Milepost 61  

 Grand Union Canal Walk — associated with Grand Union Canal

An illustrated walk along the Grand Union Canal from London to Birmingham

 THE GRAND JUNCTION CANAL - a highway laid with water. — associated with Grand Union Canal (Grand Junction Canal)

An account of the Grand Junction Canal, 1792 - 1928, with a postscript. By Ian Petticrew and Wendy Austin.

 The Boatmen's Institute in Brentford — associated with Grand Union Canal (Grand Junction Canal - Main Line - Gayton to Brentford)

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Wikipedia has a page about Milepost

A milestone is a numbered marker placed on a route such as a road, railway line, canal or boundary. They can indicate the distance to towns, cities, and other places or landmarks like mileage signs; or they can give their position on the route relative to some datum location. On roads they are typically located at the side or in a median or central reservation. They are alternatively known as mile markers (sometimes abbreviated MMs), mileposts or mile posts (sometimes abbreviated MPs). A "kilometric point" is a term used in metricated areas, where distances are commonly measured in kilometres instead of miles. "Distance marker" is a generic unit-agnostic term.

Milestones are installed to provide linear referencing points along the road. This can be used to reassure travellers that the proper path is being followed, and to indicate either distance travelled or the remaining distance to a destination. Such references are also used by maintenance engineers and emergency services to direct them to specific points where their presence is required. This term is sometimes used to denote a location on a road even if no physical sign is present. This is useful for accident reporting and other record keeping (e.g., "an accident occurred at the 13-mile mark" even if the road is only marked with a stone once every 10 miles).