Grasping Gallipoli
Page 45
When the image is adjusted to fall as required on the tracing cloth, such detail as it is desired to extract from it is traced and in this way a mapped area is composed and a plan vandyked therefrom, printed and issued.
In practice insufficient allowance was made in the trial apparatus for the variation in scale due to variation in altitudes of the ground; nor were the lighting arrangements for throwing up the image on to the tracing paper under conditions of tent working sufficiently considered. These and other imperfections will gradually be conquered and it is hoped a strong, portable and effective reducing apparatus designed, which will obviate, or at least considerably minimize (what it is hoped will prove for this purpose to be) the unnecessary stages of printing the photograph and making a patch-tracing therefrom, and will, in any case, accelerate and facilitate the utilization of the aeroplane photographs for mapping purposes.
Faith in the general correctness of a plan obtained by the procedure outlined involves at least three important assumptions, viz:
(1) That the relative positions of plate, lens and area photographed are (within small limits of error) unchanged during exposure.
(2) That the spread of view of the lens photographing vertically an approximately horizontal surface introduces no impermissible variation in scale within the portion of the photograph utilized.
(3) That the variation in distance of objects on the ground from the lens due to relief of the ground introduces no appreciable changes in scale or errors in vertical projection.
It is recognised that these assumptions are by no means always justifiable, even within the limits of error permissible to small scale and rough class of plan, but some such assumptions were necessary as a starting point to enable the cruder errors of:
(a) Variation in photographing height, and
(b) Bad vertical pointing,
To be minimised in a sufficiently simple and rapid manner to facilitate – and therefore accelerate – the composition of these trench plans, while at the same time improving their homogeneity and accuracy. The utility of the plans, even in their crudest form, can with difficulty be exaggerated: the most pressing needs were:
(c) Some measure of control so that the grosser errors were at least localized, and
(d) A systematization and facilitation of the production of the plans from the photographs.
This, the procedure provisionally adopted appreciably assists in securing.
The end kept in view by the Survey & Map Section G.H.Q. in Gallipoli is the production of a reliable 1:10,000 map of the area of operations showing all recognizable detail. On this map enemy positions and trenches, as periodically corrected, are to be shown in a coloured overprint, since the basis of the map will not appreciably change while change in enemy positions is ceaseless. Proofs of such a map of a portion of the SUVLA area were produced and circulated for criticism some little time since and final copies will probably be to hand shortly.
Appended are copies of four photographs [not reproduced here] taken by the R N A S in Gallipoli for mapping purposes, the positions of these four photos in relation to the detail on (e), four successively dated trench diagrams, on (f) the reproduction (1:20,000) of the captured Turkish map, and on (g) the 1:40,000 originally supplied, are indicated on the accompanying plans. From these it will be realized far better than from any amount of explanation the radical improvement effected by the introduction of photographic mapping.
The positions of the photographs relative to the detail on the maps can only be roughly indicated chiefly on account of inaccuracies in both maps; the photographs are poor copies as the negatives were not available and indifferent prints had to be re-photographed to obtain more copies. Photos and plans alike are only intended to be illustrative of methods and not as samples of work.
On my return from visiting G H Q Med. Force the G O C in Chief in Egypt [Maxwell] approved of the continuation of experimental work in photographic mapping in Egypt, and Major Massy, R.F.C., has since co-operated most keenly in this work. This Survey provided two of the most suitable cameras that were obtainable locally [[footnote]: Description of Camera [not present]] and a number of series of aircraft photographs were taken by Lieut C A G Mackintosh over an area on the Ismailia Canal specially selected on account of its being (1) handy (2) rich in detail (3) level, and (4) surveyed recently, accurately and on a large scale (1:2,500).
Level country was deliberately chosen for the initiation of the work but it was, at the same time, arranged that subsequent series of photographs should be taken over a well featured portion of northern Sinai of which, of course, there is an excellent map based on recent work [1:125,000, surveyed 1908–13]. I understand this has since been done, but copies of the photographs are not yet to hand, and there accordingly has been no opportunity of testing their value from a map-making point of view.
Exposure was made throughout with the camera hand-held, a bubble being attached to assist in securing verticality. At first exposure was made over the side of the plane but this was very unsatisfactory since a clear view was not attainable and the conditions were too awkward to allow the operator to watch the level bubble and make his periodic exposures properly.
It was then arranged that a trap door should be provided in an available plane, and subsequent exposures were made through this with satisfactory results. The net result of the Ismailia Canal experiments was to obtain easily and regularly sets of photographs of stretches of the country which were legible and full of detail, and which compared with the map shewed remarkable accuracy, uncorrected as they were.
From a photographic point of view the photographs were poor and each had to be “nursed” individually, but this was merely lack of experience and proper apparatus and only pointed to the need of improved manipulation and machinery.
The great difficulty of varying relief of the ground was, of course, not present, but it was desirable to free the first trials from any difficulty that could be eliminated. The study, in due course, of photographs of the well-mapped boldly-reliefed country of Northern Sinai should be most instructive.
Attached are two appendices: [Not reproduced here]
I: Notes on some individual points of immediate importance still debatable, or at least worthy of further consideration.
II: An attempt to analyse the various sources of error with a view to seeing more clearly the importance and nature of each of which, accordingly, require immediate attention and which can, for the time being, be treated as negligible.
I am very strongly persuaded of the importance of the future of mapping by means of aeroplane photography, and for normal requirements as well as those of war. The opinions and views put forward are, of course, very tentative and are hurriedly put together from a limited experience, but the whole subject is still naturally in the crudest initial stages of a new form of surveying, and we must therefore obviously be careful to keep in mind alternative methods, even if for the present they appear clearly ruled out, since an advance elsewhere may at any time call for the revival of a previously discredited instrument or method.
I would, accordingly, be grateful for any criticisms or suggestions – even the sharpest and crudest – from anyone who has had experience either in:
(1) The taking of aeroplane photographs.
(2) The construction of plans or diagrams therefrom.
(3) The use of such plans in the field.
This Survey [i.e. Survey of Egypt] being in close touch with the forces in the Near Eastern Theatres of war and, naturally, being trained in, and having the machinery – human and otherwise – for, map-making, is in a position to digest and collate such criticism and experience for the benefit of those members of the Near Eastern Forces who are concerned and interested, but who have no leisure or opportunity to collect or collate such information for themselves.
[Signed] E. M. Dowson
DIRECTOR-GENERAL
SURVEY OF EGYPT
Later Addenda
Provision of Control
For instance from one point a tree may show up standing apart from a group of trees but on proceeding to the next point it may fall in line with the group and it will be impossible to distinguish which of the trees it is.
The difficulty of recognizing the intersected objects on the photographs has almost disappeared as constant practice of picking out features of importance has led to great proficiency. When any difficulty has been experienced it has always been possible to find the object by the following method:-
The map constructed from photographs is drawn up to the nearest recognisable intersected point, and the drawing is carried on approximately in pencil by reducing each photo to fit the overlapping detail already drawn on the map. This process is continued until the point is drawn on the tracing. The relative position of the plotted intersected point is then seen with regard to the neighbouring detail and this part of the photo is searched for the object. In every case in which this method has been resorted to the point has been found within 2 millimetres of the point indicated. The point having been found on the map is re-adjusted to fit it.
An example of this method is illustrated in the specimen photograph C (not reproduced here), which was known to include the prominent blasted tree (3) shown on the profile sketch. This tree could not be found until the above method was resorted to. The neighbouring photograph to the East contained both the intersected windmills at Krithia so that it was reduced to fit the tracing 1:10,000 containing the intersected points plotted to that scale. The photograph C was then reduced until the overlapping detail agreed and the plotted intersected point was found to fall in the position shown on the back by a red circle. The tree was then picked out as the black object surrounded by a white circle. This white circle has been observed round other trees which can be easily recognised and is probably due to animals walking round it whilst rubbing themselves.
With regard to the density of fixed points these should be as thick as possible and in any case should not be separated by more than one to two kilometres in order to give a rigid check to prevent the inclusion of a distorted photograph.
Instruments for reduction of photographic detail to the intersected points plotted on the tracing. Initially great difficulty was experienced in picking out detail on the image thrown on the tracing cloth.
This was due to two causes:-
1) The lack of contrast of the majority of photographs especially the earlier ones.
2) Insufficient lighting of the photos.
These difficulties have now been overcome in the following way:-
The photos are now placed under a thin glass plate having its surface covered with a transparent varnish which will hold ink. All important detail of the photograph is then traced directly on to the glass. The travelling base board of the camera has had a hole cut through it and a dispersive reflector placed behind it throwing the light directly through the hole into the lens of the camera. The glass plate carrying the tracing is then placed over the hole and focused on the tracing cloth above. This method yields a brilliant image on the tracing cloth which more than compensates for the slight additional source of error introduced by tracing the photograph twice.
Appendix VII
Rough Running Account of Visit to M E F July 7th–24th, 1915. [dates of departure from, and arrival at, Survey of Egypt]
[Relevant extracts from Ernest Dowson’s hitherto unpublished report on Gallipoli mapping and his visit to Gallipoli, including the start of the new trig survey, July 1915. Reproduced by kind permission of Gill Dowson]
[10 July, Mudros] The handling of maps in detail from MUDROS as a centre is fairly hopeless as long as this lack of water transport prevails, and there is not the slightest apparent hope that it will be remedied as it prevails everywhere.
[11 July, Imbros] Landing from motor boat was met on pier by Cairns [printer from Survey of Egypt] who took my kit up to my tent, while Captain ______ took me up to the I. and O. (Intelligence & Operations) Mess Tent and introduced me to General Braithwaite (C G S). [Philip] Graves, recently in Cairo, came up and sat down by me and stayed while I had dinner, as also Nugent and Nicholas who came up as soon as they heard I had arrived. Among other Cairo friends and acquaintances in the Mess were [George] Lloyd, late of Egyptian Intelligence, and [Cecil] Keeling, late of Residency, now doing ciphering for I.
The Intelligence Marquee is as follows:- [sketch-plan shows tables for Col. Ward, Capt. Deed[e]s, Lloyd & Graves, Capt. Smith, Cipherers, Clerks, etc.].
Had a long walk with Nugent and Nicholas that evening and saw Nugent’s trench diagrams for the first time. Their value was obvious as (was) the danger of their uncontrolled compilation [i.e. lack of trigonometric control framework].
The next morning visited C G S [Braithwaite], Colonel Ward [Intelligence], and saw over Cairns’ work [Printing Section]. The lack of appreciation of the vital value of maps and accurate maps and of a large enough scale for the intricate detached operations to be followed was amazing. Both Nicholas and Nugent severally said that annoyance had been experienced [by the GS] when the first Turkish map was captured, and it needed much persuasion and long delay before it was allowed to be forwarded to Cairo for reproduction.
Nugent, who was then on the Staff of the VIII Corps (General Hunter Weston), came over on latter’s orders to urge the reproduction, and walked the deck of the “Arcadia” for an hour with Colonel Ward urging it. Col. Ward wishing at least to postpone decision for a week and objecting to the substitution of any other map for 1:40,000 although latter was known to be unreliable and misleading.
The C G S [Braithwaite] was equally indifferent, and that the map was not merely suppressed was apparently entirely due to the VIII Corps and to Nicholas’ continued representation.
The second Turkish map (which fortunately promises very well) was apparently subsequently chosen as a substitute on no valid ground but merely because it extended further Northward and covered probable future ground of operations. Its accuracy and reliability seem never to have been considered by the General Staff, still less was an application of any test ever imagined.
The bitter need for maps led Nugent on behalf of the Artillery of the VIII Corps to seize on a few aircraft photos that had come in and see what he could do in the way of compilation. Later General Street, Comm[anding]. Staff VIII Corps, saw the diagrams he had produced and, after trying them, the VIII Corps have been satisfied with nothing else.
The formation of a new Survey Section under Nugent, and his attachment to the G.H.Q. for Survey work, was – as far as I can see – entirely due to the urging of VIII Corps, and from no appreciation of the crying need for maps by the G.H.Q….
Tuesday, 13th. Nugent and I took the morning trawler to ‘W’ Beach … After inspecting quarters and installing ourselves [in dugout on cliff above pier], we walked out to reconnoitre, and fixed main outlines of base and first series of triangulation points. In the afternoon walked out along NW coast towards rising ground on which it was hoped to get good views back to first points and good intersections on to house or trees on Atchi Baba. Were hampered in our progress by a brisk battle in progress and eventually had to give up actually reaching the site, but were satisfied it would do. Stayed on then in the main observation post for Artillery on our left flank who were shelling guns over KRITHIA, and finished up by dining with Heavy Howitzer Battery and making our way back with some difficulty in the darkness to the dug-out….
Wednesday, 14th. Next morning in dug-out, drafting report for G.H.Q … In afternoon made our way through French rear lines to de Totts Battery and MORTO Bay. Climbed de Tott and bathed in Morto Bay. Then made our own way via SEDD EL BAHR to VIII Corps Artillery Mess, where we dined.
Thursday, 15th. Next m
orning more arrangement [of triangulation] in field and squaring up reports and back to G H Q …
Friday, 16th. Spent at G.H.Q trying to get Survey question settled. Looked through the stores which the Survey Section had bought [sic; brought from England]…
Saturday, 17th. Got the Survey and Printing proposals accepted without much difficulty, but with interminable delay and difficulty in arriving at a decision….
Sunday, 18th. Set forth with Lloyd and Nugent on T[orpedo].B[oat]. for ANZAC…. Lloyd took us all [a]long right centre and to left centre (Courtneys and Quinns Posts) but there was not time to visit the left also. We lunched with S. Butler in his dug-out …
Having previously arranged that we would, upon arrival, go off and see Commander Douglas (Hydrographer), we signalled the “Exmouth” and went off there to dinner. Douglas’ few notes appear elsewhere [location unknown]. There met dining with him Fleetwood (Navigating Lieutenant of the “Humber”) and arranged to go off to his ship the next day.
Monday, 19th. At G.H.Q. in morning writing up notes for Nugent. Nugent went over to Cape Helles to get field work properly started. In evening went off to “Humber” and had a very interesting talk on range-finding and naval gunnery, and the necessity of better methods for naval fire directed against the shore. Arranged to come off next day and go out with the ‘Humber’ bombarding. She was detailed to cover one of the new Monitors (the ‘Roberts’). Would test new map from sea and see what improvements might be made from naval point of view.