Coordinates systems

Danton uses three distinct systems of coordinates, which are outlined below, to define the position and direction of Monte Carlo particles in relation to the Earth’s geometry.

Ellipsoid

Danton coordinate systems are based on a reference ellipsoid. Depending on the selected Earth shape, the reference ellipsoid may or may not align with the geoid (see see Table 1).

Geographic coordinates

The Danton interface primarily uses GPS-like geodetic coordinates (latitude, longitude, altitude) to express positions. Directions are expressed using horizontal coordinates (azimuth, elevation), clockwise w.r.t. the geographic north. These sets of coordinates are collectively referred to as Geographic by Danton.

Note

Altitudes are expressed w.r.t. the ellipsoid rather than the geoid. This is a standard practice for GPS altitudes. Consequently, a zero altitude value may differ from the sea-level.

Geocentric coordinates

Internally, Danton uses Earth-Centred, Earth-Fixed (ECEF) Cartesian coordinates, also known as geocentric. The from_ecef and to_ecef methods of the Geometry class facilitate conversions between geographic and ECEF coordinates. It should be noted that the result is dependent on the ellipsoid, which in turn depends on the selected geoid.

Note

With the exception of the "PREM81" ellipsoid (i.e. a spherical Earth), there is a discrepancy between the geographic (orthometric) and geocentric (radial) altitudes.

Local coordinates

In some cases, it is more convenient to use local cartesian coordinates, e.g. when considering a box-bounded area of the Earth’s surface. In this case, Danton uses Local-Tangent-Plane (LTP) coordinates. Conversions between local and geographic coordinates can be performed with the from_local and to_local methods of the Box class. Note that, as with geocentric coordinates, the result depends on the ellipsoid.

Orientation

By default, Danton local coordinates are East-North-Upward (ENU) oriented, w.r.t. the geographic north. Optionally, a declination angle can be specified, in a clock-wise direction around the z-axis.