A GIS has one purpose: To allow you to quickly and easily answer questions about your data that relate to location.
The most common use of a GIS is is to produce data visualisations in the form of a map. The idea being that each visualisation is communicating something that might not be obvious from the raw data alone.
Let’s go ahead and look at some common examples.
The Category Map
The most common type of GIS map, and in many way the most simple is the category map.
A category map allows us to visualise which category each location belongs to. The example below shows sales territories, categorised by dealer. By looking at the legend we can quickly see which sales territories belong to which dealer.
Let’s Take A Look Under the Hood
A common misconception for those new to GIS, is that maps — like the category map above — are created in the same way that a graphic designer might create an illustration; i.e., starting with the outlines and then clicking on each area to change the colour based on which dealer it belongs to.
To an extent that is what’s happening, except the GIS is doing the work rather than you. You see a GIS is data driven, and we use rules to dictate the colour or appearance of each “feature” on the map.
JARGON ALERT: Feature
In GIS each location/thing on the map is generically referred to as a “feature”. So whether we are mapping States, Walmart outlets or rivers, each individual State, Walmart outlet or river can be referred to as a “feature”.
The rules — or “styles” as we call them in GIS speak — are based on data. For example our data looks like this:
Each row in the table represents a feature on the map. So we can tell our GIS that we want all sales territories that belong to Tom to be coloured red and our GIS will dutifully oblige.
In future if Tom gains more sales territories, we don’t need to touch the map. We just change the dealer value of a territory to Tom’s name and the map will magically update itself.
Update your data, and the map updates automatically!
The Quantity Map
Each feature in the map below is colour coded based on sales. The darker colour indicates more sales and lighter colours represent fewer sales.
This type of map is generally called a quantity map and if you are feeling particularly nerdy you can also call it a choropleth map. Sometimes this type of map will be referred to as a heat map, but this is incorrect terminology and shortly we will be looking at what a heat map actually is in GIS.
This type of visualisation is useful for identifying patterns and trends based on location and is most often used for business analysis and demographics mapping.
Just like the category map above, this map is data driven and uses rules based on a number “attribute” in order to style each feature.
JARGON ALERT: Attributes
To continue with the spreadsheet paradigm, if features are the equivalent of rows, then attributes are the equivalent of columns. For example, the “dealer” attribute for a sales territory could be Tom, Dick or Harry.
Creating the rules for quantities is a little more complex than it is for categories. Our legend will show a colour ramp with each colour on the ramp representing a number range.
Each colour on this colour ramp is commonly referred to as a class break. To define our class breaks we will need to tell the GIS two things; the first is how many class breaks we want and the second is is the number range for each class break.
Nerd Corner: Number ClassificationMANUAL CLASS BREAKS
When building a quantity legend there are lots of ways to decide the number range of each class break. Let’s take a look at the options that are available in most GIS systems:
This is the easiest, you just manually key in the number range for each class break. This type of class break is useful when there are pre-existing ranges that are used within your organisation for classification purposes.
EQUAL INTERVAL CLASS BREAKS
The range will just be divided by the number of class breaks.
Quantiles will adjust each class break to ensure that each class break contains exactly the same number of features.
This type of class ramp produces the most visually appealing map, as each colour in the ramp is used the same number of times on the map, but it can also be confusing for end users that don’t understand how the data is being presented.
The Jenks optimisation method, also called the Jenks natural breaks classification method, is a data clustering method designed to determine the best arrangement of values into different classes. This is done by seeking to minimise each class’s average deviation from the class mean, while maximising each class’s deviation from the means of the other groups.
In plain talk, it strikes a good balance between being pretty and sensibly distributed. If your data doesn’t fit nicely into equal intervals, then this or manual breaks is your best bet.
HEADS & TAILS
Now we’re going hardcore mathematics. Head/tail breaks is a clustering algorithm scheme for data with a heavy-tailed distribution such as power laws and log normal distributions. If you need this type of break then the chances are that you already know what it means. If not, you can safely ignore it.
Also known as a “graduated marker map”, a bubble map works in exactly the same way as a quantity map. But uses a range of bubble sizes rather than colours to visualise the variance.
A heat map is used in instances where our location data is so dense and tightly packed that we can’t make visual sense of it. Heat maps are ordinarily used when mapping “points”.
JARGON ALERT: Points, lines & polygons
There are three basic geometry types used in GIS: Points, Lines, and Polygons.
- A point is a single locations on the map represented by a coordinate such as a lat/lon position. Points are used to map locations when the boundary of the location isn’t important, such a pushpins on Google Maps.
- Lines are a series of interconnecting points. Lines are used to map linear features such as roads and rivers.
- Polygons are the same as lines except that the first point in the series of points and the last are always connected to form a closed loop. Polygons are used for boundary data such as country borders and property boundaries
A cluster map solves the same problem as the heat map. It’s a way to make sense of point layers that have a large number of features, but instead clusters nearby points together into a single point.
This cluster point usually uses a combination of colour, size and labelling to communicate how many separate points each cluster encompasses.
A GIS map is often made up of multiple layers. The map types that we looked at above can be stacked on top of each other into a single map, each map in that stack is referred to as a “layer”.
In most GIS systems each layer can be turned on and off in the map legend or moved up and down in the stack.
For example, in one map you might have one layer that contains the boundaries of each dealers territory, another layer that shows sales volume by county and another point layer showing the location of each dealers premises. The end user can then toggle each layer on and off as required.
In web GIS systems it is also common to use what is known as a base map. A base map is a map similar to the street maps and satellite maps that you are used to seeing on Google Maps. Base maps give your map layers additional context and act as supplemental data.
The layers on the map are only one half of the story. What separates a GIS map from a printed map is interactivity. The basic controls of most GIS systems work in the same way as Google Maps, which I’m sure you are already familiar with.
The map can be panned and zoomed to find an area of interest and then we can click on items on the map to find out additional information, in GIS this is called an “identify”.
An identify popup
JARGON ALERT: Identify
When we click on a feature in a GIS map and are presented with the attribute information in a table or popup for the clicked feature, this is referred to as an “identify”. As in, “perform an identify” or “I identified the feature”.
Map visualisations are just part of the story. What really separates a GIS system from a printed map is user interactivity.
As I said earlier, GIS is a very broad subject and there are some very niche GIS systems and tools, but let’s take a look at some of the tools that are common to most GIS systems, both on the desktop and the web.
Map Query Tools
A GIS query tool, or selection tool as it is often know, allows us to filter a dataset based on criteria that we define. For example we could say “show me all of the counties in which we made more than $50,000 in sales last year”, the GIS system would then highlight those features on the map.
Query tools allow maps users to do more than what the map maker envisioned when they styled the layers.
When making the map we will try and guess what data and visualisations the end user will find interesting and make sure we include layers that highlight those data points.
A query tool future-proofs our applications by allowing our users to build their own queries that we may not have envisaged when we first created the map.
In desktop GIS systems the query tools are usually very powerful and capable of building large and complex queries, the downside is that they also have a steep learning curve.
Web based GIS systems tend to have query tools that are more limited but much more intuitive, making them suitable for sharing with users that have no GIS experience.
This distinction is true across most features that are available on desktop and web GIS systems.
A desktop GIS is great for a power user that has time to invest in GIS learning activities, such as reading this book.
A web GIS system however, is a much better choice if you want to share your maps with a larger, more distributed audience. An audience which likely doesn’t have access to a desktop GIS system, never mind the time required in order to learn how one functions.
Printing maps is still a common task in GIS, especially on desktop systems.
It’s still common for companies to have a GIS department that’s responsible for creating maps and maintaining the GIS system. When non-GIS users require a map they will ask the GIS team to create a print or a PDF.
Slowly the internet is changing this workflow. It’s now becoming increasingly common for organizations to make their maps available to staff and clients via a user friendly web GIS.
This is preferred, as good web GIS systems don’t require any training or specific tools beyond a web browser.
Maps can be searched by address or by values in the attributes of their layers.
Search is an area web the web tools tend to outperform desktop GIS systems. The web systems offer Google style fuzzy search that can search across multiple layers and list the results based on the quality of the match.
Search on desktop GIS systems still tends to be much more clunky and involve first choosing the layer and column within the layer that you wish to search.
Measuring and Distance Tools
With any map it’s important to get a sense of perspective. Scale bars on a map let us know what distance those on screen pixels represent in the real world, and measuring tools allow us to accurately measure paths or areas on screen.
A web GIS measure tool
Is that it?
Nope, the tools listed here are just the tip of the iceberg, and will only give you a taste of what’s possible. Most GIS systems include a much larger range of tools.
And niche products will contain tools that are geared towards doing complex calculations based on geometry and the spatial relationship between features. We will be covering these features in more depth later in the book.
The Anatomy of a GIS Map
Every GIS program is different but they do tend to share some common DNA. Most GIS systems will have the following things in common:
Anatomy of a GIS map
The legend is where we make sense of what we are seeing on the map. The legend will list all of the layers and also show us the symbols and class breaks used in the styling of each layer.
- INFO WINDOW
Sometimes it is displayed as a popup, other times it’s show in a sidebar. The info window is used to display supplemental information, for example when you click on a map to identify a feature, the attribute data for the selected feature will be displayed here.
The toolbar will be displayed on the top or the sides of the interface. In desktop GIS systems these toolbars will be large and comprehensive, in web based systems they tend to be smaller and much more focused.