Isoazimuth

The isoazimuth is the locus of the points on the Earth's surface whose initial orthodromic course with respect to a fixed point is constant.
That is, if the initial orthodromic course Z from the starting point S to the fixed point X is 80 degrees, the associated isoazimuth is formed by all points whose initial orthodromic course with respect to point X is 80°. The isoazimuth is written using the notation isoz.
The isoazimuth is of use when navigating with respect to an object of known location, such as a radio beacon. A straight line called the azimuth line of position is drawn on a map, and on most common map projections this is a close enough approximation to the isoazimuth. On the Littrow projection, the correspondence is exact. This line is then crossed with an astronomical observation called a Sumner line, and the result gives an estimate of the navigator's position.

Isoazimutal on the spherical Earth

Let X be a fixed point on the Earth of coordinates latitude:, and longitude:. In a terrestrial spherical model, the equation of isoazimuth curve with initial course C passing through point S is:
''''''

Isoazimutal of a star

In this case the X point is the illuminating pole of the observed star, and the angle Z is its azimuth. The equation of the isoazimuthal curve for a star with coordinates, - Declination and Greenwich hour angle -, observed under an azimuth Z is given by:
where LHA is the local hour angle, and all points with latitude B and longitude L, they define the curve.