In particular I was looking pictures of the Pantheon on Google Earth (360 Cities really) and was wonder at the amazing condition the structure was in. Wikipedia mentions that the porch was meant to be taller so as to hide part of the dome. Again according to wiki it was built in only 3 years.
All this made me wonder how the design process worked. Where architecture structures made? Scale models. What kind of measurements were recorded before hand?
tl;dr See title.
http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Vitruvius/home.html
This page opens up the table of contents of de Architectura by Vitruvius (1st Cent. BCE). Each chapter opens up a link to Latin, Italian, and English versions of the text.
Vitruvius was a civil engineer or architect during the late Republic. That means that he was not one of the great architects to the emperors such as Apollodorus of Damascas (2nd Century), who was worked for Trajan. Although he never attained such heights, Vitruvius had access to a high degree of training. Fortunately for modern historians and Renaissance architects, he wrote them down.
The text of de Architectura can be seen as a technical manual of sorts for would be engineers. Despite these qualities, the work still possesses literary topoi. For example, the tract includes a prologue that contains the story of Alexander the Great's chief architect. The second half of the book, however, would be of great interest to you. In this section Vitruvius discusses how one might build pumps, baths, aqueducts, you name it. The later architects who built the great works of the principate built off of this collected knowledge.