The boundary source method is an integral method introduced in the late 1960s for solving one-dimensional one-velocity transport problems arising in cell calculations. This method was further developed in various ways since that period and found to be of particular interest for recent applications to nodal transport codes. We have developed a boundary source code in plane geometry that allows for anisotropic scattering of arbitrary high order, and it is the purpose of this paper to display the extreme accuracy of this code, showing hereby that the boundary source method is probably the most accurate transport solution method available today for solving piecewise homogeneous transport problems.