# Oblique Mercator (Hotine)

## Producer Field Guide

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Oblique Mercator is a cylindrical, conformal projection that intersects the global surface along a great circle. It is equivalent to a Mercator projection that has been altered by rotating the cylinder so that the central line of the projection is a great circle path instead of the Equator. Shape is true only within any small area. Areal enlargement increases away from the line of tangency. Projection is reasonably accurate within a 15° band along the line of tangency.

 Construction Cylinder Property Conformal Meridians Meridians are complex curves concave toward the line of tangency, except each 180th meridian is straight. Parallels Parallels are complex curves concave toward the nearest pole. Graticule spacing Graticule spacing increases away from the line of tangency and retains the property of conformality. Linear scale Linear scale is true along the line of tangency, or along two lines of equidistance from and parallel to the line of tangency. Uses Plotting linear configurations that are situated along a line oblique to the Earth’s Equator. Examples are: NASA Surveyor Satellite tracking charts, ERTS flight indexes, strip charts for navigation, and the National Geographic Society’s maps "West Indies," "Countries of the Caribbean," "Hawaii," and "New Zealand."

The USGS uses the Hotine version of Oblique Mercator. The Hotine version is based on a study of conformal projections published by British geodesist Martin Hotine in 1946-47. Prior to the implementation of the Space Oblique Mercator, the Hotine version was used for mapping Landsat satellite imagery.

This projection contains these unique parameters. Choose one of two methods for defining the central line of the projection.

Do you want to enter either:

A) Azimuth East of North for central line and the longitude of the point of origin

B) Two points - Latitude and longitude of the first and second points defining the central line

For method A, define the following:

Azimuth east for central line    Angle east of north to the desired great circle path

Longitude of point of origin    longitude of the point along the great circle path from which the angle is measured

For method B, define the following:

Longitude of 1st point    Longitude of one point along the great circle path of choice

Latitude of 1st point    Latitude of one point along the great circle path of choice

Longitude of 2nd point    Longitude of another point along the great circle path of choice

Latitude of 2nd point    Latitude of another point along the great circle path of choice

Oblique Mercator Projection

Source: Snyder and Voxland, 1989