Oblique configurations of the cylinder give rise to the
oblique Mercator projection. It is particularly useful when
mapping regions of large lateral extent in an oblique direction.
Both parallels and meridians are complex curves. The projection
was developed in the early 1900s by several workers. Several
parameters must be provided to define the projection. GMT offers three different definitions:
Our example was produced by the command
pscoast -R270/20/305/25r -JOc280/25.5/22/69/4.8i -B10g5 -Di -A250 -Glightgray -W0.25p -P \ -Tf301.5/23/0.4i/2 --HEADER_FONT_SIZE=8p --LABEL_OFFSET=0.05i > GMT_obl_merc.ps
It uses definition 3 for an oblique view of some Caribbean islands. Note that we define our region using the rectangular system described earlier. If we do not append an ``r'' to the -R string then the information provided with the -R option is assumed to be oblique degrees about the projection center rather than the usual geographic coordinates. This interpretation is chosen since in general the parallels and meridians are not very suitable as map boundaries.