Maps, Map Projections and Reading Maps

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Maps, Map Projections and Reading Maps
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• Date / Publisher / Printer
• Scale / direction /
• Distance / ratios
• Geographical Position
• Coordinates/ projection
• Features / places / legends
• Other information?

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All maps will provide you with a Arrow indicating
both truth North (the precise top axis of the
earths spheroid) and a magnetic north which
indicates where our compasses will point to as
north
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For global represented of position we use degrees
of Latitudes (parallels) and Longitude
(meridians). The largest in length degree of
latitude is the equator and forms the base line
for measurements of degrees of latitude which
increase until you reach the north or south pole
at which point a right angle has been formed
(hence the poles are 900 latitude). At 23.5
north and south latitude the tropics of Cancer
and Capricorn occur respectively.
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In contrast to these degrees of latitude which
become smaller the degrees of longitude inscribe
the same-sized circumferences but do not lie
parallel to each other
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Cylindrical projections are derived from
projecting a spherical surface onto a cylinder.
For example if you took youre orange and wrapped
an A4 sheet of paper around it. The paper can be
arranged around the orange in a variety of
arrangements A Tangent Projection would result
if you wrapped your paper vertically so that the
cylinder was parallel to the meridians (lines of
longitude).
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In the secant case, the cylinder touches the
sphere along two lines, both small circles (a
circle formed on the surface of the Earth by a
plane not passing through the center of the
Earth).
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When the cylinder upon which the sphere is
projected is at right angles to the poles, the
cylinder and resulting projection are transverse.

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When the cylinder upon which the sphere is
projected is at right angles to the poles, the
cylinder and resulting projection are transverse.

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When the cylinder is at some other,
non-orthogonal, angle with respect to the poles,
the cylinder and resulting projection is oblique.
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The Mercator projection is one of the best known
and has straight meridians and parallels that
intersect at right angles. Scale is true at the
equator or at two standard parallels equidistant
from the equator. This projection seriously
distorts distances and areas.
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The Universal Transverse Mercator (UTM) is
probably the best known projection system for
displaying large surfaces of the earth since it
provides high levels of precision. To minimize
the distortion the cylinder is wrapped around the
earth transversely and is place at 60 of rotation
East and West of 1800 meridian for each
hemisphere. Consequently 60 zones north and 60
zones south are generated and are numbered
eastward from the 1800 meridian. Cape Town is
the 34 th Zone and is referred to as UTM 34S.
The UTM system is only applied from 840 North to
800 South Latitude.
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Conic projections which result from projecting a
spherical surface onto a cone. When the cone is
tangent to the sphere contact is along a small
circle such as a latitude. You can view this by
twisting your A4 sheet into a cone and placing
over the orange.
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Albers Equal Area Conic projection allows areas
to be proportional and directions true in
limited areas but distorts scale and distance
except along standard parallels. This is one of
the most common projection used to map large
countries where the east-west distances are
greater than the north-south extent (e.g. USA and
Russia). It is often used to represent South
Africa.
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Azimuthal or Planar projections are where a flat
sheet is placed in contact with a sphere, and
points are projected from the sphere to the
sheet. You can do this by taking your A4 sheet
and pressing it against the orange.
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NON-PROJECTIONS Plane (Cartesian) - not a
projection but truth to earth surface - data may
be stored in this form, but it is not good for
accurate measurements of distance e.g. metres.
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Datums While we often refer to the earth as a
sphere, it is more correctly referred to as a
geoid (defined as a hypothetical surface of the
earth that corresponds to mean sea level). The
earth is not a sphere since it is flattened at
both poles and bulges at the equator. In
addition there are significant bulges and
depressions on the surface. The are hundreds of
different datums which have been used to estimate
the size (areas and distances) of features on the
earth. Datums have evolved from those describing
a spherical earth to ellipsoidal models derived
from years of satellite measurements.
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To best describe this geoid mathematically, we
use reference ellipsoids to approximate the size
and shape of the earth.
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Lets look at a Map - Saldanha Bay
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