Geology Reference
In-Depth Information
15
MAPS, MAPPING AND GPS
Advances in technology seldom make life easier. They make possible what
was previously impossible, but can make life more complicated in doing
so. This certainly seems to be true of the impact of the Global Positioning
System (GPS) on geophysics. Even the terminology is complicated. GPS
is just one type of Global Navigation Satellite System (GNSS) or Radio
Navigation Satellite Service (RNSS), these being generic terms for any
technology that uses satellites to determine position. GPS is the United
States' contribution and was the first in the field, but other countries have
developed or are developing alternatives.
15.1 Maps and Mapping
Before GPS became available, field crews seldom needed to worry about
such things as map projections and ellipsoids, but now they have instruments
that require that sort of information. Moreover, they have had to get to grips
with a suite of new (and constantly changing) acronyms.
15.1.1 Map projections
Mapping systems are attempts to represent on a flat piece of paper a part
of the roughly spherical surface of the Earth. Local topographic variations
are usually far more obvious than Earth curvature, but because satellite
positioning has made it possible for even small surveys to be referenced to
national mapping systems, which do take curvature into account, it is now
usually a contractual requirement that this be done. Failure to appreciate the
issues involved can result in positioning errors of several hundred metres.
Latitudes and longitudes define positions on the Earth in a spherical
polar co-ordinate system. Ideal for many purposes and essential for work
covering large parts of the Earth's surface, these
geographic
co-ordinates
are inconvenient for small areas, where a grid based on a standard unit of
length, now almost universally the metre, is preferable. To use such a system
it is necessary to treat the part of the Earth's surface under consideration
as planar. This is the process of projection, and involves distortion that can
be minimised but never eliminated. Projections incorporate scale factors for
