A month in the life of an Ordnance Survey Benchmark bagger

A couple of years ago there were stories in the news about young people, and some not so young, stepping in front of traffic and falling off cliffs whilst blindly following their mobile phones in the hunt for Pokémon.  “How foolish!”, I thought.  However, I now find myself wandering the streets, phone in hand, searching – sometimes fruitlessly – not for Pikachu and its chums but for elusive signs of a bygone technology.

I have become mildly obsessed with finding Ordnance Survey Benchmarks.

https://www.ordnancesurvey.co.uk/benchmarks/ explains what benchmarks are:

Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.

Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.

I’ve been aware of BMs for a long time. Occasionally I’d spot one and I suppose I did know they had something to do with measuring or surveying or something-or-other but I’d never realised how many there were and what a huge effort it had taken to create and manage the network.

A short evening walk had piqued my interest in how the local area had developed in the last century. Looking at old OS maps on the National Library of Scotland webpage I noticed how many BMs were marked.  I tweeted about my observations and was surprised to receive a reply from the OS telling me about the legacy list of BMs. Good grief! There’s over half a million of them . . . . .or, at least, there were although they’ve not been maintained for 50 years and many have been destroyed.

Not only is there a csv file containing every single Benchmark (apart from the Fundamental ones which are still maintained by the OS and therefore, I presume, a possible source of income) but they are also viewable as an overlay on OSMaps (Tick the Benchmarks box under Places).  Wherever I am, so long as I have a data connection on my phone, I can bring up a local map which shows the position of every benchmark.  Touching the benchmark icon brings up a description of where it is.  You do not need to be a paid subscriber to OSMaps to see the benchmarks overlay or to use the basic mapping or aerial view.

Unfortunately, this overlay is not available on the OSMaps mobile phone app.  I have asked the OS about this but it sounds like it’s a long way down their development priority list.

As a newcomer to the world of benchmark bagging there are plenty to find everywhere I go.  I’ve got into the habit of having a quick look at the map before I go somewhere so that I know which benchmarks I’m likely to be able to find.  The description is usually helpful:

Eg BRICK P E SIDE JACKSON ST 8.6M S ANG WALL

However, there are some parts I still do not understand.  For example, what is the “ANG” referred to in this one?  BLDG NO63 HAMILTON ST NE FACE N ANG

The wall faced NE-ish but I can’t figure out what angle could be referred to.

I’ve learned to look round the corner: FL BR G3955 NO138 PARK RD NORTH SE FACE S ANG

Definitely on Duke Street not Park Road North

…. and I have no idea what “PRODN” means: STO GT P NW SIDE CLAUGHTON RD 3.7M E PRODN NE FACE EMMANUEL CH SE FACE

In the last month I have found and photographed 29 benchmarks.  Most have been cut marks, ie the distinctive arrow head, but there has been 1 flush bracket; the Park Road North / Duke Street one above.  I had always assumed that the flush bracket I saw on a hill-top triangulation pillar, ie trig point, was a key part of the trig point.  In my ignorance I had no understanding of the different purpose of a trig pillar (surface distance) and a benchmark (height above Ordnance Datum / sea level).  All those photographs I’ve taken of trig pillar flush brackets over the years can now go into my Benchmark collection!

Ah, yes, the collection…….

What I would like to do is catalogue every benchmark I have found.  It needs to be ticked off a list and have the date of discovery noted along with a photograph and a description of the condition of the mark.  Of course, it is equally important to record the ones which I’ve looked for but couldn’t find…… and there have probably been over 20 over those, so far.

I’ve dallied with a few different ways of recording my finds.  An online map would be perfect but I’ve not yet found a way that seems to do the job in the way I would like.  I am geotagging my photos, so it should be possible to put them on a public online map, but I’ve not yet found the best way to do it.

I could also upload them to Geograph but I’d like a simple way to only display the BM photos on a map and I’m not sure if Geograph can do this.

Of course, no hobby – however strange – is ever new on the internet and I know there are communities of BM-baggers keeping and publishing their own records.  I’ll probably contribute to https://www.bench-marks.org.uk/.

You may have noticed that I’m quite passionate about these weird wall-scratchings.  People collect all sorts of odd things so I mustn’t beat myself up over being a Benchmark Bagger.  It doesn’t hurt anyone and it gives me a good excuse to go for a walk at lunchtime or in the evening and see what I can find.  I’m also enjoying looking at old OS maps to work out where a missing BM used to be.

It’s free, it’s harmless and it’s interesting …. and if you’re reading about benchmarks for the first time I bet you’ll have to have a little peak at OSMaps to see where your nearest one is.