Geometry of the Sphere - Spatial Mathematics: Theory and Practice through Mapping - page 9

• Longitude runs from 0° at the Prime Meridian to 180° east or west,
halfway around the globe. The International Date Line follows the
180° meridian, making a few jogs to avoid cutting through land
areas or island groups and thus separating adjacent people into
two different days.

Latitude and longitude can also be understood by their connection to the
Cartesian coordinate system. Picture a standard Cartesian coordinate system
with an X axis and a Y axis (Figure 1.7). This system, when overlaid on the
Earth, gives a framework for visualizing latitude and longitude. On the Earth,
the X axis becomes the Equator, with a value of Y = 0, or zero degrees north or
south latitude. The Y axis becomes the Prime Meridian, with a value of X = 0,
or zero degrees east or west longitude.

Longitude values are the X values as one moves left (west) or right (east) along
the Equator or along any other line of latitude north or south of the Equator.
Latitude values are the Y values as one moves up (north) or down (south)
along the Prime Meridian or along any other meridian. Confusion may arise
because in most everyday speech when referring to “latitude, longitude”, lati-
tude is typically mentioned first, and it is tempting to think of these as being
equivalent to x, y , with latitude being “ x ” and longitude being “ y. ” However,
it is important to remember that latitude is “ y ” and longitude is “ x. ” And in
location-enabled devices and tools, such as Global Positioning Systems (GPS)
and Geographic Information Systems (GIS), latitude and longitude are entered
as y and x , respectively.

The Cartesian coordinate system also helps us understand why the sign (posi-
tive or negative) of latitude and longitude is important. The Equator divides
the area above the X axis, the northern hemisphere, from the area below the

y

(2, 3)

3

2

(-3, 1)

1

(0, 0)

x

-3 -2 -1 1 2 3

-1

-2

(-1.5, -2.5)

-3

Figure 1.7 Cartesian coordinate system. Source: Based on material from K. Bolino,
2008, released into the public domain, Wikipedia, http://en.wikipedia.org/wiki/
File:Cartesian-coordinate-system.svg