The results of an experimental study of the magnetic structure in the H-1 heliac are presented. Electron beam magnetic mapping has confirmed the existence of closed nested flux surfaces, in good agreement with a computer model. Measurements over a wide range of helical winding currents demonstrated a variety of attainable magnetic configurations within a rotational transform range of 0.6 ⩽ t0 ⩽ 1.8. The observed islands can be attributed to deduced small errors in the coil alignment in H-1. A magnetic island study and a correction in the model to fit the experimental observations revealed the error sources in the magnetic field