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The War Department 1:2500 sheets feature contours, unlike the normal public editions of these plans that only have bench marks and spot heights. The contemporaneous 6-inch sheet, Devon CXVII, is stated to have been surveyed and contoured by Captain Cameron R.E. in 1856, so this 1:2500 sheet is very likely to also be Cameron’s work. In fact, the survey would have been conducted at the 1:2500 scale and then photo-reduced at Southampton to enable the 6-inch plates to be engraved.

Lieutenant-Colonel John Cameron C.B., R.E., F.R.S. (1817-78) joined the Royal Engineers in 1839, rising to the post of Director General of the Ordnance Survey in 1875. One of his key areas of interest was the determination and recording of altitudes using spot heights, bench marks and contours, as borne out on this sheet (although the height datum is not identified in the marginalia. During this period, the datum was the Mean Sea Level measured at Victoria Dock, Liverpool, recorded over just nine days in 1844).

From the ‘Report on the Ordnance Survey of the United Kingdom for 1855/56’, the then Director, Henry James, writes:

‘A special survey has been made and engraved of Devonport, Stonehouse, and Plymouth and of a large district round these towns, for the Government and for the local Board of Health, which is to pay 500l towards the expense of this survey. An extension of this survey, including the ground to the west-ward, on which it is proposed to erect some works of defence, has since been ordered, and is in progress.’

‘It has been the practice hitherto to make such plans as the above as independent surveys, having no relation to each other, and simply in manuscript: but I have made the above-mentioned surveys as parts of the surveys of the counties to which they belong, occupying the sheets to which they belong if the whole counties were complete. By this arrangement, and by printing them on the scales now adopted for the national survey, every special survey so made extends the general survey on a uniform system.’

Other ‘War Department’ surveys at this time included:

- Aldershot (see this Bodleian Library Maplist, searching for Aldershot, and this Aldershot map held by the NLS)

- Portsmouth / Gosport / Northern parts of the Isle of Wight

- Forest of Dean

This particular sheet seems to have extremely limited relevance to Plymouth’s defences, the bulk of which were much closer to the city itself (see Plymouth Fortifications). Portsmouth was similarly surveyed i.e. over a large area surrounding the city. But in both cases, at the time of the 1856 survey, the locations of new defence works had yet to be established and so the maps were presumably intended to be for planning purposes. By the time the plans were printed, work on the new fortifications had already begun.

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