REPRESENTATION OF THE EARTH

Representation of the Earth

            Maps are representations of all, or part, of the Earth on a flat surface. The study and practice of making maps is called cartography.

            We use maps in different ways: to locate a place, to identify geographical features, such as land relief or water, and to collect other types of information. All maps have the following characteristics:

     1.      They are reduced because geographical space is represented on a small surface. 

     2.      They are simplified because they only show certain features.

     3.      They take a standard form, using specific colours and symbols defined in a key.

LINES AND GEOGRAPHIC COORDINATES

            We draw imaginary lines on the globe called parallels and meridians to locate any place on the Earth's surface. To make map-making and map-reading easier, we use a limited number of lines separated by the same distance measured in degrees.

                        Parallels: Are lines drawn east to west around the Earth. Parallel 0º is the Equator. The other lines are numbered from 0º to 90º to the north and to the south. Other important parallels are the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle.

                        

Meridians: Are north to south semicircles drawn from pole to pole. The prime meridian or 0º is the Greenwich meridian, which is the reference for all the other meridians. These are numbered from 0º to 180º, both to the east and to the west of Greenwich.

            The geographic coordinates (the exact location of a specific point on the Earth) are called latitude and longitude.

                        Latitude: Is the distance between any point on the Earth's surface and parallel 0º (the Equator)

·         It is measured in degrees, minutes and seconds.

·         It can be north or south.

·         Its maximum value is 90º, which corresponds to the poles.

                        Longitude: Is the distance between any point on the Earth's surface and the 0º meridian (Greenwich)

·         It is also measured in degrees, minutes and seconds.

·         It can be east or west.

·         Its maximum value is 180º.

            Time zones

            The Earth revolves on its axis and takes 24 hours to make one complete revolution. This movement causes the progression of days and nights. When it is night-time in some places, it is day-time in others.

            In order to establish an official time in every country, the Earth's sphere (360º) is divided into 24 equal sections of 15º (24x15º=360º).

            These sections are called time zones and each one equals one hour. The time we use as a reference is called Coordinated Universal Time (UTC) formerly known as Greenwich Mean Time (GMT)

            As we move east from the Greenwich meridian, clocks move forward the same number of hours as time zones travelled. If we move west from the Greenwich meridian, clocks go back the same number of hours as time zones travelled.

            Some countries in two time zones have the same time for the whole country (e.g. Saudi Arabia, Argentina). Others have several time zones: in the United States, when it is 7 a.m. in New York, it is only 4 a.m. in Los Angeles.

           

           

Map projections

            Maps distort the Earth's surface because they are representations of a three-dimensional object on a flat surface. In fact, globes show the Earth's surface more accurately because they are spherical like Earth. We use a technique called map projection to make maps.

           

            Scale

            Scale is the ratio between a distance on the ground and its representation on a map. There are two types of scales:

§  A graphic scale is a line divided into equal parts that allows us to use a ruler to measure distances on a map.

§  A numerical scale is the relationship between a unit of distance on a map and the real distance. For example, a scale of 1: 100,000 shows that one centimetre on the map is equivalent to one kilometre on the ground. We use different scales:

·         Large scale, between 1: 5,000 and 1: 50,000

·         Intermediate scale, between 1: 50,000 and 1: 100,000

·         Small-scale, from 1: 100,000

How to measure distances on a map

§  Using a graphic scale, we first measure the distance on the map and then compare it to the scale.

§  A numerical scale shows the relationship between centimetres on the map and kilometres on the ground.

            Maps and geographic technologies

            There are several types of maps, including the following:

·         Topographic maps show the main natural and man-made features of an area, such as mountains, rivers and lakes, towns and roads

·         Thematic maps show a particular feature of an area, using symbols or colours. They can give information such as climate, economic activities, natural resources, political frontiers and population density.

·         Plans are very large scale maps of a small area, like a town or neighbourhood, and include a lot of detail.

            Other representations of the Earth.

            Computers and satellites have made possible to improve representations of geographic data. A Geographic information system (GIS) brings together many different types of information.

·         Aerial photography gives us images of the Earth from aeroplanes, while space images are taken from man-made satellites in space. We can use this information to see meteorological phenomena, or observe landscapes.

·         Remote sensing is the collection of information from a distance. It can help to predict natural disasters such as hurricanes. Aeroplanes and satellites collect information, which is then processed by a computer. It can be converted into digital images.

·         Virtual maps are created on computers. They can show great detail and may be three-dimensional.