Unit 03

MAPPING THE EARTH'S SURFACE

Unit Overview

This unit focuses on the transmission of geographical information through maps. The main sections pertain to:

  • The spherical Earth
  • Map projections
  • Map interpretation
  • Evolving cartographic technology

The Earth is a spheroid that can be divided into 360 intervals both vertically and horizontally. There are 360 degrees of latitude and 360 degrees of longitude. One degree is comprised of 60 minutes and one minute is comprised of 60 seconds. Imaginary lines can be drawn on the Earth that correspond to specific latitudes and specific longitudes. The lines of latitude are known as parallels and these lines are parallel to the equator up to the poles. Lines of longitude are known as meridians and these span the entire circumference of the Earth just as the prime meridian does. So, since the Earth is approximately spherical, latitude and longitude lines can be drawn around the globe to form a locational reference system. Latitude is a measure of the angular distance from the equator in the North-South directions, while longitude is a measure of the angular distance from the prime meridian in the East-West directions.

Even though the length of one degree of latitude does not change significantly across the globe, the length of one degree of longitude decreases substantially from the equator to the poles. The meridians converge at each pole, where the length of one degree of longitude is zero.

Representing a sphere, such as the globe, as a two-dimensional map is a frustrating task. No matter what projection technique is used distortion always occurs. Size, shape, direction, and proportion cannot all be preserved simultaneously in a projection, thus a map is an abstraction of reality: it not only disregards certain details but it also introduces distortion. The four major classes of map projections are cylindrical, conic, planar, and equal-area. Cylindrical projections are useful for navigation purposes because direction is preserved. Conic projections are suitable for showing the middle latitudes. Planar projections are used to show polar regions. Equal-area projections show all areas in correct proportion to one another.

Maps can also be used to display quantitative information. Points, lines, areas, and surfaces can show the spatial variation of phenomena across the Earth. Finally, remote-sense imagery coupled with geographic information systems (GIS) and digital cartography has enabled the development of extremely high-quality map products.

Unit Objectives

  • To introduce the reference system for locations on the Earth's surface
  • To describe the most important characteristics of maps and the features of common classes of map projections
  • To discuss the elements of map interpretation and contemporary cartographic techniques


Glossary of Key Terms

Cartography The science, art, and technology of mapmaking and map use.
Conic projection One in which the transfer of the Earth grid is from a globe onto a cone, which is then cut and laid flat.
Contouring The representation of surface relief using isolines of elevation above sea level; an important basis of topographic mapping.
Cylindrical projection One in which the transfer of the Earth grid is from a globe onto a cylinder, which is then cut and laid flat.
Equal-area projection One in which all the areas mapped are represented in correct proportion to one another.
Equator The parallel of latitude running around the exact middle of the globe, defined as 0° latitude.
Geographic information system (GIS) A collection of computer hardware and software that permit spatial data to be collected, recorded, stored, retrieved, manipulated, analyzed, and displayed to the user.
Interactive Mapping In geographic information systems (GIS) methodology, the constant dialogue via computer demands and feedback to queries between the map user and the map.
Isarithmic (isoline) mapping A commonly-used cartographic device to represent three dimensional volumetric data on a two-dimensional map; involves the use of isolines to show the surfaces that are mapped.
Isoline A line connecting all places possessing the same value of a given phenomenon, or ¿height¿ above the flat base of the surface being mapped.
Latitude The angular distance, measured in degrees north or south, of a point along a parallel from the Equator.
Legend The portion of a map where its point, line, area, and volume symbols are identified.
Longitude The angular distance, measured in degrees east or west, of a point along a meridian from the prime meridian.
Map projection An orderly arrangement of meridians and parallels, produced by any systematic method, which can be used for drawing a map of the spherical Earth on a flat surface.
Mercator projection The most famous of the cylindrical projections, the only one on which any straight line is a line of true compass direction.
Meridians On the Earth grid, a north-south line of longitude; these range from 0° (Prime Meridian) to 180° E and W.
Parallels On the Earth grid, an east-west line of latitude; parallels of latitude range from 0° (Equator) to 90° N and S (the North and South poles, respectively, where the east-west line shrinks to a point).
Planar projection One in which the transfer of the Earth grid is from a globe onto a plane, involving a single point of tangency.
Prime Meridian The north-south line on the Earth grid, passing through the Royal Observatory at Greenwich in London, defined as having a longitude of 0°.
Remote sensing A technique for imaging objects without the sensor being in immediate contact with the local scene.
Scale The ratio of the size of an object on a map to the actual size of the object it represents.
Standard parallel The parallel of tangency between a globe and the surface onto which it is projected.


Unit Outline

  • The Spherical Earth
    • Dividing the Earth
      • degrees
      • parallels
      • meridians
    • Latitude and Longitude
      • the Equator
      • latitude measures distance north or south of the Equator
      • the Prime Meridian
      • longitude measures distance east or west of the Prime Meridian
  • Map projections
    • Properties of map projections
      • scale
      • area
      • shape
    • Types of map projections
      • cylindrical projection
      • mercator projection
      • conic projection
      • planar projections
      • equal-area projections
  • Map interpretation
    • Use of symbols in mapping
      • point symbols map locations
      • line symbols represent linkages
      • area symbols map quantitative ranges
      • volume symbols describe surfaces
        • isarithmic (isoline) mapping
        • contouring
        • Evolving Cartographic Technology
    • Geographic information systems allow users to collect, store, analyze, and manipulate data to produce layered and integrated maps
    • Remote sensing allows data to be collected from high-altitude locations
    • Interactive mapping provides for dialogue between the user and the map


Review Questions

  1. Describe the properties of a Mercator projection, and why it is a valuable tool for navigation.
  2. What is a global positioning system (GPS) and what are its uses in the field?
  3. How is the Earth divided into a grid system of degrees, hours, and minutes?