Initial great-circle bearing between positions

The bearing, in degrees clockwise from true north, along which a great circle leaves the first position heading for the second.

Usage

gc_bearing(lat1, lon1, lat2, lon2)

Arguments

lat1, lon1

Numeric vectors of latitudes and longitudes of the start positions, in decimal degrees. West longitudes are negative.

lat2, lon2

Numeric vectors of the positions bearing is taken to. Recycled against lat1/lon1.

Value

A numeric vector of bearings in [0, 360). NA where any coordinate is NA, and NA where the two positions coincide, which has no bearing.

Initial, not constant

A great circle changes bearing as it is followed, so this is the bearing at the first position. Over the distances between consecutive survey positions the change is negligible — a tenth of a degree over 10 km at these latitudes — but it is the reason cross_track_distance() takes a bearing and a point together rather than a bearing alone.

See also

gc_distance(), cross_track_distance(), track_bearing()

Examples

# Due north along a meridian
gc_bearing(43, -69, 44, -69)
#> [1] 0

# Due east, at the moment of departure
gc_bearing(43, -69, 43, -68)
#> [1] 89.659

# A position has no bearing to itself
gc_bearing(43, -69, 43, -69)
#> [1] NA