The profession of land surveying is referred to in the earliest texts with references to land ownership (The Iliad), the marking of property boundaries (modern Israel), re-establishment of boundaries (Egypt, after Nile floods), setting out of towns (Greece) and determination of the size of the Earth (Greeks in Egypt). Romans used Agrimensors to set out their forts and their roads.
The modern equivalents of these still exist and the work may be undertaken by land surveyors or other specialists (geodesists, civil engineers) who concentrate on one small part of the topic.
When Francis Drake needed to determine a gentle waterflow from the River Meavy (the weir & take-off are now under Burrator Reservoir) to Plymouth, he employed Robert Lampan (Hemery, 1986), who found a route with a gradient steep enough for the water to flow downhill but not so steep as to run out of height before reaching Plymouth. Along its route, Drake had sufficient power to run 6 mills. In doing so it crossed out of one valley (the Meavy) at a height that would bring it down to Millbay. This crossing of a watershed is a finely balanced achievement.
Many leats were dug around the country, to supply water for power, for washing (as in the leat on the left bank of the Erme for the Mill at Ivybridge) or for drinking water (possibly the upper leat on the right bank of the Erme). One that clearly displays a slow descent is Kings Gutter, which nearly follows various contour lines but gradually drops down as it makes its way towards Dinnaton