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AscensionUltra / Coordinate System

Coordinate System

Since Ascension Ultra uses a combined base/vessel approach to get the island's topography into place, it is essential to agree on a common coordinate system. In addition, Orbiter's limitation of putting shadows under vessels projected onto a plane on the planet's sphere is putting a limit on the places of the origin.

Plane-to-sphere relation

Consider the following diagram for plane-to-sphere correlations:

Plane on sphere

You can see the red point as what should be called "landing-point" or LP. It is the point in longitude/latitude coordinates on the sphere where the map plane is touching the sphere.

The green point is the defined origin for the map plane's coordinate system (depicted by blue arrows).

Base layout

The actual base layout visualizes Ascension Ultra's tarmac and landing stripes. It is done merely to have Orbiter's rendering engine show shadows of vessels on the ground, as this is not the case on other vessel meshes.

It looks like this:

Base layout

The red point is showing the (abstract) LP of the base map, the green point the actual (logical) origin.

Generalized base and vessel layout

The vessel is "landed" somewhere near the base in order to show up with the topography and additional meshes (e.g. TA1). Obviously, we'll end up with a bunch of points and coordinate-systems here:

General vessel and base layout

  • 4 reference points
    • Red point is LP of base map
    • Green point is origin of base map
    • Violet point is LP of vessel
    • Yellow point is origin of vessel
  • 3 coordinate-systems
    • Earth's system (longitude/latitude)
    • Base map system (blue arrows on green point)
    • Vessel system (blue arrows on yellow point)

Unify base and vessel layout

All layouts should be unified as good as possible. This could be done by choosing one long/lat LP for both the base and the vessel. This LP should be the one of the vessel, as there it causes the most problems if located elsewhere (plane-to-sphere-problem). In addition, there should be only one origin and coordinate system. This should be the one of the base map, as it is the working environment of the designer. The solution will then look like this:

Common vessel and base layout

  • 2 reference points
    • Red and Violet as LPs of base and vessel
    • Green and Yellow as origins of base and vessel
  • 2 coordinate-systems
    • Earth's system
    • Base and vessel system This way, every object in the system can be absolutely located in Orbiter by means of 3 parameter:
  1. Longitude/Latitude of the base/vessel on the planet (Earth, mostly)
  2. Offset of origin wrt. landing point - this is fixed
  3. Position of the object in the base/vessel coordinate system