from the center of Earth
to a plane tangent at any point on the
surface
• Polar projections are common
because meridians radiate from
the pole and parallels are
concentric to each other
Stopped here on Thursday
• So far, we have reviewed the concepts of
– Latitude
– Longitude
• Then, we discussed the following map projections:
–
–
–
–
Mercator
Conic
Polyconic
Gnomonic (Great Circle Charts)
MAPPING SEAFLOOR TOPOGRAPHY
• Perspective diagrams & drawings are common, especially for
– Non-technical purposes
– When large areas are represented
• Terms:
– Elevation & depth are relative to sea level as a datum
– Relief is the difference between highest & lowest elevations/depths
MAPPING SEAFLOOR TOPOGRAPHY
• Contour maps
–
–
–
–
–
Are more accurate than perspective diagrams
Contour lines are lines of equal depth
Contour lines are labeled as to depth represented
Contour interval is usually constant, but should vary if data is sparse
Contour patterns:
• Assume uniform slopes
• Adjacent contours tend to parallel each other
• Close spacing = steep slope
• Wide spacing= gentle slope
INSTRUCTIONS FOR DRAWING
DEPTH CONTOUR LINES ON A MAP (Ex. #3-R)
• Contour lines can be estimated easily by interpolation between points of known
depth.
• Interpolation is used to find the spacing of a number of contours or important
intermediate depths on an assumed constant slope.
• In the sketch (below, repeated on next slide, & p. 20 of Study Guide),
a straight guideline is drawn lightly in pencil between each pair of control
points (= known depths) where a contour line on the map will be intersected.
INSTRUCTIONS FOR DRAWING
DEPTH CONTOUR LINES ON A MAP (cont.)
• Important depths nearest to the control points are estimated and marked on
the guidelines.
• Intervening important depths or contours are estimated, evenly spaced along
the guidelines, and marked.
• Now you draw your contour lines by smoothly connecting those depths chosen
for your contour lines, e.g., 500 m and 1 000 m, as below
• Do not forget to label your contours.
• Erase the guidelines.
INSTRUCTIONS FOR DRAWING
DEPTH CONTOUR LINES ON A MAP (cont.)
• On the E3 answer sheet, a portion of several contours, e. g., the
1,000-fathom contour, have been drawn to assist you in getting
started.
• Keep your lines smooth and label them as you work to avoid
errors & omissions.
• Bottom features will become apparent as you draw.
Required
• Suggested contour lines (in fathoms) to be used are listed below:
shallow water
intermediate depths
10
200
20
300
50
400
100
500
(If appropriate, i. e., if the depth is applicable.)
deep water
1000
1500
2000
etc.
• An uneven contour interval is used to indicate the uncertainties
associated with fewer data points in deep water.
Example 1 of Exercise #3 in the Study Guide
(A Different Exercise (#3-R) has been handed out)
Example 2 of EXERCISE #3
• Use these
slides to
correct your
copy if some
of the depth
numbers are
too faint to
read in the
Study Guide.
The NEW Ex. #3-R
Note: A closed contour line will be in this
area, indicating a depression. Indicate
this with a series of “tick” marks on
the downslope side of the labelled
contour line
END OF FILE
Greenwich, England
• Old Royal Observatory,
Greenwich, England
• The Greenwich Observatory
has long been involved with
navigation and the
establishment of a
standardized worldwide
longitude network.
• The prime meridian, the
north-south trending line
along which the world is
divided into eastern and
western halves has been
defined as passing directly
through the observatory and
is marked on the side of
this building.