As for most painters in all times, very likely Giovanni Paolo Panini's career may have started doing drawings, as it always has been a more basic activity requiring less working equipment, skill and money to experiment.
In Panini's case, drawing seems to be mostly a testing device to experiment, and preparatory to paintings.
Panini's drawings very often deal with the same situations / atmospheres which we can see in the paintings.
And in his drawings we see the more explicit and precise architectural elements, the research of lines and depths, which could be found in Canaletto's work.
Giovanni Paolo Panini, very proficient in his plastic re-creation, had a different line style, of course, but always stayed in those Rococo standards which define the artistic tendencies of the XVIII century.