Editing Spirit level (section)

==Surveyor's leveling instrument==
{{Unreferenced section|date=December 2024}}
{{main|Level (instrument)}}
[[File:Us land survey officer.jpg|thumb|Modern automatic level in use on a construction site]]
Combining a spirit level with an [[optical telescope]] results in a ''tilting level'' or ''[[dumpy level]]''.<ref>{{cite web|url=http://www.sli.unimelb.edu.au/planesurvey/prot/equip/equip.html|title=Equipment Database Menu |publisher=Sli.unimelb.edu.au |date=1998-10-19 |access-date=2009-07-29|url-status=dead |archive-url=https://web.archive.org/web/20090710064317/http://www.sli.unimelb.edu.au/planesurvey/prot/equip/equip.html |archive-date=July 10, 2009 }}</ref> These leveling instruments as used in [[surveying]] to measure height differences over larger distances. A surveyor's leveling instrument has a spirit level mounted on a telescope (perhaps 30 power) with cross-hairs, itself mounted on a [[tripod]]. The observer reads height values off two graduated vertical rods, one 'behind' and one 'in front', to obtain the height difference between the ground points on which the rods are resting. Starting from a point with a known [[elevation]] and going cross country (successive points being perhaps {{convert|100|m|ft|0|sp=us}} apart) height differences can be measured cumulatively over long distances and elevations can be calculated. Precise levelling is supposed to give the difference in elevation between two points {{convert|1|km|mi|abbr=off|sp=us|spell=in}} apart correct to within a few millimeters.