Applying symbology to maps

When you first load a vector layer into your map it will be assigned a default colour. For instance, if you have a outline map of the UK, in a vector polygon format, it will load with a different colour each time you bring it into your map. Vector maps are built up from a combination of point, line and polygon layers. QGIS and ArcGIS only allow a layer to contain either point, line or polygon objects. MapInfo is different in that it will allow vector data for more than one object type to be present within the same layer. You can still import a MapInfo tab file, containing a layer with more than just one object type present, directly into QGIS. QGIS will check to see if more than one object type is present within the MapInfo layer and will then separate the various object types into separate point, line and polygon layers.

When you assign colours, sizes, patterns and transparency for example, to your vector data layers you are making your map more intelligible for the user. For instance, a point layer will normally default to a small filled circle which may represent airports, cities, trees or fire hydrants. To make a point layer more meaningful you could apply a specific symbol type which would be more relevant. For instance, airports could be represented by an aeroplane symbol. As far as possible you should try to have symbology which is representative of the features displayed within your map. So lines representing rivers are normally blue, railway lines are black with dashes through them and polygon shapes of land areas may have colour shading representing different types of vegetation.

Attribute data associated with the layer can be used to symbolise a map. For instance, as previously mentioned, data on vegetation could be used to symbolise a map with a different colour for each polygon area. This would then be able to show which areas of the map represent different types of vegetation based on the attribute data.

The same basic outline layer of a country could be used to create many different kinds of maps depending upon any associated attribute data and the aims of the person creating it. There are many techniques available for customising maps by using symbol functionality.  This use of symbology, available within modern desktop GIS, enables the right message to be presented to the target audience.

If you would like to further explore how GIS could help your organisation jps services offers one and two day training courses in QGIS, ArcGIS and MapInfo.