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 know-ledge was retained in some areas.

LATITUDE

side view

• 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)

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 DRAWINGDEPTH 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 DRAWINGDEPTH 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 DRAWINGDEPTH 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 10 200 1000 20 300 1500 50 400 2000 100 500 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

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-RNote: 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.