ARINC661 Map projection

An ARINC 661 Server will use a Map projection to convert MapItems (Latitude, Longitude) coordinates to their associated positions in 100th of mm.

Overview

The projection algorithm use the following characteristics:

• The reference position of the Map (the PRP), both on the real world in (Latitude, Longitude)^[1]
  The (Latitude, Longitude) coordinates must be defined in degrees
  , but also in screen coordinates in the reference system of the MapHorz widget
• The range of the Map, both on the real world in nautical Miles, but also in screen coordinates
• The North orientation of the Map^[2]
  The orientation must be defined in degrees

Algorithm

The projection algorithm between (Latitude, Longitude) to (East, North) coordinates in nautical miles uses a simplified version of the Lambert conformal conic projection.

Reference of the Map

These formulas allow to compute only once the overall values associated with the Map.

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Computing the (East, North) coordinates from the (Latitude, Longitude) coordinates

The simplified formulas for computing the (East, North) coordinate of a mapItem are:

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Computing the (East, North) coordinates from the (Latitude, Longitude) coordinates

The inverse of the previous formulas are:

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Computing the screen coordinates

Computing the screen coordinates involves the orientation of the Map.