Being the founder of Mango, I think it’s fair to say that I spend more time looking at web maps than is probably healthy. In the course of a day I see the whole spectrum, from beautiful maps with interesting data and a clear message, right through to “Test Map” showing a few random points and everything in between.
One of biggest areas for improvement for most maps is the legend. Here’s my list of the three most common mistakes.
1. Not Changing the Labels in the Legend
Thanks to Shapefiles only allowing column names to be ten characters long, we in the GIS world have invented a new language over the years which allows us to express complex ideas in under ten characters, which is great. The problem is that people outside of the GIS community don’t speak that special language and have no idea what “popasian” or “langnoteng” mean.
Also the default name of a layer is the name of the file which you uploaded (if you didn’t choose another name on upload). Once again most users won’t have any idea what a layer called “us_tiger_cen_2011_v2” is.
When making a web map remember to give an alias name to your layers, classes and attributes, it will make you map much more user friendly and accessible.
2. Too Many Toggle Layers in the Legend
In a regular desktop GIS we can turn all layers on and off in the legend, we don’t have to worry too much about performance as the data is on our hard drive and being displayed as vectors by a program that has full use of our computers resources. A web map is quite different, it’s sending image tiles across the internet which are displayed in the confines of your web browser which only has access to limited resources.
To be able to switch items on and off in the legend in a web map we need to stack on tile layer on top of another to achieve the same effect. Every new layer in the stack effectively doubles the number of tiles that need to be downloaded to render the map therefore doubling the load time.
At MangoMap we get around that by offering layer groups, a layer group can be turned on or off in the legend but can contain more than one layer, all the layers in a group are rendered and sent across the internet as a single image which is much more efficient.
Layer groups are a great addition to a web map, but remember to use them sparingly in order to ensure fast load time and good performance.
2 layer groups = Fast
4 layer groups – Slow
3. Too Many Class Breaks
This isn’t a problem that’s unique to web mapping but in the low attention span world of the internet the problem certainly is compounded. Try to keep the number of classes in your legend to a minimum, that way there will be greater contrast between the colors and the visual impact will be increased.
3 class breaks = increased contrast
11 class breaks = confusion
As I discussed in my recent post Think like a web designer, not a cartographer, we sometimes have to make concessions in terms of traditional cartography in order to create a high impact map that instantly engages the user and prompts them to explore further.