1. INTRODUCTION TO MAPS AND CHARTS
GEOL 1033,
General Oceanography
Review Lesson 3 in the Study Guide

2. 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.

3. 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.

4. World Map ~700 AD
World map ca. 700 A.D. attributed to St. Isidore. Demonstrates Greek geographic know-ledge was retained in some areas.

5. 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

6. LONGITUDE Meridians = lines of longitude Time zones:
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

7. 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”)

8. 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

9. MERCATOR PROJECTION
High latitudes are distorted, so there is no simple distance scale

10. 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

11. MERCATOR PROJECTION Advantage of this map projection: Disadvantage:
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.

12. CONIC PROJECTION Shows less high latitude distortion than
Mercator projection.

13. 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

14. 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

15. 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)

16. 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

17. 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

18. 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.

19. 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 m, as below
Do not forget to label your contours.
Erase the guidelines.

20. 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.
Suggested contour lines (in fathoms) to be used are listed below:
shallow water intermediate depths deep water
etc.
(If appropriate, i. e., if the depth is applicable.)
An uneven contour interval is used to indicate the uncertainties associated with fewer data points in deep water.
Required

21. Example 1 of Exercise #3 in the Study Guide (A Different Exercise (#3-R) has been handed out)

22. 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.

23. 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

24. END OF FILE

25. 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.