http://www.fao.org/3/r4082e/r4082e04.htm A bench mark is a permanent mark established in a field to use as a reference point. A bench mark can be a concrete base in which an iron bar is fixed, indicating the exact place of the reference point. A bench mark can also be a permanent object on the farm, such as the top of a concrete structure. In most countries the topographical departments have established a national network of bench marks with officially registered elevations. All bench mark heights are given in relationship to the one national datum plane which in general is the mean sea level (MSL) (see Fig. 52). https://northernwoodlands.org/articles/article/does_an_acre_of_hilly_land_contain_more_land_than_an_acre_of_flat_land https://alonhadat.com.vn/nha-dat/can-ban/dat-nong-lam-nghiep/ha-noi/423/huyen-hoai-duc.html http://dmcland.com/duyet-ke-hoach-su-dung-dat-huyen-hoai-duc-nam-2021-hoan-thien-du-an-duong-vanh-dai-3-5-dn1302.html