Determination of the three-dimensional (3D) shape of microphase-separated block copolymers (BCPs) is essential to investigate the "packing frustration" of the constituent blocks, which dominates their self-assembled nanostructures. Electron tomography (ET) is often used to visualize the 3D shape of BCP interfaces in real space, with staining often employed to enhance contrast between domains of similar electron density. As the number of blocks in the BCP structure is increased and the accompanying microphase-separated structure becomes more complicated, precise determination of interfacial structure from ET methods becomes progressively more difficult. Herein, the precise location of the interface was investigated for an intriguing complex double-helical structure formed by an ABC-type triblock terpolymer. The structure was characterized through the use of a novel structural analysis method combining the advantages of two dissimilar methods: intuitive real-space 3D observations provided by ET and quantitative, statistically accurate, nondestructive Fourier-space analysis provided by grazing-incidence small-angle X-ray scattering (GI-SAXS). The effect of staining on the helical structure is also discussed.