Free standing, nanoporous alumina templates were fabricated as transmission masks from aluminium using a two-step anodization process followed by acid etching. The resulting membrane comprises self-ordered, periodic arrays of non-connecting circular channels which can be prepared with pore diameters <100 nm and with minimal occlusion. Aspect ratios greater than 300:1 were measured directly using electron transmission and the channels were shown to be highly aligned (angular) over membrane thicknesses of tens of microns. Also evident is some local order associated with both azimuthal and angular domain structure giving rise to local channel tilt which has not previously been reported. Transmission electron microscopy has been shown to be an important characterization tool for these nanomasks as the channels are transparent to electrons, providing a means of directly measuring their thickness and aspect ratio. Expressions for determining their thickness and aspect ratio are also presented and evaluated in this work. These membranes are well suited for use as nanotemplates in transmission lithography applications including ion implantation and ion or electron beam collimation.