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Diffstat (limited to 'docs/source/operations/projections/omerc.rst')
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1 files changed, 97 insertions, 1 deletions
diff --git a/docs/source/operations/projections/omerc.rst b/docs/source/operations/projections/omerc.rst index 7131df62..ba2edf45 100644 --- a/docs/source/operations/projections/omerc.rst +++ b/docs/source/operations/projections/omerc.rst @@ -4,6 +4,27 @@ Oblique Mercator ******************************************************************************** +The Oblique Mercator projection is a cylindrical map projection that closes the +gap between the Mercator and the Transverse Mercator projections. + ++---------------------+----------------------------------------------------------+ +| **Classification** | Conformal cylindrical | ++---------------------+----------------------------------------------------------+ +| **Available forms** | Forward and inverse, spherical and elliptical projection | ++---------------------+----------------------------------------------------------+ +| **Defined area** | Global, but reasonably accurate only within 15 degrees | +| | of the oblique central line | ++---------------------+----------------------------------------------------------+ +| **Alias** | omerc | ++---------------------+----------------------------------------------------------+ +| **Domain** | 2D | ++---------------------+----------------------------------------------------------+ +| **Input type** | Geodetic coordinates | ++---------------------+----------------------------------------------------------+ +| **Output type** | Projected coordinates | ++---------------------+----------------------------------------------------------+ + + .. figure:: ./images/omerc.png :width: 500 px :align: center @@ -11,6 +32,77 @@ Oblique Mercator proj-string: ``+proj=omerc +lat_1=45 +lat_2=55`` + +Figuratively, the cylinder used for developing the Mercator projection touches +the planet along the Equator, while that of the Transverse Mercator touches the +planet along a meridian, i.e. along a line perpendicular to the Equator. + +The cylinder for the Oblique Mercator, however, touches the planet along a line +at an arbitrary angle with the Equator. Hence, the Oblique Mercator projection +is useful for mapping areas having their greatest extent along a direction that +is neither north-south, nor east-west. + +The Mercator and the Transverse Mercator projections are both limiting forms of +the Oblique Mercator: The Mercator projection is equivalent to an Oblique Mercator +with central line along the Equator, while the Transverse Mercator is equivalent +to an Oblique Mercator with central line along a meridian. + +For the sphere, the construction of the Oblique Mercator projection can be +imagined as "tilting the cylinder of a plain Mercator projection", +so the cylinder, instead of touching the equator, touches an arbitrary great circle +on the sphere. The great circle is defined by the tilt angle of the central line, +hence putting land masses along that great circle near the centre of the map, +where the Equator would go in the plain Mercator case. + +The ellipsoidal case, developed by Hotine, and refined by Snyder :cite:`Snyder1987` +is more complex, involving initial steps projecting from the ellipsoid to another +curved surface, the "aposphere", then projection from the aposphere to the skew +uv-plane, before finally rectifying the skew uv-plane onto the map XY plane. + + +Usage +######## + +The tilt angle (azimuth) of the central line can be given in two different ways. +In the first case, the azimuth is given directly, using the option :option:`+alpha` +and defining the centre of projection using the options :option:`+lonc` and +:option:`+lat_0`. +In the second case, the azimuth is given indirectly by specifying two points on +the central line, using the options +:option:`+lat_1`, :option:`+lon_1`, :option:`+lat_2`, and :option:`+lon_2`. + +Example: Verify that the Mercator projection is a limiting form of the Oblique +Mercator + +:: + + $ echo 12 55 | proj +proj=merc +ellps=GRS80 + 1335833.89 7326837.71 + + $ echo 12 55 | proj +proj=omerc +lonc=0 +alpha=90 +ellps=GRS80 + 1335833.89 7326837.71 + +Example: Second case - indirectly given azimuth + +:: + + $ echo 12 55 | proj +proj=omerc +lon_1=-1 +lat_1=1 +lon_2=0 +lat_2=0 +ellps=GRS80 + 349567.57 6839490.50 + + +Example: An approximation of the Danish "System 34" from :cite:`Rittri2012` + +:: + + $ echo 10.536498003 56.229892362 | proj +proj=omerc +axis=wnu +lonc=9.46 +lat_0=56.13333333 +x_0=-266906.229 +y_0=189617.957 +k=0.9999537 +alpha=-0.76324 +gamma=0 +ellps=GRS80 + 200000.13 199999.89 + +The input coordinate represents the System 34 datum point "Agri Bavnehoj", with coordinates +(200000, 200000) by definition. So at the datum point, the approximation is off by about 17 cm. +This use case represents a datum shift from a cylinder projection on an old, slightly +misaligned datum, to a similar projection on a modern datum. + + Parameters ################################################################################ @@ -62,7 +154,11 @@ Optional .. option:: +no_rot - Do not rotate axis. + No rectification (not "no rotation" as one may well assume). + Do not take the last step from the skew uv-plane to the map + XY plane. + + .. note:: This option is probably only marginally useful, but remains for (mostly) historical reasons. .. option:: +no_off |