Using GIS software

Geographic Information Systems (GIS) are important tools to display and analyse complex data.

There are a few starting points you need to consider to make the most of your GIS:

What does your business or organisation need?
What problem are you trying to solve? How are you expecting GIS to solve your problem?
What data do you have available? Addresses or locations perhaps?
How does digital mapping relate to your problem? Do you need to map street names, route networks and land contours for instance?
Do you know what would be the most appropriate GIS to meet your business need?
What skills and IT resources do you have, to manage and work with geographic data?

You need to clearly define your needs before choosing the right GIS and deciding how to use it. If you are a department in a larger organisation, you no doubt will need to create a business case. Answering the above questions will help you do this.

Getting started

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

Geospatial data includes attributes that could be determined from the original source material. Information 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 this feature to any other information about it in other systems, such as surface type, average width, status for gritting, or number of accidents in the last year. This can lead to very powerful applications.

Who can use 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 of the geographical data that can be interrogated, but all kinds of information.

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

For example, Ordnance Survey (OS) 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.

This kind of application depends 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 (TOIDs). These are unique 16-digit numbers applying to every feature in its large-scale database. TOID makes 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).

GIS software

What are attributes?

‘Attribute’ describes any piece of information about an object that can be stored in addition to its geographic properties.

For instance, a road may have a number, a name, a maximum width, a speed limit and so on. GIS can work with this descriptive attribute information and present it in an intelligent format – superior to what can be achieved by placing text on a paper map.

With GIS, you are no longer restricted by how many text descriptions can fit into the available space to convey information about the objects in an area. Tabular information can be stored about each of the objects just as in a database, so allowing an almost infinite array of attributes to be recorded. All GIS have simple tools that allow the interrogation of the features. Hence, by using your mouse to click on an object, a full set of attributes can be displayed without that information having to be on screen all the time. The object of interest can be identified by the visual map graphic and then that object can tell you its own attributes.

Attribute tables

Most GIS enable the user to view the data in tabular form without necessarily using map graphics at all. This is equivalent to using spreadsheet software. Often you may know the name of an object but not necessarily where it is – hence you can use the table to find the object and then switch to the map to see where it is.

The GIS forms a constant link between the attributes and the geographical properties of each of the features: you can get either one of these if you know something about the other. This is the basis of location-finding mapping services on the Internet: you can generate a map for any location because there is a data layer with a link between the postcode attribute and the geographical coordinates.

GIS can be used to link to any piece of information that may exist about an object from other systems.

Using GIS? Be selective

Features in GIS map layers can be queried by selecting and viewing those that satisfy particular criteria.

Data stored in tables can be difficult to digest all at once. It is necessary to filter out relevant sets of information corresponding to a particular group of conditions. For example, imagine you are visiting a city with which you are unfamiliar. You might have a map showing the city centre and the location of all the restaurants. With GIS a range of information about those restaurants could be stored – what type of food, average cost per head, etc.

The GIS can help you decide which restaurant you want to visit. You are in the mood for something a bit spicy, so you decide to select Mexican restaurants. The records from the table will be identified and any Mexican restaurants in the city will be highlighted on the map screen. You can refine your search further. You are on a budget, so you do not want the bill to be too pricey: you can therefore add an extra filter to the query by requesting a list of all Mexican restaurants with a medium cost. The display now shows just those records that fit your present needs and you can maybe choose which of the selections you want based on where they are in the city.

You can make this type of query as simple or as complicated as you like, as long as the data fields are there to interrogate. This ability is not unique to GIS software; many different types of information system will allow you to perform selections. However, only GIS can provide a visual representation of the location of the query results. Furthermore, GIS can apply geographical criteria to the selection filter such that objects are selected based on where they are.

Ways to use GIS

Geocoding

Geocoding describes another way of importing non-map data into the GIS such that its geographic properties can be identified and the records positioned in space. However, unlike the linking method in which the additional attribute table remains external to the mapped layer, when a table is geocoded it becomes a new map layer in its own right. Coordinate points are assigned to the geocoded table so that it can be used on its own to display the locations of the objects concerned.

Geocoding is often applied to address lists. Not many people know the National Grid coordinates of where they live so any list of people's addresses needs to be geocoded to load it into a GIS. Any company – a bank for example – which holds an address list of customers can geocode this information and instantly analyse their geographic distribution. This may reveal trends that the bank would otherwise be unaware of – areas of particularly high or low customer density – which may suggest reasons for the successful recruitment of customers.

The significance of structure

We know that to link environmental information about rivers we needed a river map layer possessing names as attributes; and to geocode a set of addresses we needed a geographic layer with the coordinates of postcode locations together with the postcode text itself.

In these examples the GIS must hold location and attribute data that corresponds to the physical objects that people want to analyse. This means that the structure of the physical object data in the GIS data must be attuned to the types of application that the system is meant for.

The thematic map

By bringing together data from a wide range of different sources, you can visualise trends in the data by creating thematic maps.

Unlike a paper map, GIS does not require every piece of information to be visible at the same time. It can also change the depiction of a particular object depending on the value of one of its attributes. This function is known as thematic mapping. You will already be familiar with thematic maps from atlases and geography textbooks. For example, a map of parliamentary constituencies shaded in different colours can show the number of seats held by different political parties. GIS can build this kind of map automatically from the data values (number of seats), and in many different ways.

Basic spatial querying

GIS can not only tell you what information exists about particular features in the map data but it can also analyse where things are in relation to each other.

The software can identify trends across a given area as well as performing specific queries. Such queries can select attribute data depending on its geographical location and then interrogate the attributes by performing calculations and statistical analysis. Selecting data based on the geometry of objects is known as performing a spatial query. An example might be, find all of the addresses within 1km of a supermarket or cinema.

Buffers

It is possible to analyse how close objects are to one another using a buffer. A buffer is a shape based on any other existing object (point, line or area) that can be generated by the GIS. The buffer object represents the total area within a certain distance of a given feature.

You can use the GIS to generate buffer zones and then identify all features that lie within a particular distance. For instance, you can select all addresses within a 500m buffer of a busy road and compare these with data about the incidence of asthma. By comparing both sets of data you can work out if there are statistically more asthma sufferers living in the buffer region than in the general population; helping you to analyse if the busy road is a likely factor .

Overlay operations

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).

For example, a farmer may require 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.

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. 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.

Network analysis

One of the most far-reaching applications of GIS is in network analysis. This is the mathematical processing of the geometry of a link/node layer, enabling the identification of all possible routes around that network, along with the distances and times involved. Put simply, this means that using an accurate road data layer, the computer can identify possible routes between two locations and calculate the shortest.

Drive-time analysis

With network analysis you can calculate drive times – how far you can travel in a certain amount of time.

A typical drive-time map – for a pizza delivery for example – would show a central point surrounded by a series of circles. This assumes an as-the-crow-flies route in each location, to estimate how long it takes to get to places within that radius.

GIS can be much more accurate – network analysis can be used to generate isochrones (lines that join up points of equal travel time). This takes into account the true road network, providing a true measurement of how far you can get over a set time.

This method can even take into account the average speed on each road, so that the area appears stretched along faster roads.

Many different organisations use this kind of drive-time analysis to plan their operations, from the siting of new stores to the planning of distribution networks.

Optimum-path analysis

Network analysis doesn't have to be carried out on vector link and node data. Raster data can be effective in describing continuous varied surfaces. This quality of raster data is useful for identifying the optimum path (path of least resistance or shortest path) through a continuous surface.

For example, a company needs to erect electricity pylons from A to B. They need to make sure that they disrupt the forest areas as little as possible. The GIS calculates the path of least resistance, by finding the path that adds up to the lowest value. A vector line can then be added to show the location of the proposed route.