There’s a set of boundary stones encircling the rifle range on the south bank at the far east of the map, so that’s one possible reason.
Not just the coastline - they were used on survey benchmarks for *all * Ordnance Survey activities, from what I can tell. In the 1800s (Napoleonic Wars) , the entire country was mapped (as a quasi-military activity, hence the Ordnance moniker, which originated in the Scottish wars)
The flood defences along the south bank of the Thames need raising slightly now, again sometime in the 2040s, then again in the 2070s. Providing this happens you should be OK, but (assuming they are still responsible in 20/50 years) will the Environment Agency and Kent County Council (as Lead Local Flood Authority) be able to fund it? My gut feeling is yes, as it’s “London”, but even if I was a betting man, I wouldn’t put house money on it.
If private flood defences are an option for you, I would look into it. You can get very discrete floating barriers installed at your doors, and clever air bricks that seal up again by floating. Expensive, but you will reap some money back on your home insurance.
Also, those flood defences are designed to protect against a 1 in 100 year flood event. A 1 in 200 year event could happen tomorrow.
Not trying to put you off buying, but, if you haven’t already, maybe speak to some home insurers and also the Environment Agency.
The land up to the high water mark belongs to the W.D. and comes under the relevant Military Lands Act. They may have acquired it as part of the military barracks to the south, and laid it out as sports fields etc. In an era before Land Registry there was probably a legal requirement to mark the WD’s boundary so that people did not encroach on it.
Thanks, good advice. Surveys came back last week indicating the house is just outside the flood risk zones - just. We’ll look into flood insurance, and defences sound a wise idea.
If not posted:
Europe geography and history
Lots of fighting
Good luck with it. I had significant input in the survey for the EA’s flood maps after the floods 17 years ago in Devon and Cornwall. A lot of work went into them and they were very accurate.
Well, SOMEONE had to do it.
What a shame I didn’t encounter you 20 years ago. I could have introduced you to our neighbor, who was a cartographer. He loved to talk about his work…in great detail. lol Perhaps too much for most social occasions, but it would probably have been fascinating for you. 
they were indeed war department stones. there was one in my back garden and a picture is available here
http://www.medwaymemories.co.uk/?page_id=278
this stone marked the end of military land, there was an army camp build between the wars here but now gone. The army land was used from Napoleonic times.
The stone here also marks a parlimentary division (or marked) and even more so the MARK for a man of kent/kentish man. rainham mark
BSW^D
wd = war depardment
Bs = boundary stone
^
is the OS arrow
depicting 'Get Orf My Land" (or something like it!)
GAWD, I’m going to have fun with all the data in this thread! I made maps for over twenty years for CalTrans! Researching historical maps was the best part of the job!
~VOW
Then why would you need maps?
So that the imagination can travel?
“Bondage, Sadism, Whipping Up Dere”
Old maps provide the means to travel into the past. What you are looking at comes alive. I’ve been looking at a 1909 map of where I was in 1959. And the memories come flooding back.
Even though your post is seven years old, it has today provided the answer to a question that has just arisen from looking at an OS map dated 1909. The British army had barracks at Bordon in Hampshire, England, at the time - hence a large number of BS and BM (Bench Mark?) letters dotting the area.
1a.) Welcome to our silly little community! Please be welcome.
2b.) The fact that this thread helped some random person years after the fact is part of why some of us stick around.
Also we like to nitpick, argue the stoopitest piddly nonsense, and occasionally, help each other out.
Boundary stones mark property corners, and were typically placed by surveyors. Nowadays, brass or aluminum caps are used.
Bench marks (or stones) identify elevation at that point.
So to completely clear up the mystery, BSW^D means "Boundary Survey War Department.
~VOW