Lab 7: Topographic Maps and Profiles
Maps are the basic form of data representation in geology but they must represent three-
dimensional objects in two-dimensional forms.
First let us consider
. Scale is commonly given in one of three forms:
of scale, such as 6 inches to a mile (6 inches on the map represents
one mile on the ground);
, such as 1:1,000,000 (one unit on the map represents one million units on the
ground, or 1 mm in the map represents 1,000,000 mm or 1 km on the ground), or;
a bar scale
, such as shown below. The length of scale on the map represents the
distance indicated on the ground; in this example, the bar represents a distance of 10 km.
Another form of scale may be given by
lines or tick marks on the map.
The spacing of lines of longitude (the north-south lines) changes from the equator to the poles as
they converge at the poles. These lines are therefore inconvenient for determining scale. If we
assume a planet to be perfectly spherical, however, the spacing of lines of latitude (the east-west
lines) is uniform. As there are 90
of latitude north of the equator and 90
of latitude south of the
equator, the spacing of the lines of latitude is half the circumference of the sphere divided by
(2 x 90
) per degree of latitude. If R is the radius of the planet then 1
of latitude is given by:
is mathematical pi (= 3.14159). For Earth, with a radius of 6378.15 km, 1
of latitude is
approximately 111.3 km. The advantage of using a bar scale or a latitude/longitude scale on a
map is that if the map is enlarged or reduced, the scale remains correct as it is enlarged or
reduced with the map. A direct statement of scale or a ratio scale becomes invalid if the map is
enlarged or reduced.
A wide variety of information may be displayed on a map, including cultural features (buildings,
roads, administrative boundaries), biological features (vegetation or animal ranges), or
geological information (rock type and geological structures). Unless modified by human
activities, the shape of the land surface is a direct result of the interactions among the surface
geological process, such as weathering, erosion, deposition and impacts, subsurface geological
process, such as faulting, folding and volcanism, and the underlying mechanical structure of the
near-surface rocks and their resistance to the surface processes. Thus, perhaps the most basic
geological information is the shape of the land surface.
This information is generally known as topography, and is represented on maps by topographic