GIS Files: Adding real-world information

3.2: GIS can tell you everything worth knowing about anything

Once a feature is loaded into a GIS, any piece of information about that object can be linked to it. How does this work?

When you start with geospatial data you often only have attributes that could be determined from the original source material. Information gleaned from the original map might, for example, show a line feature as an A road, numbered A11. However, the GIS can be used to link to any piece of information that may exist about the object from other systems. This can often lead to very powerful applications of GIS.

Any organisation which holds information about geographical objects can load that information into a GIS as long as they have some map data containing the relevant objects. Therefore it is not just the attributes that come with the geographical data that can be interrogated but any other item of information known about the object.

For this to work it is necessary to have some kind of common referencing system so that the correct record in the geospatial data can be matched with the corresponding record in the non-geospatial data.

For example, Ordnance Survey holds a table of spatial information about rivers (name, location, length) and an environmental group holds a table of environmental information about rivers (name, sourcetype, nitrate pollution, flow rates). The two tables both contain the name of the river, so the two tables can be joined using the name attribute as the common reference. The joining of the tables gives the environmental information a spatial reference (location), so now the environmental information can be viewed on a GIS display.

Use the graphic and links to run through this example.

Link 1 shows you the Ordnance Survey data about this river and a map showing its location.
Link 2 shows the Ordnance Survey data and the environmental data - notice the different information stored in each table and the common reference.
This shows all the information joined together and its location on the map.

This process has allowed us to link other attributes (environmental information) to the map.

This kind of application is very much dependent on the ability to establish links between the entities in the two sets of information. Often it is better to use a numerical referencing system understood by all users of a particular type of information, so that the specific features can be identified unambiguously. If you just use text names this can fall down if one set of information has a misspelt name or if there are duplicate entries. There are, for example, many stretches of river in Britain with the name attribute River Avon.

Ordnance Survey has developed its own common reference system using millions of Topographic Identifiers (TOID^®s). These are unique 16-digit numbers applying to every feature in its large-scale database. TOID will make it a lot easier for users to link, combine or transfer information quickly and efficiently. This system is part of a massive project known as the Digital National Framework™ (DNF™). We will cover the DNF in later chapters.

Once the GIS is populated with feature attributes, the layers can be analysed in many different ways using queries and selections. We will now look at that in section 3.3 - Using GIS? Be selective.

< 3.1: The attributes of map features (2) | 3.3: Using GIS? Be selective >