Hammer retroazimuthal projection

The Hammer retroazimuthal projection is a modified azimuthal proposed by Ernst Hermann Heinrich Hammer in 1910. As a retroazimuthal projection, azimuths (directions) are correct from any point to the designated center point.^[1] Additionally, all distances from the center of the map are proportional to what they are on the globe. In whole-world presentation, the back and front hemispheres overlap, making the projection a non-injective function. The back hemisphere can be rotated 180° to avoid overlap, but in this case, any azimuths measured from the back hemisphere must be corrected.

Given a radius R for the projecting globe, the projection is defined as: $${\displaystyle {\begin{aligned}x&=RK\cos \varphi _{1}\sin(\lambda -\lambda _{0})\\y&=-RK{\big (}\sin \varphi _{1}\cos \varphi -\cos \varphi _{1}\sin \varphi \cos(\lambda -\lambda _{0}){\big )}\end{aligned}}}$$ where $${\displaystyle K={\frac {z}{\sin z}}}$$ and $${\displaystyle \cos z=\sin \varphi _{1}\sin \varphi +\cos \varphi _{1}\cos \varphi \cos(\lambda -\lambda _{0})}$$

The latitude and longitude of the point to be plotted are φ and λ respectively, and the center point to which all azimuths are to be correct is given as φ₁ and λ₀.