Source of ela in chain survey?

The source of error in chain surveying can come from various factors such as incorrect measurement of chain length, sag in the chain, misalignment of the chain, and human error in recording measurements. Other sources of error can include environmental factors such as uneven terrain, obstacles in the survey area, and weather conditions. It is important for surveyors to be aware of these potential sources of error and take steps to minimize them in order to ensure accurate survey results.

Elevation above mean sea level