  Boundary element methods for the Helmholtz equation lead to large dense
matrices that can only be handled if efficient compression techniques are used.
Directional compression techniques can reach good compression rates even for
high-frequency problems.
  Currently there are two approaches to directional compression: analytic
methods approximate the kernel function, while algebraic methods approximate
submatrices. Analytic methods are quite fast and proven to be robust, while
algebraic methods yield significantly better compression rates.
  We present a hybrid method that combines the speed and reliability of
analytic methods with the good compression rates of algebraic methods.
