Jerry’s top ten crime mapping tips

Some tips for crime mappers…

Tip 1: Include a scale bar. A map is all about geography, and what is the point if a map reader cannot tell how far one place is from another? Use sensible numbers such as 0 – 5 – 10 miles, and adjust your scale bar accordingly. Be careful when using automatic scale bars; they are rarely spot on first time. If you are presenting to an international audience, they will appreciate a map showing miles and kilometers. 5 miles is close enough to 8 kilometers for presentation purposes.

Tip 2: Include a North arrow. It may not seem much, but it takes up very little room, is easy to do and does help a few viewers. Some people say there is a limit to the value of North arrows. For example, it is probably not necessary to show a North arrow on a map of the of the US; but if in doubt, include a North arrow.

Tip 3: Simple and clear titles. Don’t forget a title for your map, and use a simple one that means something to a range of people. You never know who will use your map later on and may misinterpret what they are seeing (alas, I speak from experience). When deciding on a title, use the KISS principle (Keep It Simple Stupid!). Often the type of crime, the place, and the date range – is enough, but sometimes a more provocative title garners attention. An explanatory sub-title can be helpful.

Tip 4: Use color carefully. Color is a marvel – that should be used sparingly. Think about how appropriate your color use is, and use color for those things that you want to emphasize. Strong colored backgrounds tend to destroy any hope of seeing symbols on top of them. Try and use paler (more insipid) backgrounds to shade regions (if they have to be shaded at all) with dark or bold bright symbols over them. Don’t be afraid to experiment (and make improvements), but remember the maxim “less is more”. You might also want to view the companion web pages on color and presentations.

Tip 5: Really understand your data. Know what you are presenting, and understand the limitations of the data. This is especially true if you are presenting maps of data created by someone else. Having a map showing minute by minute burglary patterns is useless if your burglary data (like most) has start and end times hours or days apart. Repeat victimization is also a real consideration and most GIS will simply place one dot on top of another. Do something about this, or at least be ready to explain to your audience, either in a caveat or in person.

Tip 6: Use thematic mapping cautiously. Thematic mapping really simplifies what used to be a complex procedure, but the automatic settings used by most GIS still leave a lot to be desired. The automatic settings when making maps using ‘quantiling’ or ‘equal count’ for example, tend to end up with categories that use fractions that, while technically accurate, mean little to most viewers. Be prepared to customize them to more sensible values. GIS are also stupid in that they will let you make choropleth maps of things that should just not be mapped in that way. The automatic features should not be invoked without understanding your data and the thematic map processes. Graduated symbols such as circles should only be used to denote increasing values of something (e.g. value of goods stolen, or number of burglaries at a location), while area shading is preferable for showing proportional rates (assaults per 100,000 people) than raw rates (because the map can be skewed by different sized areas and therefore populations).

Tip 7: Legends are generally essential. A legend is essential if you have any type of shading or symbology. It will also help you remember what the map portrays months later. Use sensible numbers. 1 to less than 5 means something to most people. 1.000325 to 2.4352 is in the realm of nonsense, unless you have a very technical audience. In fact, I know a few technical people and they get offended by this. If you have complex numbers, you could always change the scale from ‘low criminal activity’ to ‘high criminal activity’ (or similar) and lose the numbers. The audience will appreciate it.

Tip 8: Caveats mean you are not lying. To make a map with a title saying; “Melbourne burglaries, 2014” implies that you are mapping all of the recorded burglaries. However it is still an unfortunate reality that geocoding rates are rarely 100% and you should tell the reader of the real rate, and any other caveats. This is especially the case if the geocoding hit rate is less than 85%. It saves embarrassing questions later, when someone points to an area of known burglaries that is featureless due to geocoding problems. You should append the caveat to the map itself, as maps and text can often be separated by others, either by accident or nefarious design.

Tip 9: Limit the information you show. As map complexity increases, a limit is reached beyond which map comprehension in the reader actually decreases. Sometimes it might be better to produce two or more maps instead of one monster that loses all meaning. A person can differentiate about 5 different types of symbol at a glance, and any more needs to be constantly checked back to the legend. Why put them through that? Also consider the function of additional features such as national parks, public toilets & railway lines. Are railway lines relevant and therefore necessary to your map? If you suspect that burglars are using them to gain access to properties then perhaps yes, but they are hardly relevant for a map of drug sale locations.

Tip 10: Check the map appearance in grayscale. If you map is a real success then it will be copied and disseminated – this is the real mark of success. Unfortunately until color copiers become standard you should run your map under a photocopier to see what comes out of the other end. This will give you an idea of what becomes indistinguishable or illegible after reproduction. Small, italicized text is particularly vulnerable, as are similar shades of color.