Two new tools for computer assisted assignment of rotational spectra with the PGOPHER program are presented, aimed particularly at spectra where many individual lines are resolved. The first tool tries many different assignments, presenting a small number for possible refinement and a preliminary version of this has already been presented. The second tool, the nearest lines plot (a new style of residual plot) provides a clear indication as to whether a trial calculation is plausible, and allows rapid assignment of sets of related lines. It gives good results even for dense spectra with no obvious structure and in the presence of multiple interfering absorptions. The effectiveness of these tools is demonstrated by the analysis of high resolution IR spectra of 8 bands of cis- and trans-1,2-dichloroethene where, including hot bands and isotopologues, 31 vibrational transitions and 158 316 individual lines have been analysed, including perturbations for the higher energy states. Walkthroughs are presented to show this process is rapid.