Page 68 - Ranger Demo

The Block Plot – Survey Targeting in World War Two
By Mike Nolan.
Preamble
The contribution of “Survey” to the effectiveness of artillery in the closing stages of the First World War is well documented and the Field Survey Association, later renamed the Defence Surveyors’ Association, grew from the early old comrades’ reunions of many of those involved. After the war, Artillery survey passed to the Royal Artillery and, again, the story of artillery survey in World War Two is well recorded. The R.E. survey contribution to targeting in World War Two is less obvious, its main effort being in map production, map printing and map supply, but from the beginnings in World War One RE Survey was instrumental in the inter-war period in developing the techniques of aerial survey. An offshoot of aerial survey for mapping was the use of the principles of the radial line technique to allow the co-ordination of targets identified on reconnaissance photography by using the principal point locations on a “block plot”.
This short article is simply and mainly, a collection of relevant references to the block plot and its use in World War Two, taken from various sources. Inevitably there is some repetition of the basic facts, this, incidentally, being a feature of Clough’s work in Maps and Survey.1
Introduction
Without knowing it, I first came across the concept of block plots in 1961 when using maps of Kenya and Uganda. On the face of these sheets were small crosses indicating the principal points of the vertical photographs used in their construction together with the sortie and photo number typically in the form 0025/81/RAF/1465. At the time they were merely a curiosity. Only later did it occur to me that they might have a use and be something to do with the “Radial Line Method” of detail plotting.
Much later, in 1977, an explanation was found when I was given a copy of Brigadier Clough’s 1952 historical monograph “Maps and Survey”.
However, by way of introduction, a succinct description is provided in a 1958 Survey Manual: -
A Block plot consists of a set of photographs of an area with the co-ordinates of the principal points either listed or plotted directly on an accompanying grid, generally 1/25,000 or larger. The intention is to have a set of photographs readily available from which co- ordinate values of other points, which can be identified on the photographs, can be determined by intersection from the appropriate principal points. This does not imply that the photographs of the block plot only can be used. Any other photograph within the area, at any scale, can be compared with the plot and by inspection the point required can be identified on the photographs of the block plot.2
Thus, the position of a new target, such as a gun battery, identified on reconnaissance photography could be pricked on each of the three block plot survey photographs covering the same area. Each, in succession, could then be laid over the base line between successive principal points on the block plot and the target rayed in on the block plot from each principal point. The co-ordinates of the intersection on the block plot were thus the co-ordinates of the battery target.
The block plot had a much earlier inception. The first reference to the use of the term that I have come across is in Salt’s paper for the Air Survey Committee. Describing rapid map production from air photographs by the radial line method he refers to;
Compilation of block plots on master grid, assuming not more than 10 block plots, 2 hours.
In his Summary of Terms used in air survey, he defined Block Plot as;
The celluloid plot carrying the principal point traverses traced off the master grid compilation. And on which detail plotting is carried out.3
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