War Department Ordnance Survey Marker
War Department Ordnance Survey Marker, Bermuda
Photo notes: A stone inscribed WD below a Broad Arrow symbol, one of hundreds of similar War Department Ordnance Survey Markers found in Bermuda. The Survey Department was responsible for military maps and fortification surveys; the Ordnance Department's responsibilities included the supply of locomotives and rolling stock for use on railways in the United Kingdom, parts of the British Empire, and in theatres of conflict. This marker is located beside the South Shore Road, near Spittal Pond, Smith’s Parish, Bermuda. Photographer Seán Pòl Ó Creachmhaoil, April 2006. 
There are a surprising number of sites devoted to "military boundary stones". Of the sites surveyed, we liked:
Frontline Ulster: Military History in the North of Ireland. https://frontlineulster.co.uk/wd-boundary-stones/ The site has a good outline of the types, styles and variety of markers, as well as a guide to dating them. There is also a reference chart to the markings, along with photographs.
The City of Plymouth (UK), has a page https://www.plymouth.gov.uk/boundarystone/militaryboundarystones that includes downloadable multi-page PDF documents with a glossary of marks and abbreviations found on War Department, Royal Navy, Board of Ordnance and Air Ministry stones.
Notes & Sources
 Wikipedia; search “Broad Arrow”. Retrieved January 2019.