Representing Data - Data Points: Visualization That Means Something - page 449

This system is used less than the Cartesian coordinate system, but it can be
useful in cases in which the angle or direction is important.

Geographic
Location data has the added benefit of a connection to the physical world,
which in turn lends instant context and a relationship to that point, relative
to where you are. With a geographic coordinate system, you can map these
points. Location data comes in many forms, but it's most commonly described
as latitude and longitude, which are angles relative to the Equator and Prime
Meridian, respectively. Sometimes elevation is also included.

Latitude lines run east and west, which indicates north and south position on
a globe. Longitude lines run north and south and indicate the east and west
position. Elevation can be thought of as a third dimension. Compared with
Cartesian coordinates, latitude is like the horizontal axis, and longitude is like
the vertical axis. That is, if you use a flat projection.

The tricky part about mapping the surface of Earth is that it's wrapped around a
spherical mass, but you usually need to display it on a two-dimensional surface,
like a computer screen. The variety of ways to do this are called projections,
and as shown in Figure 3-14, each has its advantages and disadvantages.

When you project something that is three-dimensional onto a two-dimensional
plane, some information is lost, whereas other information is preserved.

The Mercator projection, for example, preserves angles in local regions. It
was created in the 16 th century by cartographer Geradus Mercator primarily
for navigation on the seas and is still the most-used projection for online
direction lookup. On the other hand, the Albers projection preserves area
but distorts shape. So the projection you choose depends on what you
want to focus on.

SCALES

Whereas coordinate systems dictate the dimensions of a visualization, scale
dictates where in those dimensions your data maps to. There's a variety of
them, and you can even define your own scales based on mathematical func-
tions, but most likely you'll rarely stray from the ones in Figure 3-15. These can
be grouped into three categories: numeric, categorical, and time.

FI G U R E 3-14 (following page)
Map projections