SpatiaLite Cookbook Chapter 07: Exotic Cooking aspects, which are often forgotten (or avoided)

For Geodesic functions such as:

• GeodesicArcLength
    Surface measurement from point 1 to 2, on the outer circle arc / earth surface.
    The distance along the curved line making up the arc.

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• GeodesicChordLength
    Shortest line (chord) from point 1 to 2, through the circle / earth.
    The region between a chord of a circle and its associated arc.

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• GeodesicCentralAngle
    Central angle from arc length in meters together with the radius (a) in meters.
    Central Angle - the center of a circle by two given points on the circle.

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• GeodesicArcHeight
    Height of segment [short-sagitta] in meters yellow line in image -->
    Sagitta - The height of an arc or segment.

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• GeodesicArcArea
    Area of arc [short-sagitta] in meters cyan area in image-->
    Area of a circle segment given the central angle.

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Note:
  since all projections use meters for the earth's radius, the length/area results are always in meters

Sample using Wsg84 [4326] in New Zealand:
   radius[6378137.0]: in meters (Wsg84)

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From: North/Western point of the North Island.
  point_from: MakePoint(172.677712,-34.426461,4326)
To:     Southern most point of the South Island.
  point_to: MakePoint(167.836789,-46.969656,4326)

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SELECT GeodesicArcLength(point_from,point_to,1);
  1450994.109302 meters
     surface distance, without any hills/moutains

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SELECT GeodesicChordLength(point_from,point_to,1);
   1447867.187222 meters
     shortest distance, through the surface

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SELECT GeodesicCentralAngle(point_from,point_to,0);
  13.034502 degrees (0.227495 radians)
     angle from the center of the earth to the points

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SELECT GeodesicArcHeight(point_from,point_to);
  41217.263818 meters
     from the middle point of the shortest line to the surface (ocean)

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SELECT GeodesicArcArea(point_from,point_to);
  39810530621.627159 meters
     area between points, along Shortest Line and Surface measurement