Benchmarks Layer


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By TallPaul on 11/10/16 at 8:32am

Benchmarks are trig points' little brother which, until recent times, the OS still relied on for tracking height above Ordnance Datum and there's still a lot of them scattered about Great Britain.

Here's an example: one carved into the buttress of a church tower:

Benchmark on a church

You can read more about them here and you'll notice at the bottom that they offer the complete database of about 500,000 benchmarks under an Open Government Licence. So I'm thinking about us adding them as a layer (not least for nerdy purposes of my own).

But would anyone else find that interesting? Like the water levels layer you would be able to turn it on and off and, when on, clicking on it a benchmark marker would show you information about the benchmark taken from the database.


By Lancashire Lad on 11/10/16 at 12:12pm

. . . . . But would anyone else find that interesting? . . . .

Yes indeed!

In fact only yesterday I was looking at that very spreadsheet to see if it included any info on the Bowfell summit bench mark I'd photographed on my "Bowfell via Ore Gap" walk (see walk report).

Unfortunately I couldn't find anything there.

On the premise that the actual location of Bowfell's "arrowed boulder" is approximately Grid Ref NY 2446 0645, I searched the spreadsheet and looked at all results beginning with Grid Ref NY 24 (of which there are many), but none seemed to be relative to NY 24 06 (or close enough to that specific location).

Which is odd, given that the spreadsheet is stated as being the complete bench mark archive.

(I recall reading somewhere that the bench mark was put there well over a hundred years ago, but haven't yet been able to discover the specific year).

Best regards, Mike.


By Lancashire Lad on 14/10/16 at 12:43pm

. . . In fact only yesterday I was looking at that very spreadsheet to see if it included any info on the Bowfell summit bench mark I'd photographed on my "Bowfell via Ore Gap" walk (see walk report).

Unfortunately I couldn't find anything there. . . . .

As a follow on to the above, I've had some email dialogue about this particular bench mark with Ordnance Survey, and received a reply which included " . . . . we can find no record of a Benchmark on Bow Fell. It may be the mark is associated with the Principle Triangulation (as opposed the retriangulation) or levelling associated with the 1st Geodetic Levelling, i.e. pre 20th century, for which we have no digital records.
Can you please send us a photograph so we can identify and investigate this further. . . . "

I have sent them the requested photos, and have asked them to inform me of the results/conclusions of their further investigations.

I will update again here, if I hear anything further.

Regards, Mike.


By TallPaul on 14/10/16 at 1:41pm

Well my news is that we now have a working benchmark layer on our test system which I hope we can release to supporters very soon.

However it's becoming increasingly clear, having looked at what we're seeing and comparing it to what I've found on the ground that there are a lot more benchmarks out there than are shown in their database. I've been looking at some OS 25 inch to the mile (so about 1:2,500) maps from 1892 to 1905 and those show lots and lots of benchmarks and, having done a quick survey of the local ones, quite a few still exist but very few are in the OS database so I assume, based on what they told you, that these were part of the 1st Geodetic Levelling.

I can feel a project, and possibly another web site, coming on.


By Lancashire Lad on 14/10/16 at 2:10pm

Hi Paul,

I assume that you are aware of this website? - http://www.geog.port.ac.uk/webmap/thela ... kemenu.htm

which includes this page on Bench Marks: - http://www.geog.port.ac.uk/webmap/thela ... enmrkf.htm

On that page, (which does actually have two references to the Bowfell summit benchmark), is this comment - "Although you automatically think Ordnance Survey, bench marks are made by other surveyors, sometimes just for an immediate project" - so it may well be that some of the bench marks we see, and have always assumed were made by the Ordnance Survey, may well have originated elsewhere.

Regards, Mike.


By TallPaul on 14/10/16 at 3:14pm

Yup. I've come across that issue before too and indeed one the benchmarks I found while looking for local ones looked like an OS one but wasn't on the 25in to the mile map so Bob alone knows what that was.

One of the things that intrigues me in all this is that a lot of the OS benchmarks are carved into private property. Presumably they had some sort of wayleave that let them do that without having to secure the landlord's permission? ...


By TallPaul on 15/10/16 at 1:36pm

Well my news is that we now have a working benchmark layer on our test system which I hope we can release to supporters very soon.

The good news is I think we're now happy with the software so it's ready to go live however we're having to reload all 500,000 benchmarks into our database first which is going to take a few hours to do so it will probably be late today or some time tomorrow before it goes live.


By TallPaul on 16/10/16 at 7:11pm

OK, this is now live. There's a new option the menu which lets you toggle it on and off. You only see the option if you're a supporter and you only see benchmarks if you're zoomed in to 1:50K or 1:25K.

There are 517,009 benchmarks in the database so we're only displaying those in a 10km square around the centre of your map, which should be off the edge of your screen unless you have a very high resolution screen. It re-loads the benchmarks every time you move the map. If you move it a lot it may take up to five seconds before they are refreshed (to keep the load on the server down).

I've also found details of the 190 FBMs mentioned on this page which I'm in the process of importing. Once that's done we'll put those online too.

Anyway let me know what you think of it.


By Lancashire Lad on 17/10/16 at 12:48pm

Hi Paul,

I've just had a bit of a play, and it is working perfectly.

I assume that the numbers shown on the map layer, against each bench mark, refers to the "order" as noted on the description when an individual bench mark is clicked on?

To be honest, I don't know what that "order" actually means. I'd hazard a guess that you know a lot more about bench marks than I do. Just a thought, but perhaps there is some sort of basic "glossary" of bench mark terminology that might be able to be uploaded and accessed on Walk Lakes to help people who are using this new facility?

Whilst browsing I came across this by pure chance - proving that even the OS are not beyond the odd error or two ;) : -

The description for the bench mark in question says it is of the rivet type, verified in 1978, at 23.829 metres height, and -1m above ground??? Clearly incorrect, since it is located almost on the 310m contour line for that location. (and how would one verify a bench mark at minus 1 metre above ground? :shock: :D ). Obviously nothing that you can do about such things, as your markers will be picked up from the grid references given on the OS database.

Anyway, OS database errors apart, the layer is working perfectly.

Regards, Mike.


By TallPaul on 17/10/16 at 3:31pm

I've just had a bit of a play, and it is working perfectly.

Excellent.

I assume that the numbers shown on the map layer, against each bench mark, refers to the "order" as noted on the description when an individual bench mark is clicked on?

Yup. Essentially it's a hierarchy, so the fundamental benchmarks (FBMs) are considered to be error free and the rest have an accuracy based on their order and their distance, d, in km from the nearest FBM (I hope you're all paying attention at the back, I'll be asking questions later ;-) ) thus:

+-------------------+----------------+
| Type of bench mark| Maximum error  |
+-------------------+----------------+
| fundamental       |  Error free    |
| Geodetic          |  ±2mm × ?d     |
| Secondary         |  ±5mm × ?d     |
| Tertiary          | ±12mm × ?d     |
+-------------------+----------------+

For more on this see the OS document A guide to coordinate systems in Great Britain and in particular 5.3.3.

Whilst browsing I came across this by pure chance - proving that even the OS are not beyond the odd error or two ;) [...] The description for the bench mark in question says it is of the rivet type, verified in 1978, at 23.829 metres height, and -1m above ground??? Clearly incorrect, since it is located almost on the 310m contour line for that location. (and how would one verify a bench mark at minus 1 metre above ground? :shock: :D ). Obviously nothing that you can do about such things, as your markers will be picked up from the grid references given on the OS database.

Yes, I was aware of the subterranean benchmarks. They became obvious when I was analysing the data prior to loading it. There are 6,718 of them as follows:

+------------+----------+
|     Above  |    Total |
+------------+----------+
|    -3.5000 |        1 |
|    -3.0000 |        2 |
|    -2.9000 |        1 |
|    -2.8000 |        3 |
|    -2.7000 |        2 |
|    -2.6000 |        1 |
|    -2.4000 |        6 |
|    -2.3000 |        2 |
|    -2.2000 |        2 |
|    -2.1000 |       10 |
|    -2.0000 |        4 |
|    -1.9000 |        4 |
|    -1.8000 |        9 |
|    -1.7000 |       10 |
|    -1.6000 |        9 |
|    -1.5000 |       30 |
|    -1.4000 |       39 |
|    -1.3000 |       45 |
|    -1.2000 |       64 |
|    -1.1000 |       81 |
|    -1.0000 |      114 |
|    -0.9000 |      121 |
|    -0.8000 |      237 |
|    -0.7000 |      221 |
|    -0.6000 |      428 |
|    -0.5000 |      632 |
|    -0.4000 |      673 |
|    -0.3000 |      979 |
|    -0.2000 |     1600 |
|    -0.1000 |     1388 |
+------------+----------+

What this means on the ground I've no idea as yet. My plan is to pick a couple and go take a look.

As for that particular benchmark also being way off height-wise (if you right click at that location the OS data suggests it's at about 306m not 24m, which looks about right from the surrounding contours) again I'm not sure what that's all about but I did find other typos, for example "197A" for the verified date, so this dataset is clearly not completely accurate. I suspect it may have been entered from paper records and was perhaps outsourced ...


By TallPaul on 18/10/16 at 10:44am

I've also found details of the 190 FBMs mentioned on this page which I'm in the process of importing. Once that's done we'll put those online too.

And those are now being shown, with a pin of "0". Enjoy.


By Lancashire Lad on 18/10/16 at 11:11am

. . . In fact only yesterday I was looking at that very spreadsheet to see if it included any info on the Bowfell summit bench mark I'd photographed on my "Bowfell via Ore Gap" walk (see walk report). Unfortunately I couldn't find anything there. . . . .

As a follow on to the above, I've had some email dialogue about this particular bench mark with Ordnance Survey, and received a reply which included " . . . . we can find no record of a Benchmark on Bow Fell. It may be the mark is associated with the Principle Triangulation (as opposed the retriangulation) or levelling associated with the 1st Geodetic Levelling, i.e. pre 20th century, for which we have no digital records.
Can you please send us a photograph so we can identify and investigate this further. . . . "

I have sent them the requested photos, and have asked them to inform me of the results/conclusions of their further investigations.

I will update again here, if I hear anything further.

Regards,
Mike.

Ordnance Survey received my photos, and, having further investigated the particular mark, have replied as below: -

Thank you again for your follow-up email regarding this benchmark.

We have now received feedback from the Geodetic Team which I summarise below.

First they thank you for providing further details including photographs. From the size it could be a very early benchmark but if so it is damaged (as there is no height reference point) and predates any of the information we now hold.

There are two types of cut marks for either horizontal or vertical surfaces, the most common are the latter which have a horizontal line across the top of the 'Government broad arrow' mark, which forms the 'bench' in the benchmark. For horizontal surfaces at the point of the Government broad arrow a brass rivet or bolt is installed or alternatively a small indentation which is designed to accept a 5/8" ball bearing. This latter type of mark is known as a pivot type benchmark (see attached diagram).

The mark in the photograph may be a pivot type but their guess is that it is not. In its current condition, it is clearly not suitable as a levelling reference point and appears to lack either the hole or brass bolt to which the height is actually referenced. It should also be noted that many government agencies have used the 'Government broad arrow' mark and their guess is that this may well be a boundary marker of some kind.

Not the definitive result that I'd hoped for, as it now looks highly unlikely that I'll ever discover exactly by whom, and in what year, this mark was made. But at least I have more information than previously, and have learned a bit more about Bench Marks along the way. (I have thanked those at the OS for their time and efforts in trying to answer this query - It is gratifying to know that there are still some organisations out there that do try to help when your average "Joe Bloggs" has a genuine query).

Regards, Mike.


By TallPaul on 20/10/16 at 9:33am

I have tweaked the benchmark popup to translate some of the OS shorthand descriptions of benchmark types, in particular the cryptic "FL BR" which is as Flush Bracket. I've also linked to this page which gives a good explanation of the various types, with examples.


By Lancashire Lad on 04/10/19 at 2:03pm (last edited 04/10/19 at 2:09pm)

Hi Paul,

With reference to your last post, (immediately above), I've been using the WalkLakes GPS Mapping "Benchmark Layer" of late, and wanted to get more information on various types of benchmark.
The "this page" link in your above mentioned post no longer takes you to the webpage that you intended.

I note also, that when any individual (red) benchmark icon is clicked on, (on the WalkLakes benchmark layer of the map), there, amongst the description, (beside the type of benchmark in question), is a hyperlink to "Jochta.com" where you mention that descriptions of the various types of benchmarks can be found.

It looks like that website has been revamped somewhat, as the links you have provided now redirect to "The buttons of my mind" webpage.
Still part of that same Jochta website I think, because scrolling down on that page reveals a hyperlink to "Jochta's Benchmarks".

However, I have clicked on that link and had a good browse around from there, but I cannot find anything at all on that website that actually describes in detail, the various different types of benchmark.
(All the various types are mentioned in a list, but when clicked on they just take you to those benchmarks of the particular type that have been recorded by the person running the website).

Do you still have any "viable" link(s) that currently go to the original page(s) where the various types of Benchmark were described?

Regards,
Mike.


By TallPaul on 04/10/19 at 2:46pm (last edited 04/10/19 at 2:46pm)

Damn, you're right. Looks like he's got rid of it. I've found the original page in the Internet Archive so if I can't find anything better to link to I'll re-use that page (with his permission hopefully).


By Lancashire Lad on 09/10/19 at 3:18pm (last edited 09/10/19 at 3:20pm)

Hi Paul, - (A tentative enquiry).

Might it be at all possible to somehow differentiate between the various types of benchmark, when viewing that layer on WalkLakes GPS Mapping page?

i.e. cut-mark, cut-mark with bolt, OSBM bolt, rivet, pivot, flush bracket, projecting bracket, & fundamental benchmark.

I'm doing a bit of personal research at the moment, and I'm finding the benchmarks layer very useful.
But, (even after having downloaded the entire national benchmark database from OS website), there seems to be no way, in any readily available manner to choose only a certain specific type of benchmark for any given geographical area.

I'm wondering whether there's a possibility of having a different coloured marker icons, or perhaps the ability to "switch on" or "switch off" each type so that only the specific types you are interested in at the time are overlaid onto the map.

Obviously, I don't know what might be involved in trying to do this, and it may be far too much work to achieve. But I for one, would find it very useful if it can be done.

Kind regards,
Mike.

PS: Some of the benchmarks are designated as PA Bolt. - Do you have any idea what the PA stands for?
(I've looked everywhere I can think of for an answer - I've even contacted OS directly, but no response as yet).


By TallPaul on 09/10/19 at 5:34pm

Yup, I could colour code them. I'll add it to the list.

PS: Some of the benchmarks are designated as PA Bolt. - Do you have any idea what the PA stands for?

No ... perhaps you should go look at one ;-)


By Lancashire Lad on 11/10/19 at 12:15pm

No ... perhaps you should go look at one ;-)

Hi Paul,

I don't think that the "PA" reference is an indication of some physical difference that would immediately identify the mark as that specific type.

There are numerous images of PA Bolt benchmarks to be found by doing an image search online, and they don't look any different to the "ordinary" cut mark with bolt type.

I have looked at a number of those images, and have verified by cross reference to the national benchmark database, that the marks in question are definitely listed on the database as "PA bolt" type, and not simply as "bolt".

There doesn't appear to be any PA types locally to where I am, but I still do intend to have a look at one or two if I find myself in an area that has them. – Just in case!

NB: Some of the online PA bolt images show the bolt located off the right hand end of the horizontal cut mark, rather than at its centre, (i.e. where it would have been immediately above the point of the arrow mark).
However, that cannot be any indication that the mark in question is a PA type, as many of the marks which are simply attributed as normal "bolt" types on the national database, also have the bolt located off to one side of the horizontal cut mark.

And that in itself raises the question as to what would be the reason for some of these bolt type benchmarks having the bolt off to one side of the mark, and not at its centre?
Something which again, I've tried to find out, but have had no luck whatsoever. :?

Regards,
Mike.


By SheepFarmer on 12/10/19 at 7:14am

According to this site PA bolt is "Published Abstract Bolt". This pic has a slot in the bolt if this has any bearing I've no idea.

Plus with regard to an earlier bit about negative height above ground, what if the mark was on the side of a canal lock wouldn't that be below ground level, or a wall with the ground higher on one side than the other, those are just two examples I thought of and they can't map every single change in ground level, any contour must be a mean value within some limit.


By Lancashire Lad on 12/10/19 at 9:07am

According to this site PA bolt is "Published Abstract Bolt". . . . . . . . . . . . .

Sheepfarmer, with regards to this particular little mystery, you are, without doubt correct!

The reference is, as I suspected, not relating to any physical difference of the actual marks, but rather the method by which the levelling relative to those marks was carried out.

I have, this morning found the answer, and was just about to log in to make my post - but you beat me to it !!! ;) :D

I have just taken delivery of a book: - "Ordnance Survey, Map Makers to Britain since 1791" - published by HMSO.

In the section on benchmarks, is the following text, which relates to levelling operations carried out with reference to a just found discrepancy from a datum level which at the time was located at Liverpool Victoria Dock (referenced from tidal readings taken between 1859 and 1860.
The text reads: -
" Compilation of results was carried out by a staff of men using pencils, paper and ten figure logarithmic tables. They used a mathematical method known as "least squares" which enabled them to determine the best values of unknown quantities. For the levelling it was used to find ninety-one unknowns in one adjustment block (England and Wales) and seventy-seven in another (Scotland). The results of the initial levelling were finally published in a weighty Abstract in 1861 and the bench-marks listed within have ever since been known as PA (published abstract) marks. "

With regards to your observation of horizontal groove in a bolt, I'm reasonably sure that most, if not all bolts have that groove. (Certainly all the ones I've personally seen do). - A couple of pics of my own sightings below.

Now, onwards, to try and solve the mystery of why some benchmark bolts are offset to one side of the actual cutmarks - and a million other such little head-scratchers!!!!

Regards,
Mike.


By SheepFarmer on 12/10/19 at 3:27pm

Mike,

It didn't take me long with google to find what the letters PAB stood for & knew what a Published Abstract was but couldn't link it in my mind a summary of an article (I should have been thinking report instead) with benchmarks out & about to search with, but you've come up with the definition.

Back to the subject of negative heights above ground could this -

This was a phosphor-bronze ring, encased in concrete and buried below surface level over a secondary trig (buried block).

from here and

Buried blocks are part of the network of Passive Stations. There are 2667 buried blocks in the T:UK database.

from here be part of the answer. But this last site does provided good descriptions but not the defintions of the various letters used that the missing site you & Paul refered to earlier.

I'll admit I'm just throwing suggestions at you to keep you head scratching. lets just hope you've been a fast walker read this:lol: Although what this says about Paul who seems to leave all the walking to Beth I don't know.:lol::lol:


By Lancashire Lad on 12/10/19 at 6:25pm (last edited 14/10/19 at 12:22pm)

SheepFarmer,

Trig point markers, such as the one at Blencathra summit, plus all of the usual stone or concrete trig pillars, along with buried blocks and all the other types of triangulation station mentioned on the TrigpointingUK website aren't included on the Ordnance Survey's national benchmark database.

The national benchmark database, and the national triangulation stations database are two separate entities. (Both can be downloaded from OS website).
Interestingly, (or perhaps I should more accurately say "frustratingly"), the trig point database only gives trig locations by northings and eastings and not by national grid reference.

Maybe TallPaul can figure out a way of adding all the UK trig points onto the benchmark layer, or perhaps a separate trigpoint layer?
(Hint, Hint, :lol:).

I would accept though, that there is an obvious relationship between the two, as trig pillars have flush bracket benchmarks attached.
But yet those mountain top trig pillars, along with their uniquely numbered flush bracket benchmarks, are not included on the national benchmark database. - Again, another mystery for which I have no answer.

As for the specific case of Blencathra's (original OS) "concrete detector ring" trig point, Ordnance Survey consistently state that it was one of these: -

I can accept that others of that type may have been, but to my knowledge, that particular concrete ring and its associated brass rod, were never buried sixteen inches below ground level at Blencathra summit.
That specific concrete ring was sat on rock, so unless someone can prove to me that at least sixteen inches of height has been eroded away from the entire area of the top of Blencathra since that ring was placed there, I will not accept that it was ever sixteen inches below ground level!!!

I haven't personally searched for any "below ground" benchmarks as there have been none related to my particular interest. I can understand the logic of your thoughts re places such as canal locks etc. and I don't doubt that some benchmarks might legitimately have negative height above ground measurements, (those in culverts for example), but I do suspect that many of those negative height listings will be erroneous.

As for walking speed - I do tend to meander when walking on the fells. I'm out to enjoy the walk, and I'm generally on the look out for good photography viewpoints.
However, I also walk at least twenty miles per week locally, (905 miles and 78728 ft of ascent so far this year), and my average speed then is around 16-minutes per mile.

Regards,
Mike.

EDIT - An afterthought! - Not sure if you are aware of this, but both benchmarks and trigpoints can be shown on the online version of OSMaps. - When viewing OSMaps, just go to the menu, click on places, and then scroll down to switch on benchmarks and/or trigpoints.


By TallPaul on 09/10/19 at 5:33pm

I've spoken to the author or the original page - it is still there but I can't deep link to it at the moment as it only appears in a JavaScript popup. He's promised me a direct link however ...



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