A theodolite is that gizmo on a tripod used by land surveyors. This explainer shows how they work, and discusses how they shifted from analog devices to the modern digital ones.
Conventional measurement tools like a tape measure and protractor don't work for large civil structures and public works projects. Surveying is essentially the science of measuring big stuff. In this video I give a quick explanation of how surveying works and show a few ways you can do your own leveling survey at home. No sines, cosines, or tangents required!
Here's another one in use: