| Universal Transverse Mercator
| The Northern Hemisphere projections for the infamous UTM system consisting of
120 zones (60 different zones with North and South variants of each).
Originally developed for military use and now widely misused in civil mapping.
|
| Universal Transverse Mercator (South)
| The Southern Hemisphere projections for UTM. These are mainly distinguished
by each having a Northing parameter of 10 million so that no coordinates need involve negative numbers.
|
Near the Equator, the Mercator projection provides low distortion. Away from
the Equator distortion becomes very high. This limits the utility of the
Mercator projection to regions near the Equator. That is a big limitation because
most places that people live (and thus, most of the regions that people most
frequently map) are located not along the Equator but along North-South
directions, such as from North America to South America.
Turning the Mercator projection's cylinder so that it is tangent to the Earth
along a meridian (longitude line) instead of the Equator results in what is
called a Transverse Mercator projection. If we created a Transverse Mercator projection that had a
meridian as the central ring of the cylinder we could make local maps anywhere along
the North-South line of tangency. If the maps are limited to the thin,
vertical region near the meridian of tangency they will be relatively free of
distortion.
The problem is that any Transverse Mercator projection created by choosing any
one meridian as a line of tangency is useful only near that meridian. If we
pick a North-South line running through Athens we can make maps all the way from
Scandinavia down the length of Africa, but any maps using this projection in
North and South America would be hopelessly distorted.
The Universal Transverse Mercator system of projections deals with this by defining 60 different standard
projections, each one of which is a different Transverse Mercator projection that is
slightly rotated to use a different meridian as the central line of tangency.
Each different centerline defines a UTM Zone. The "UTM Zone" is a shorthand way of naming a specific, different
projection that consists of a Transverse Mercator projection using a different meridian
as the centerline. By rotating the cylinder in 60 steps (six degrees per step)
UTM assures that all spots on the Earth will be within 3 degrees of the
centerline of one of the 60 cylindrical projections.
To map any spot on Earth, one picks the UTM Zone centerline that is closest to
it and then makes a map using that "UTM Zone" cylindrical projection.
UTM Zones should not be Combined
Novice UTM users usually do not realize that each UTM Zone is in fact a
different projection using a different system of coordinates. New users of UTM
therefore will frequently attempt to "combine" different maps created in different
UTM zones into one map with the expectation that the combined map will show
all objects with low distortion as did the original maps. The motivating factor
is often a desire to create a map centered on a region of interest that spans
several UTM zones or which is centered between two zones. Such plans fail to
take into account that UTM is an intrinsically inflexible system. In effect, the
UTM system assumes objects from different zones will never be seen together in
the same map.
Combining objects from different UTM zones into a map that is projected using
only one of those UTM zones will result in distortion in the locations and
shapes of the objects that originated in a different zone map. Geographic shapes
that look good in a transverse Mercator projection centered upon a given UTM
zone line will be very distorted when illustrated in a UTM projection centered
upon a different zone line.
The illustration above shows part of Europe projected into UTM Zone 2 in the
yellow map. Overlaid on the yellow map is an Orthographic projection centered
on the same map center shown in blue color. The numbers are positioned at the
center of UTM Zones 1, 2 and 3.
The Orthographic map is essentially accurate over the entire illustration. In
contrast, the UTM map is highly inaccurate only one half zone away from the
"home" zone. Note that it distorts the coast of France so much that it has
France (in the yellow, UTM projection color) crossing the Channel.
If we need to combine objects from several different UTM zones, the correct
solution is to choose a different projection (such as a conic or azimuthal
projection) for the combined map that provides low distortion over the entire region
of interest. The illustration above shows a Lambert Conformal Conic projection
in black outline and darker blue color overlaid over the Orthographic
projection. Note that both projections are so close to each other it is difficult to
pick out places where they differ. For example, in the region of France where
the UTM projection had the continental landmass crossing the channel there is a
very slight North/South offset but otherwise the two projections are virtually
the same.
Remember, although no projection is perfect for all uses some projections are
better than others in the uses for which they were designed. UTM was designed
to map objects within one zone at a time. It is a very bad choice if objects
from several zones must be shown together on the same map.
Comments
Like the State Plane Coordinate System or Gauss Kruger, UTM is a living
fossil. It was created for use in expert hands as a means of dealing with the
technological limitations of an earlier era. When used with skill as originally
intended it still functions well within its intended uses. The problem with UTM
(as with the State Plane and Gauss Kruger systems) is that it is constantly
misused in civil applications by inexpert users who do not realize the limitations
built into the system.