With this type of benchmark
https://upload.wikimedia.org/wikiped...cket_G4863.jpg

Is the level,recorded on maps, given by the flat ledge immediately above
the upward pointing broad arrow, or half way up the recess involving
that ledge or some proprietary position, only known within OS, relative
to the 5 location recesses in the plate

N_Cook used his keyboard to write :
With this type of benchmark
https://upload.wikimedia.org/wikiped...cket_G4863.jpg

Is the level,recorded on maps, given by the flat ledge immediately above the
upward pointing broad arrow, or half way up the recess involving that ledge
or some proprietary position, only known within OS, relative to the 5
location recesses in the plate

The horizontal line is the datum.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

On Saturday, 9 January 2016 13:20:54 UTC, Harry Bloomfield wrote:
N_Cook used his keyboard to write :
With this type of benchmark
https://upload.wikimedia.org/wikiped...cket_G4863.jpg

Is the level,recorded on maps, given by the flat ledge immediately above the
upward pointing broad arrow, or half way up the recess involving that ledge
or some proprietary position, only known within OS, relative to the 5
location recesses in the plate

The horizontal line is the datum.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

Yes but by todays standards, they're not that accurate.
ISTR all the physical datum lines and trig. point have been abandoned by OS some time ago.
They even depend on private individuals to maintain them.
I used to maintain a trig point myself.

On 09/01/2016 15:54, harry wrote:
On Saturday, 9 January 2016 13:20:54 UTC, Harry Bloomfield wrote:
N_Cook used his keyboard to write :
With this type of benchmark
https://upload.wikimedia.org/wikiped...cket_G4863.jpg

Is the level,recorded on maps, given by the flat ledge immediately above the
upward pointing broad arrow, or half way up the recess involving that ledge
or some proprietary position, only known within OS, relative to the 5
location recesses in the plate

The horizontal line is the datum.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

Yes but by todays standards, they're not that accurate.
ISTR all the physical datum lines and trig. point have been abandoned by OS some time ago.
They even depend on private individuals to maintain them.
I used to maintain a trig point myself.


I thought dGPS/total station 2 station GPS had taken over but I was
wrong, according to the head of local archaeological unit.
They've had so much error with dGPS , that they now stump up 20 GBP a
pop for a convenient OS reference point, X,Y,Z data close to any dig
they now start. Never using dGPS for any basepoint determination of a
site, later on site yes, but not the initial absolute base reference
X,Y,Z. They could not even trust dGPS X and Y fixes as the worst error
on that, placed one of their sites in a neighbouring building, it
emerged later on.

On 09/01/2016 13:20, Harry Bloomfield wrote:
N_Cook used his keyboard to write :
With this type of benchmark
https://upload.wikimedia.org/wikiped...cket_G4863.jpg


Is the level,recorded on maps, given by the flat ledge immediately
above the upward pointing broad arrow, or half way up the recess
involving that ledge or some proprietary position, only known within
OS, relative to the 5 location recesses in the plate

The horizontal line is the datum.


The traditional chisselled benchmark, the level was the centreline of
the horizontal chissel mark, ie most recessed, rather than the lower
edge, equating to the horizontal ledge of the flush bracket type.

N_Cook presented the following explanation :
On 09/01/2016 13:20, Harry Bloomfield wrote:
N_Cook used his keyboard to write :
With this type of benchmark
https://upload.wikimedia.org/wikiped...cket_G4863.jpg


Is the level,recorded on maps, given by the flat ledge immediately
above the upward pointing broad arrow, or half way up the recess
involving that ledge or some proprietary position, only known within
OS, relative to the 5 location recesses in the plate

The horizontal line is the datum.


The traditional chisselled benchmark, the level was the centreline of the
horizontal chissel mark, ie most recessed, rather than the lower edge,
equating to the horizontal ledge of the flush bracket type.

Very conveniently, we have one just 100 yards from here.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

On 10/01/2016 12:18, Harry Bloomfield wrote:
N_Cook presented the following explanation :
On 09/01/2016 13:20, Harry Bloomfield wrote:
N_Cook used his keyboard to write :
With this type of benchmark
https://upload.wikimedia.org/wikiped...cket_G4863.jpg



Is the level,recorded on maps, given by the flat ledge immediately
above the upward pointing broad arrow, or half way up the recess
involving that ledge or some proprietary position, only known within
OS, relative to the 5 location recesses in the plate

The horizontal line is the datum.


The traditional chisselled benchmark, the level was the centreline of
the horizontal chissel mark, ie most recessed, rather than the lower
edge, equating to the horizontal ledge of the flush bracket type.

Very conveniently, we have one just 100 yards from here.


I was doing some laser levelling from a flush bracket benchmark,
yesterday. With beeper 1200m range the specs say, but +/-0.5mm per 10m
not so good despite an espensive model, I wonder what dumpy level specs are

150m distance for me yesterday, despite a road choc a block full of
cars, phone poles , lamp posts, walls etc managed to find one clear
optical path over/around that lot