GIS Files 4: Putting it all in a system

4.3: What happens where? - The power of the spatial query (3)

Overlay operations

As well as drawing simple shapes and calculating buffers to select objects, it is possible to place layers on top of each other (remember we looked at combining layers in chapter 2, section 2.5: Looking at multiple layers) and select all objects from one layer that lie within an object from another layer.

Chapter 2, section 2.5 - Looking at multiple layers

The tricky but important bit...
A key advantage of being able to layer data in a GIS is to carry out overlay operations. These can be quite complex, but simply mean combining layers of data to create one new layer (similar to the way a mathematical sum or calculation creates a new value or answer).

In the example below a farmer needs a certain level of rainfall and a type of soil to successfully grow a crop. By combining the rainfall map and the soil type map it is much easier to find the best location. In this example the GIS assigns a numeric value to each soil type and to the amount of rainfall. This makes it easier, on the resulting map, to see where the optimum growing area is located.

The point is that by combining layers of information, the farmer creates a new map that is much more useful.

Overlay operations are particularly effective when using raster data. As discussed in chapter 1, raster data is good for representing the continuous varied surface of the earth, whereas vector data makes assumptions that the edge of an area has a defined boundary. An example of a vector overlay can be seen in the graphic above. The soil type areas are clearly defined, whereas in reality we know where two soil types meet they often gradually blend into each other. Another good example is using aerial photography to analyse vegetation. It would be very difficult to draw onto a photograph where one area of vegetation stopped and another began: the vegetated areas would be blurred into each other and probably produce a speckled effect in the photograph.

GIS users must never forget that the result that a GIS provides is only as accurate as the data that was used for the query. Don't forget that rubbish in equals rubbish out.

Spatial querying is used in many different ways to help understand the world around us. Often the answer to a particular problem can only be unravelled by comparing two layers of information in a way that would be almost impossible to achieve without the GIS software. The next page looks at a much more specific GIS application that is increasingly becoming a part of everyday life.

One of the most dramatic successes of the GIS revolution has been the development of intelligent navigation systems using digital models of road networks: section 4.4: Show me the way to go.

< 4.3: What happens where? - The power of the spatial query (2) | 4.4: Show me the way to go (1) >