Day: 29 April 2018

In an important training exercise, Greg begins today to practice ‘zone calls’.

Log book entry

Date: 29.4.18 
Hour: 2.35 
Machine type and No.: DH 5155 
Passenger: – 
Time: 55 m 
Height: 3000 
Course: Aerodrome 
Remarks: Practice.

Date: 29.4.18 
Hour: 5.55 
Machine type and No.: DH 5155 
Passenger: – 
Time: 1 h 20 m 
Height: 3000 
Course: – 
Remarks: Zone calls

Date: 29.4.18 
Hour: 8.0 
Machine type and No.: RE 5146 
Passenger: – 
Time: 30 m 
Height: 2500 
Course: Aerodrome 
Remarks: Practice 1 landing.

Zone Calls

Zone calls are one way wireless messages by Morse code from aircraft to artillery batteries giving information about targets, such as enemy batteries firing, enemy transport and troop movements. 

A zone call might be something like:

NF L 26 c 2 0

This would mean:  Guns Now Firing from a position at map reference L 26 c 2 0. Maps of the Western Front (and presumably maps used by Greg’s Training Squadron at Yatesbury) used a reference system that was a combination of squares and a grid. 

• Each 1:40,000 map sheet was divided into twenty-four 6,000 yd squares, arranged in a 6 x 4 array and lettered A to X – in our case, L. 
• Each 6,000 yd square was in turn subdivided into thirty-six 1,000 yd squares, numbered 1 to 36 – in our case, 26.
• Each 1,000 yd square was then subdivided into four quadrants (a, b, c, d) – in our case, c.  
• Finally, a decimal grid reference (eastings and then northings) was used to specify the intended position with the required degree of precision.  This could be to one significant figure as in our example (2 0), which identified a 50 yd square.  Or it could be to two significant figures if greater precision was desired and achievable (22 01, for example), which would identify a 5 yd square within the 50 yd square denoted by 2 0.

Fine levels of detail are not visualisable on a 1:40,000 map.  The artillery and infantry tended to use 1:20,000 and 1:10,000 scale maps, depending on the purpose in hand, which were revised periodically as trenches and other ground features changed.  These revisions, incidentally, would rely heavily on the aerial photography of work of the corps squadrons.  The complete map reference would include the number of the 1:40,000 sheet (eg Sheet 36A, to give 36A  L 26 c 2 0), but the sheet number was typically understood in context and therefore omitted in zone calls.

If this combination of squares and grid references sounds complicated, it is not actually different in principle from an Ordnance Survey grid reference, which in one of its incarnations uses a combination of an identified square and a decimal grid reference within the square.  For example, the OS reference SU 053711 is based on a 100 km square designated SU and then, within that square, a three significant figure decimal grid reference 053 711. This in turn identifies a 100 m square – which, as it happens, is on the former Yatesbury airfield (1:50,000 Landranger sheet 173 in today’s OS maps).

A zone call was a one way wireless message from the aircraft to an artillery battery on the ground.  Ground to air communication was done by ‘ground strips’, as explained in this earlier post:

Friday 5 April 1918 – Landings and Ground Strips