During the past four years of the U.S.S. Nokomis’ survey along the north coast of Cuba we have developed a system of shore-line establishment by airplane photography which has proved very satisfactory.
At the start in 1924 our photographs were taken from an altitude of 5,000 feet, giving a scale of about 1 to 5,000. The practice then was to connect up the air pictures with a triangulation scheme by means of numerous “ground signals,” which were horizontal marks of considerable ground spread to insure photographing from altitudes. These ground signals require center tripods or high flags for visibility in locating them from triangulation towers. It was evident that this method would incur entirely too much extra labor and expense and greatly impede progress, so we eliminated ground signals, hoping that our ordinary surveying tripods (already erected for general use) would serve the same purpose. This they did, and together with the triangulation towers, showed plainly enough on these 5,000-foot pictures. The tryout with aerial photography was made in the harbor survey of Cardenas, Cuba; and since it was experimental, the shore line was also run by the standard method, requiring tedious sketches and instrument work. The results of both methods were plotted on the same sheet at a scale of 1 to 15,000 and found to practically coincide, except in inaccessible places, where of course the airplane pictures were far more reliable. Obviously, the pictures represent a much truer line of the shore because the old method requires sketching in between fixed points, and not all observers are adept at these sketches. Captain R. E. Ingersoll, U. S. Navy, was then in command of the Nokomis and took a great interest in the aerial photography. With his idea of taking the pictures before establishing the shore line, so that they could be used in the actual field work, we have developed the following system of airplane hydrography which has tested out to our entire satisfaction.
First, blue-print maps of small scale for use in the planes are prepared for guidance of the pilots and photographers and to serve as key charts for plotting the various runs of photographs. A roll of aerial film will take about one hundred pictures and each roll is given a key letter, A, B, C, etc., with respective pictures numbered consecutively, A-1, A-2, A-3, etc., and so entered on the key charts. If the old map of the required section can be secured we use a tracing of it for these blue prints; otherwise an air reconnaissance is made and rough sketches done for this purpose.
Weather permitting, the pictures are taken. We tried out and found (at the suggestion of Lieutenant C. H. Schildhauer, U.S.N.) that an altitude of 10,000 feet, with successive pictures timed so the advance of each negative would be one third its width, was ideal for the work, as these conditions permit accurate orientation and afford the opportunity of using the central portions of the pictures when assembling them. In addition to the general photography, oblique views are also taken to assist in assembling the various runs, particularly in sections of isolated cays so common to our territory. Two or three thousand feet altitude is sufficient for the oblique pictures.
The prints are made in duplicate, one set for the field work and the other for office reference. The shore-line officer starts out on an area about ten miles square with appropriate field set of pictures and blue print of the area showing the established surveying signals. Objects, natural or artificial, which appear distinctly on the pictures and can be identified in the field, are “occupied” with sextant or transit, and angles observed on the surveying signals. An object so occupied is indicated on the picture by a cross or small circle, given a reference number, and the angles and notes recorded in a field note book. It is not difficult in most instances to find some mark in the field discernible on the photographs such as a small cay, sharp point, house, dock, cleared or bare spot, large tree, etc., and if it is not accessible to the observer, an azimuth and distance is taken to it from a nearby point which can be properly located, and this data is entered on this picture or in the note book. These instrument locations or “fixes” serve as the major control of the areas and are taken about every mile, except that a fix is always observed on a prominent point, and on an isolated cay the minimum number of fixes is two, as far apart as possible, so the cay may be properly orientated on the plotting sheet. Along with the instrument work a study of the shore-line characteristics is made, such as mangroves, sand beaches, kind of tree growth, rocky cliffs with estimated heights, etc., and notes concerning them are entered on the photographs in the field. On returning to the ship or station after a day’s work the observer plots his angles as a check, so if any errors are found they may be rectified.
During the plotting season at our home port the field pictures are matched together in a continuous or mosaic formation by tracing with ink on a thin sheet of transparent celluloid the portions of shore line common to connecting pictures, and fitting them together under the celluloid tracing which serves as a sort of assembling template. This part of the work is a little tedious at times when variation in altitude or turning of the plane changes the scale of adjoining pictures, but we average these differences on some selected line so the orientation will be affected as little as possible. This error, however, is not cumulative to any practical extent as the instrument fixes, surveying signals, and tangents from triangulation towers and secondary stations provide excellent control.
The points or objects located by the observer in his field work, as entered on the pictures, are also located on our smooth plotting sheets by his field angles, so that we have corresponding points on both the assembled pictures and the plotting sheet. Checking on these points, the picture data is transferred to a section of thin tracing paper with a precision pantograph, adjusted to compensate the difference in scale between the assembled pictures and the plotting sheet. The pantograph work is best done by the shore-line observer (who naturally would be familiar with the territory) and another operator, one of the two following the shore line and details with the steel point for that purpose, and the other tending the pencil attachment which enters the data on the section of thin tracing paper. The tracing is then transferred to the smooth plotting sheet, making any necessary small adjustments so that the shore line will fit in with the plotted tangents and fixes. We use a prepared transfer paper for this purpose which is slipped under the tracing in position, marking through by impressing a hard drawing pencil or stylus, after the' manner of a carbon copy. The transferred data on the smooth sheet is now checked with the pictures for omissions or accidental errors and is then inked in as final. After this another reference is made to the photographs for characteristics of the shore line.
By having the airplane photographs in advance of the actual field work we have prepared some very useful reconnaissance sheets for a coming season’s work. Some of these have proved surprisingly accurate and saved many days of reconnaissance. We now have on hand sufficient pictures for probably more area than we can finish in our next season’s survey, and with them are enabled to plan ahead and to have a lot of useful data for the work in general.
In 1927, under the command of Commander Benjamin Dutton, U. S. Navy, a remarkable saving of time and acquisition of valuable coastal data resulted from a run of aerial photographs between Habana and Matanzas, while locating the prominent Hershey Sugar Mill about midway between these two Cuban cities, situated about fifty miles apart. Our other alternative was by triangulation, which would have required at least two weeks, whereas with plane and ship we obtained this location in about four hours, and in addition the coast line and a great deal of new navigational data for the general chart of Cuba. We believe our photographic position of this mill is more than sufficiently accurate for a navigator’s needs.
By the old method of running shore line, four miles a day was fast work for one party, but now with the aerial pictures fifteen miles by one party can be controlled without extra effort, and also a large amount of useful chart data is secured from the photographs.