Database Reference
In-Depth Information
Chapter 10. Maps and Location
“There's just something hypnotic about maps.”
—Ken Jennings, 74-time Jeopardy winner
Not only are maps hypnotic, but they are also very practical. Simply, maps are our way of
communicating where things are in relation to one another. When I say the word “map,” you
probably think of a world map, but the earliest known maps are of the heavens. Dots on the
walls of the Lascaux caves in southwest France date back to 16,500 B.C, and are interpreted
to depict constellations: the
position of stars in relation to one another
.
There are many, many different types of maps. Architectural floor plans are also maps, as are
dental charts and heat maps that show the location of pitches in a baseball game. What they
all have in common is that at least one of their encodings is location in physical space.
In this chapter, we'll focus on maps with two positional encodings: latitude and longitude, or
more generally, x and y. Of course the earth is not actually a flat plane, so we will need to
make use of what's called a
standard projection
to avoid requiring a three-dimensional medi-
um. There are many different types of projections, and Tableau uses the Web Mercator
(sometimes referred to as EPSG 3857) projection common in online mapping applications
such as Google Maps and Bing Maps.
One Special Map
One year before he died at the age of 89, French civil engineer Charles Joseph Minard left
the world with a truly remarkable map depicting the ill-fated march of Napoleon's army to
Moscow and back in the winter of 1812, shown in
Figure 10-1
.
