  Mapping the positions of single electron spins is a highly desired capability
for applications such as nanoscale magnetic resonance imaging and quantum
network characterization. Here, we demonstrate a method based on rotating an
external magnetic field to identify the precise location of single electron
spins in the vicinity of a quantum spin sensor. We use a nitrogen-vacancy
center in diamond as a quantum sensor and modulate the dipolar coupling to a
proximate electron spin in the crystal by varying the magnetic field vector.
The modulation of the dipolar coupling contains information on the coordinates
of the spin, from which we extract its position with an uncertainty of
0.9\,\AA. We show that the method can be used to locate electron spins with
nanometer precision up to 10\,nm away from the sensor. We discuss the method's
applicability to mapping hyperfine coupled electron spins, and show it may be
applied to locating nitroxide radicals. The magnetic tomography method can be
utilized for distance measurements for studying the structure of individual
molecules.
