Large, complex thematic datasets and web maps

The Guardian recently published an interactive web map (built by ITO World) of the deaths and injuries from road traffic accidents 2000-2010 in Great Britain. That's nearly 33,000 killed and 3 million injured people...represented as separate points on the map.

Viewing the overall picture at zoom level 4 (a scale of about 1:36m) we see the all too familiar mass of points on a web map with a legend that doesn't relate to the map we're seeing. Points overlap and coalesce. It shows very little other than the fact that there's an awful lot of death and injury. At this scale the data need aggregating in some sensible way. For instance, cluster analysis would spatially summarize the data and could be mapped using an isopleth technique for a more suitable small scale map. At any of the lower zoom levels (smaller-scale) the map is less useful still.

At zoom level 9 (about 1:1m) we start to see a little more clearly that the dots have different colours but they are all still the same shape. If we look a little more carefully in the background we can just make out some lighter symbols. What are these? Are they part of the basemap or part of the data? It's worth mentioning here that the underlying basemap labels are obscured by the dots. If they cannot be placed on top of the data then why have them at all?

At zoom level 11 (about 1:280,000) we see some symbols appear that resemble those in the legend but they are illegible. There are a lot of background symbols though...and with all that overlapping transparency the colours on the basemap are heavily compromised. Here then, we start to see that a neutral basemap in a single hue (e.g. light grey) containing very little detail would provide a more uniform background and allow the data to be seen more clearly rather than melt into the background.

It's not until zoom level 15 (about 1:18,000) we start to see the symbols as they appear in the legend but is it any clearer? There are 12 variations of the symbols and they all overlap. Actually that's not strictly true as yellow circles and triangles seem to be the lowest layer and blue circles and triangles and pink triangles the highest so the latter take visual priority even though they represent categorical data.






I've never been able to zoom in further than level 15 since it fails to draw any symbols beyond that scale but what can we take from this map in terms of its cartography? The importance of designing for specific scales cannot be underestimated. Putting a mass of points on a map simply doesn't work at most if not all scales. At smaller scales, data needs manipulating so it is in a form suited to a small scale thematic map type. At larger scales, symbols need to be simple and clear. That said, I like the map for one simple reason...it's one of the first I have seen that has attempted to show a very complex data set by type rather than the use of a single coloured generic marker symbol. At larger scales the symbol design is generally good and gives a mechanism to visually disentangle incidents by type, transport, age, date and gender. It's doing what cartography was designed to do..allow the map maker to take complex data and classify, symbolize and provide a picture so patterns can be seen that goes beyond what a table, graph or uniform point marker web map can provide. It's not perfect as I've pointed out but it's pleasing to see web maps begin to show signs of cartographic thinking and design.