Engineering is one of the skills for which the Romans are most renowned. Some of their works, such as bridges carrying roads or water, are visually spectacular because of their sheer scale and daring. Others are equally impressive for the less obvious reason that they required very precise surveying. Examples which leap to mind are roads which cut across country as straight as an arrow, kilometre-long tunnels whose headings met deep underground without significant error, and aqueducts on gradients that can average 1 in 8000 for twenty-five kilometres or 1 in 20,000 for eight. Such feats of engineering would have been impossible without good surveying techniques and good instruments. That these existed has of course long been recognised, and many historians of technology have commented on them, although there has been no fundamental discussion of the evidence for many years. The regular conclusion has been that the standard instrument for laying out straight lines and right angles was the groma, that the standard instrument for levelling was the chorobates, and that Hero's dioptra was a non-starter. Constant repetition has almost sanctified this opinion into a dogma. But while it is partly true, it is also partly wrong, and it is very incomplete in that it is biased towards the Romans and ignores much of the evidence available in Greek.

One of my aims is to remedy the deficiency, and in the process to restore to the Greeks their rightful share of the credit.