Five things you always wanted to know about GIS, but were afraid to ask
October 2009
GIS - it's a minefield, isn't it? We put some taxing questions to our specialist, Jake, to find out what it's really all about...“GIS” – is it just a buzz word for maps?
It is a “buzz word” but it’s much more than just maps. GIS is the creation and use of spatial data in a variety of ways, to allow in depth analysis. A map is only one of the ways in which spatial data can be presented. GIS can also use spatial data at a very fundamental level to perform calculations, analysis and to create hybrid datasets. For example, an Ordnance Survey map can be used by a walker to plot their route using a compass; GIS can analyse the terrain and path networks to find the most efficient route. It could also be used to find the most challenging route!
There seem to be GIS datasets for everything and in so much detail – I’m overwhelmed. How do I get to see just what I need on the screen?
It’s all about asking the right question. For example, you may want to find out which supermarkets are within a mile of a new development or the town centre, over 2000sq m in size and more than 10 miles from a motorway junction. You could manually plot all of that information onto a map to see and measure for yourself, or you could ask the GIS to only show the supermarkets that meet your criteria.
I’ve seen people opening up several GIS datasets at once. How is it that all the layers “know” where they are on the screen?
Everything on earth has a grid coordinate. When a GIS dataset is opened, it is aligned to this coordinate system and, as long as all the datasets are referenced to it, they should all match up. This data is said to be “georeferenced”. Having a real world coordinate system is a big advantage as it allows different organisations to create data that will work together, using real world calculations.
I had a spreadsheet that listed survey respondents within a town along with their postcode. How did I end up with a map showing where they all lived?
In this instance, the GIS used a process called “geocoding”. It compared and linked each of the respondents’ postcodes to an existing list that held the grid coordinates of every postcode, and overlaid them onto a street map. The geocoding process can be used with any two datasets that share a common value, as long as one of them has already been georeferenced. For example, a spreadsheet of house prices by ward can be geocoded to a set of ward boundaries.
I once asked our GIS staff how much of our district was covered by ancient woodlands. How did they do it so quickly and how accurate was it?
There are many GIS datasets, such as the ancient woodland dataset, that have been created and maintained over the years. All the ancient woodlands in a particular district can be identified and the total area calculated in real world units such as hectares. Combining the dataset with other information, for example population count, allows us to calculate how many people live within a certain distance of each of the ancient woodlands.
What do you do for your colleagues at RTP?
My core role is using GIS to provide support on our projects. This support varies as much in its type as it does in its intensity. Typically it is split between supplying cartographic support and some form of GIS derived spatial intelligence. Some examples of this are:
Housing: calculating the many thousands of spatial relationships between proposed sites and constraining factors.
Retail: building maps that allow detailed analysis of the socio-economic data specific to particular study zones and market catchments
Regeneration: accurate translation of data sets from one particular type of geography (eg wards) to another (eg parishes). I can also include other datasets, such as population or employment floorspace, to allow for wider analysis.
Much of the data I provide feeds into the overall evidence base for our work and getting it right from the start is crucial. By far the most important part of the work I do is ensuring that the information I provide is accurate and reliable.
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