When a spacecraft detector measures particle flux as a function of look direction in a plane (the scan plane) anisotropy is often seen. This anisotropy is caused by spatial gradients, by E¿B particle drift, and by various spectral and geometric effects. This paper treats all of these effects systematically, starting from the nonrelativistic Vlasov equation. The general analysis is applied to a simple model of an anisotropic distribution to give a relation between the E¿B drift, the gradient and the experimentally observed first, second, and third harmonics of the flux as a function of angle in the scan plane. Even with an assumed model, anisotropy observation in one plane alone do not suffice to determine the E¿B drift velocity and the spatial gradient independently. If one is assumed, the other follows. If the E¿B velocity is assumed (e.g., the corotational velocity in a rotating magnetosphere), the spatial gradient may be deduced, and from it the time rate of change of flux in a nonrotating frame of reference.