# MPA: THE CONCEPT OF GEOGRAPHICAL FEATURES/DATA

##### This unit represents geographical feature representation on a map

Types of Geographical Features/Data

Maps are produced using geographical or spatial data. The data give us locational and other useful information about the geographical features represented on the map. With respect to their geometric dimensions, geographical features are generally categorised as point, line, or area (polygon). Similarly, geographical data could be point, line or polygon. These geometric descriptions are the basic data elements of a map. Each of the geographical feature/data type is discussed below.

Points

They are represented as a single ‘dot’ on the map (Figure 2.0).

• Points are used to indicate discrete locations.

• They have no length or area at the given scale.

• They usually have a single X, Y coordinate.

• Used to represent a feature that is too small to be displayed as a line or area.

Arcs/Lines

Arcs are ordered sets of points that have the look of a straight line or a curved arc depending upon the feature it describes (Figure 2.1).

They are considered to have a length but no width.

They are accompanied by a set of coordinates.

They are used to represent a geographical feature that is too narrow to have area, such as a stream or a road.

Polygons/Areas

They are closed features whose boundary encloses a homogenous area (Figure 2.2).

They have an area that is given by the arcs/lines that make the boundary.

They are used to represent features that have area (e.g. lakes, large cities and islands)

Sources of Data for Map Making

There are various sources of data for map making. The sources, however could be grouped into two broad categories namely primary sources (hence primary data), and secondary sources (hence secondary data). Primary data are original data collected and used for the first time by the person using the data. On the other hand, secondary data is already existing data which was collected and used previously.

The major primary and secondary sources of data for map making include:

Traveller’s Note (e.g. data collected during a field trip or excursion).

Existing maps

Aerial photographs  (these  are  photographs  of  portions  of  the

Earth’s surface taken from aeroplanes)

Satellite images (these are images of portions of the Earth’s surface obtained using remote sensing satellites that are orbiting the earth in space)

Land survey (this is the method of physical measurement of distance, direction or height on the Earth’s surface).

Questionnaire survey

Official Statistical records

Field observation (this involves personal physical observation and recording of information about a phenomenon being studied)

Global Positioning System (GPS) (this is an electronic or digital device used to record information on the latitude and longitude of any location on the surface of the Earth).

Digital environmental information files (these are digital information about different aspects of man’s physical environment; such information is usually compiled by various agencies).

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