Geology Reference
In-Depth Information
transforming between geographic and grid co-ordinates and, because lines
of longitude on the surface of the Earth converge but lines of latitude do not,
at least one of the scale factors must change over the surface of any map,
no matter what the projection. A projection in which both scale factors are
adjusted in the same way, so that scale is independent of direction, is said to
be
orthomorphic
. Most modern maps use orthomorphic projections.
The two main projection systems map the spherical Earth surface onto
the surfaces of either cylinders or cones. In the orthomorphic Mercator
projection, the mapping information is projected onto a cylindrical surface
that is tangential to the Earth at the Equator. Distortion is minimal near the
Equator but increases towards the poles (rendering Greenland enormous on
Mercator maps).
The low distortion that characterises Mercator maps in equatorial regions
can be achieved in other areas by redefining the Equator. For the Trans-
verse Mercator (TM) projection, the regular Mercator projection is rotated
through 90
◦
so that one of the lines of longitude (
meridians
) effectively
replaces the Equator, and distortion is small for points within about three
longitude degrees of this line. The system is ideal for countries such as Chile,
which cover a wide range of latitudes but only a small range of longitudes.
Countries, or areas, that are elongated E-W rather than N-S are less well
served, and mapping onto the surface of a cone, generally with its apex on
an extension of the Earth's spin axis, may be preferred. The surface of the
cone may either be tangential to the Earth's surface, giving zero distortion
at a single latitude, or may cut the surface. The first option is the simpler
but the second, with zero distortion at two latitudes, is preferred where the
mapped area has significant N-S as well as the E-W extent.
Once a projection has been chosen, a linear Cartesian grid can be estab-
lished, and this requires a point of origin. This point should be in a region
of minimal distortion, e.g. on the central meridian for a transverse Mercator
system or on the contact latitude in a conic system with one standard paral-
lel. The origin defined in this way is not usually assigned zero co-ordinate
values. Instead, values are chosen so that negative co-ordinates do not occur
anywhere within the region of interest.
The Universal Transverse Mercator (UTM) system, which is centred on
meridians spaced at 6-degree intervals, beginning at 3
◦
EandWofGreen-
wich, is now widely accepted as a global standard. The prime meridian
through Greenwich is a zone boundary in this system, and UTM co-ordinates
therefore change abruptly and discontinuously in east London. This would
create enormous problems for mapping in the UK, and the central meridian
of the British National Grid is therefore set at 2
◦
W - i.e. the grid uses a TM,
but not the UTM, projection.
