ROC Plotting Instrument, Height Corrector

Observer Corps - ROC ..  

IWM Non Commercial Licence .

A typical mid-war Post Plotting Instrument. Note the two pointers on the map (near the dashed circle), indicating the addition of the Micklethwait Height Corrector. 

A number of details of the post and Post Instrument can be seen in this image. The map on the Post Instrument shows the UK Grid Coordinate system for the area around the post. As the instrument is rotated azimuth and moved in altitude, the pointers move over the map - they are currently pointing near the grey disk just to the left of the large 1 in the centre of the map. This allows the teller to easily report positions without having to calculate them.

On the map are markings for 11 Group and Watford Centre, the reporting chain this post is part of (Watford is a short distance to the southeast, 11 Group covered the Midlands). The large C1 centered on the map is the ID of this post. Four grey disks can also be seen on the map with markings for L2, F3?, C3 and C4. These indicate the location of other nearby posts, and were used with the Micklethwait Height Corrector to calculate the altitude of the aircraft.

Arrows at the edges indicate the direction to two other posts, F2 and D3?, which are off the map area. The dashed ring on the map is the "sound line", used to estimate locations based on sound alone.

The Observer 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.

[image] Post Plotting Instrument & Micklethwait Height Corrector

'‘The Observer Instrument Mk 2A, designed by an Engineer named Captain Ball of Hampshire. Part of the instrument comprised ‘The Micklethwait Height Correction Attachment’, which allowed for heights to be corrected between two posts plotting the same aircraft.

The Correction Height Procedure was as follows :-

1. An observer estimated the height of an aircraft and set the height on the height bar.
2. The aircraft was sighted and the grid position shown by the instrument pointer was reported to the Ops Room. It should be noted that the aircraft grid position was only as accurate as the accuracy of the estimated height. It was necessary for at least one other post to carryout this procedure as well.
3. Using cross plotting the Ops Room plotter could ascertain the true position of the aircraft and he would quickly inform the posts of this grid position.
4. The post observer would then slide the Micklethwait cranked pointer on to the true grid position and read off the true height of the aircraft on the Micklethwait Height bar. This would then be set on the instrument height bar for all subsequent plots.

http://www.rocassoc.org/open/items/09/mickle.htm
http://www.roc-heritage.co.uk/post-instrument.html
http://www.roc-heritage.co.uk/uploads/7/6/8/9/7689271/use_of_post_instrument.pdf
https://en.wikipedia.org/wiki/Post_Instrument