A map can display information about countries, rivers, roads, cities. On a paper map all this information is displayed as a single piece of information. In a GIS data such as country information is displayed on a separate layer. In fact within the GIS system a map often can consist of many layers depending on the complexity of the data.
Data displayed within a layer such as a city or river for example is known as a feature. This kind of data is also known as vector data. Vector data comes in three formats: points, lines and polygons. Vector data has both shape and size. For example, in a GIS cities could be represented as points. Roads and rivers could be represented as lines and land parcels, countries could be represented as polygons.
Apart from layers containing features GIS systems can also display layers with data in a form often referred to as surfaces. This type of data has a numerical value rather a shape. Surfaces consist of tiny dots called raster. Here each dot will represent a piece of information which could refer to depth, as in an ocean layer, or height as in an elevation layer. Each dot would have a numeric value which could be represented by different colours or shades to represent height, depth or some other value such as temperature.
Each feature on a map will have a location. To find a feature on a map the GIS uses a co-ordinate system. This is represented by a series of parallel and vertical lines which create a grid system. Within this system each point is represented by an X and Y co-ordinate, similarly a line would have at least two points and a polygon would have at least three points.
When you look at a paper map you know that the map is representing an area which significantly larger than the map is actually printed. In order to retain accuracy the map uses a scale to represent the real world. So a map of the world may be shown at a scale of 1: 1,000,000. A map of Europe could be shown at a scale of 1:400,000. In a GIS we can zoom in to an area of interest and this will affect the scale at which the map is drawn. Features will therefore display at different sizes depending on the scale at which we are viewing. To avoid a map becoming cluttered layers can be set up so that they are only visible when a certain scale is reached. In this way cities, roads and other information such as labelling can be seen without other features causing confusion.
Besides shape and location a feature such as a road or city can contain much more information. This data may include for instance population, income or crime statistics. All this kind of information is stored in a tabular form which can be attached to the layer. For each feature there is a record or row in the table and a field or column for each piece of attribute data. This information known as attribute data can be displayed in the map by clicking on the appropriate record in the table. Through attribute data it is possible to create queries which can be displayed on the map. For instance population density could be displayed within each country with varying depth of colour depending on size of population.
Relationships between various features can be shown. This enables queries concerning which features are near other features, which features cross or enclose other features. For instance which countries share a border or have lakes within their borders. By analysing data in this way it is possible to create new features from areas which overlap within other layers which meet specific conditions you have set.