I don't have a drawing, however, if I needed one, I would look around for one and contact the water municipality in charge of it for the specs and ask permission to go to the site for measurements; and don't foget about each footing supporting each leg.
..." I would look around for one ..." by that statement I meant that I would look for elevated tanks in surrounding areas. In my neck of the woods, there are many elevated water tanks owned and operated by municipaltites.