RM meaning in Governmental ?

A river mile is a measure of distance in miles along a river from its mouth. River mile numbers begin at zero and increase further upstream. The corresponding metric unit using kilometers is the river kilometer. They are analogous to vehicle roadway mile markers, except that river miles are rarely marked on the physical river; instead they are marked on navigation charts, and topographic maps. Riverfront properties are sometimes partially legally described by their river mile.

The river mile is not the same as the length of the river, rather it is a means of locating any feature along the river relative to its distance from the mouth, when measured along the course (or navigable channel) of the river.

River mile zero may not be exactly at the mouth. For example, the Willamette River (which discharges into the Columbia River) has its river mile zero at the edge of the navigable channel in the Columbia, some 900 feet (270 m) beyond the mouth. Also, the river mile zero for the Lower Mississippi River is located at Head of Passes, where the main stem of the Mississippi splits into three major branches before flowing into the Gulf of Mexico. Mileages are indicated as AHP (Above Head of Passes) or BHP (Below Head of Passes).