Future mobile networks will need new tools to deal with the challenges of emerging technologies. In particular, more flexible networks will require localization of transmitters in the networks. In this work we present a novel method for transmitter localization, suitable for rich multipath mmWave 5G scenarios such as dense urban environments. Our work combines stochastic estimation of Radio Environmental Maps (REMs) with the well known concept of triangulation, generalizing the latter into a method for localization in anisotropic propagation environments. It can be considered a conceptual bridge from classical distance-based triangulation into a generalized version where the propagation environment is taken into account. The result is a highly flexible tool for network planning in general and transmitter localization in particular.