Chapter Three – Models of the Earth
Cartography is the science of mapmaking. Cartographers use a grid system to locate exact
positions on Earth. Lines of latitude refer to distances north or south of the equator. Lines of
longitude refer to distances east and west of the prime meridian. Latitude and longitude are
measured in degrees. One degree can be divided into sixty minutes
(symbol '), and one minute can be divided into sixty seconds (symbol "). The distance of one
degree of latitude is 111 km, but since longitude lines are not parallel, one degree of longitude
ranges from 111 km at the equator, to 0 km at the poles. The equator is the zero degree line of
latitude which divides the Earth into northern and southern hemispheres. The prime meridian is
the zero degree line of longitude, running through Greenwhich, England, separating the Earth
into eastern and western hemispheres. The 180o line of longitude opposite the prime meridian is
the International Date Line.
Maps are flat models of Earth's curved surface. All map projections therefore contain some sort
of distortion in the shapes or areas of landmasses. When using a map, be sure you understand its
legend (the symbols on the map and their meaning), and its scale (the relationship between
distances on the map and actual distances on Earth). A topographic map shows changes in
elevation using brown contour lines (lines connecting points of equal elevation). The difference
in elevation between two adjacent contour lines is known as the contour interval, and is the same
for the entire map. To make reading elevations easier, every fifth contour line is bold and known
as an index contour. Circular contour lines represent hills, while circular contours with lines
pointing into the circle show depressions. Some locations have specific elevation measurements
and are called benchmarks. They are shown on the map with the symbols "BM", "x", or "∆". The
elevation of the ocean is zero.