Sperry Marine VISIONMASTER FT ECDIS-E, User Manual

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Charts ECDIS-E User Guide
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Chart Projections
Effect of Projections
Since the earth's surface is not flat a projection algorithm is required to map
objects on the earth's surface onto the flat display screen.
On the ECDIS watch mode the projection used for presenting all LAT/LON
referenced objects optimises their alignment with the chart. Navigators
should be aware of the resultant small offset between radar video and other
objects such as the chart, AIS targets, tracked targets, the cursor LAT/LON
readout and routes.
These offsets are negligible at or near own ship's position and at the
equator, and increase with range from own ship and increasing latitude. As
a guide, an object that is 20NM from own ship at a latitude of 75 degrees
may exhibit an offset of approximately 4 pixels (depending on screen
resolution).
Projection Types
There are two primary projection types
supported by the VisionMaster system:

Mercator
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
Polar Stereographic
6
Mercator is the default chart projection, used to
display geographic areas between 0 degrees
and 80 degrees latitude north or south. The
exception to that is with non-navigational
scales (1:1,000,000 or above) where Mercator
can view regions up to 85 degrees
North/South.
5
Mercator projection is a method of showing a map of the globe on a flat surface.
On a globe, the lines of longitude (measuring east-west position) converge at the poles and
the lines of latitudes (measuring north-south position) are equal distance apart. In a Mercator
projection, the lines of longitude are straight vertical lines equal distance apart at all latitudes,
and horizontal distances are stretched above and below the equator, this stretching is
exaggerated near the poles.
6
In a Polar Stereographic projection the lines of longitude emanate from either the north or
south pole (depending on which hemisphere is being displayed), and lines of latitude are
displayed as concentric circles around the pole.