The widths of 284 lines in a 193 nm resist were measured by two methods and the results compared. One method was scanning electron microscopy (SEM) of cross-sections. The other was a model-based library (MBL) approach in which top-down CD-SEM line scans of structures are compared to a library of simulated line scans, each one of which corresponds to a well-defined sample structure. Feature edge shapes and locations are determined by matching measured to simulated images. This way of determining critical dimensions makes use of known physics of the interaction of the electron beam with the sample, thereby removing some of the ambiguity in sample edge positions that are assigned by more arbitrary methods. Thus far, MBL has shown promise on polycrystalline silicon samples [Villarrubia et al., Proc. SPIE 4689, pp. 304-312 (2002)]. Resist lines, though important in semiconductor manufacturing, pose a more difficult problem because resist tends to shrink and charge upon electron beam exposure. These phenomena are not well characterized, and hence are difficult to include in the models used to construct libraries. Differences between the techniques had a systematic component of 3.5 nm and a random component of about 5 nm. These differences are an upper bound on measurement errors attributable to resist properties, since they are partly attributable to other causes (e.g,. linewidth roughness).