What are the number seen in this photograph?

[Maury Markowitz]

I am trying to understand what the numbers painted on the walls of this OC post might be.

The number near S is 30, which implies N is perhaps 60. These would not be degrees or grads, nor any variation of mils that I can think of. Nor does it seem to have any relation to the OS grid numbers on the chart.

Anyone know what this might be?

 

[Hood]

It must be some kind of bearing as the same numbers are painted on the base of the plotting board, you can just see them if you zoom in and they align with those on the walls.

 

[Maury Markowitz]

I never noticed those numbers before! But they do not appear to line up.

If we continue the numbers we see on the walls, we would have 15 at east, 30 south, 45 west and 0/60 north.

We can see numbers at two places on the instrument, by the wheel on the left of the image, and beside his hand on the right.

The numbers on the right are in the 40s, which might work if the wheel is labeled in the reverse direction of the walls.

But even then, there's no way they'd be in the 20's by the point we see them on the disk, that would have to be next to the telephone operator's arm.

 

[perttime]

I see 26, 25, 24 on the edge of the board, lower right near the hand of the "aimer". That seems to be pretty close to what is on the walls. What the aimer sees on the board edge vs. what is on the wall.

A 6000 unit circle would be a perfect match for the "mil" used by Soviet Union and Finnish military, at least previously.

Milliradian - Wikipedia

en.wikipedia.org

The badges on the operators say Royal Observer Corps. Why would thy use a 6000 unit circle?

 

[HoHun]

Hi Maury,

The number near S is 30, which implies N is perhaps 60. These would not be degrees or grads, nor any variation of mils that I can think of. Nor does it seem to have any relation to the OS grid numbers on the chart.

Mils might fit. I guess there might be an offset between the "South" marker and the numeric scale because one of them might be a "magnetic" value and the other a true value.

Milliradian might give better resolution than degrees. Trying to imagine the process of observation, I think there would probably a spotter with hand-held glasses on the platform, and he could read the mil values the railing and pass them to the tracker, whose scope most likely has a smaller field of view.

However, why they would use a magnetic direction, I am not sure ... the interceptors would fly magnetic headings obviously, but there would be several processing steps down the chains, and I suspect that the reporting would be based on map squares anyway, as the instrument with the scope serves to directly convert sightings to map locations.

Regards,

Henning (HoHun)

 

[BlackBat242]

I never noticed those numbers before! But they do not appear to line up.

If we continue the numbers we see on the walls, we would have 15 at east, 30 south, 45 west and 0/60 north.

We can see numbers at two places on the instrument, by the wheel on the left of the image, and beside his hand on the right.

The numbers on the right are in the 40s, which might work if the wheel is labeled in the reverse direction of the walls.

But even then, there's no way they'd be in the 20's by the point we see them on the disk, that would have to be next to the telephone operator's arm.

These numbers - on the side of the circular platform:

 

[Maury Markowitz]

Ok, I found similar numbers on the other photo I have:

In this case they are printed on the white ring on the surface as opposed to around the outside. The image resolution is not very high, but you can make out that the number nearest the camera, aligned north, is "00", and "50" can be seen on the right. Although it's not easily visible, the number on the east is "45".

So first off, has anyone ever heard of a measurement system like this before? It seems to be 6 degrees each?

And then why is the numbering different than on the example in the first page, which is running the other direction (counterclockwise) and appears to be a different unit?!

 

[Maury Markowitz]

These numbers - on the side of the circular platform:

Yeah, those ones, they don't line up with the numbers around the post. 20 is about where 10 should be, and even then, that's assuming the numbers are reversed, running counter-clockwise instead of clockwise on the walls.

 

[HoHun]

Mi Maury,

Yeah, those ones, they don't line up with the numbers around the post. 20 is about where 10 should be, and even then, that's assuming the numbers are reversed, running counter-clockwise instead of clockwise on the walls.

The closest number appears to be a 26 in my opinion, and if you draw a line from the 26 through the pivot of the tracking scope to the opposite railing, it should intersect the railing in a point out of frame where the 26 on the railing should be. The spotter would read the number off the railing opposite to him, while the tracker would align the eyepiece side of the tracking scope with the number on the mounting facing his way.

From the new photograph, it looks as if there are 60 divisions to a circle, which would match the 6000 per circle definition of some artillery measurements given in the Wikipedia article.

I would speculate that the reason for using these divisions might be that the observer corps were using instruments designed for use by anti-aircraft artillery, which were built according to artillery conventions, thus the "deciradian" scale.

Regards,

Henning (HoHun)

 

[Flame2512]

I spent a fair while pondering this before realising that the answer appears to be right there, buried in the image description on Wikimedia Commons (which is where I assume the image in this thread was downloaded from, going off the file name): https://commons.wikimedia.org/wiki/File:The_Royal_Observer_Corps,_1939-1945._CH8215.jpg

The Observer, Smith, appears to be sighting along the iron sights on the top of the sighting unit, as opposed to using the telescope eyepiece. The walls of the post are printed with numbers representing 600ths of a circle, a short-lived measurement system that was later replaced with angular "mils", 400ths of a circle. Posts were normally manned by two, as in this case, but sometimes included a third who acted as a lookout and helper.

Also if anyone is interested here is a link to the Royal Observer Corps Instructions for Observer Posts manual dated 1941 (I read through this before thinking to check the image description). The same website also has a whole load of other ROC manuals covering from 1928 all the way up to 1991.

 

[Maury Markowitz]

I spent a fair while pondering this before realising that the answer appears to be right there, buried in the image description on Wikimedia Commons (which is where I assume the image in this thread was downloaded from, going off the file name): https://commons.wikimedia.org/wiki/File:The_Royal_Observer_Corps,_1939-1945._CH8215.jpg

And the most breathtaking thing about this is that I wrote that in 2019.

In the four years since, I've managed to completely forget doing so!