Slide the Map to Compare

I love the slide control in these maps in this Urban Institute interactive. Washington DC - Our Changing City is a report on the population changes happening in Washington DC.

The interactive report includes a series of maps comparing population and demographic changes in the American capital between 2000 and 2010. Each map includes a slide control that allows the user to swipe across the map to compare the differences in dot maps from the year 2000 and 2010.

I've seen this type of slide control used before to compare before and after photographs. For example, I used a similar control to compare aerial imagery from before and after the 2011 tornado in Tuscaloosa. However this is the first time I've seen a slide control used to compare two different map layers.