Earth locations have distances and elevations {surveying measurements}. Measured great-circle arc {level, survey} can be surveying basis.
Finding elevation difference uses instrument {level, instrument} {spirit level}| that measures angles.
High objects have angle {acclivity} to horizontal.
Low objects have angle {declivity} to horizontal.
Surveying instruments {theodolite}| can measure horizontal and vertical angles.
Observers above ground can see Earth surface out to distance {horizon, Earth}. If Earth is spherical, horizon relates to height above ground: r = 1.23 * h^0.5, where r is radius from observation point to horizon, and h is height above ground.
Surveyors place metal plates {benchmark, surveying}| at locations with precise elevation.
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Date Modified: 2022.0225