What do these lines on OS map mean?

[B17_Fan]

Can anyone help me with this map. If you look at the very top, in the middle, there's a place called knavingcots. There are 3 lines jutting out, like half a windmill almost. Does anyone know what it means? http://maps.nls.uk/view/101453424

[groom]

I think it's a Benchmark symbol with the BM next to it showing height.

[Greensleeves]

As Groom says, it looks like a benchmark, showing height above sea level in feet.  Probably indicates that the ground at this point was higher than the surrounding land. 

Regards
GS

[matthewj64]

Bottom of the map describes what the arrows indicate - from what I can make out they're the position of marks on walls, buildings etc and their height above "mean water level at Liverpool"

M

[jess5athome]

Hi, I agree, it's a "Benchmark"

Frank.

[ReadyDale]

Yes, it is a benchmark - used for levelling when surveying the maps. It cannot be said that the location was higher than surrounding land (as this one is on the corner of the outbuilding north of Knavingcots), but was certainly more visible from surrounding benchmarks. As mentioned, heights were from Liverpool on this map. Later (after 1921) heights were taken from mean sea level at Newlyn in Cornwall.
BTW, The symbol used for benchmarks is from a similar origin as the arrows on convict clothing way back when.

[stanmapstone]

Bottom of the map describes what the arrows indicate - from what I can make out they're the position of marks on walls, buildings etc and their height above "mean water level at Liverpool"

M

See http://maps.nls.uk/view/101453424#zoom=4&lat=717&lon=4851&layers=BT

[Andrew Tarr]

As Groom says, it looks like a benchmark, showing height above sea level in feet.  Probably indicates that the ground at this point was higher than the surrounding land.

Indisputably a benchmark, indicated by B.M.  All it may indicate is a surveyed height, from which other nearby places may be measured.  The small town where I live has well over a dozen, usually on solid stonework such as the corner of a railway bridge or a bank.  A glance at an Edwardian or Victorian 6-inch map will show them.  The sad thing is that they are all relics now since the arrival of GPS mapping, but it can be interesting to search for them.  Look for the same 'broad arrow' symbol with a horizontal line above, marking the actual height.

[stanmapstone]

Bench marks are permanent marks provided at intervals of one-third of a mile or less along a line of levelling. They are cut on vertical serfaces at about knee hight usually in durable material such as stone. An incised horizontal bar defines the level to which the height is referred and a broad arrow is positioned immediately below the bar; the whole mark occupies a space of about four inches square. The name derives from the angle iron which is fitted into the horizontal cut  to give a 'bench' or support for a levelling staff.
See http://www.geograph.org.uk/photo/2395132
Stan