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6.3.4 Oblique Mercator projection (-Jo -JO)

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:

  1. Option -Joa or -JOa:

  2. Option -Job or -JOb:

  3. Option -Joc or -JOc:

Our example was produced by the command




pscoast -R270/20/305/25r -JOc280/25.5/22/69/4.8i -B10g5 -Di -A250 -Glightgray -Wthinnest -P \
        -Tf301.5/23/0.4i/2 --HEADER_FONT_SIZE=8p --HEADER_OFFSET=0.05i > GMT_obl_merc.ps


Figure 6.19: Oblique Mercator map using -Joc. We make it clear which direction is North by adding a star rose with the -T option.
\includegraphics{scripts/GMT_obl_merc}

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.


next up previous contents index
Next: 6.3.5 Cassini cylindrical projection Up: 6.3 Cylindrical projections Previous: 6.3.3 Universal Transverse Mercator   Contents   Index
Paul Wessel 2010-07-14