AIR SURVEY Long before aeroplanes were invented it was confidently hoped that vertical photographs would some day be taken, and it was felt certain that, if so, they would greatly assist archaeology. Maj. Elsdale was the pioneer of air-photography in the British army. Between about 188o and 1887 he carried out many experiments from free balloons; but ballooning was not much in favour then and, although some progress was made, he received little or no official support in his experiments. During several years pre ceding the World War, Henry S. Wellcome successfully used large box-kites, with specially devised automatic control cameras, for photographing his archaeological sites and excavations in the upper Nile regions of the Anglo-Egyptian Sudan.
During the World War, when aeroplane photographs first be came common, it was expected that archaeological features would be observed; but in the British sector in France none were seen. The photographs were often taken at a great height, over country which is archaeologically barren, or which was too rankly over grown to show results. Only on other fronts was time found for archaeology in the air. Col. Beazeley observed and photographed in `Iraq, in 1917, cities whose ruins were unintelligible on the ground. On an air-photograph these ruins were seen as an orderly arrangement of streets and houses. This definitely proved that air-photography could add to knowledge, and that it would be an invaluable aid to excavation. To Col. Beazeley, therefore, is due the credit for the first actual application of aeroplane photog raphy to archaeology.
The birth of the new study in England, however, dates from 1922 when Air Commodore Clark Hall observed certain curious marks on R.A.F. air-photos taken in Hampshire. With him must be mentioned Flight-lieut. Haslam, who took a number of photo graphs near Winchester showing what turned out to be prehistoric fields. Air Commodore Clark Hall showed these photographs to Dr. Williams-Freeman and O. G. S. Crawford, archaeology officer of the Ordnance Survey, who saw that their expectations were fulfilled, and even surpassed, by what was revealed. It was possible from these photographs to make a map of the pre historic field-system near Winchester. Popular interest was first aroused by O. G. S. Crawford's discovery and publication of negatives showing, for the first time, the complete course of the Stonehenge avenue (eastern branch). The photographs were taken in the dry year of 1921 by the Old Sarum squadron; but their archaeological importance was not recognized until two years later, in 1923. The most important archaeological air-photographs obtained are those taken during 1924 by O. G. S. Crawford and Alexander Keiller. An aeroplane was specially hired, and about 30o photographs of archaeological sites were taken.
Outside England little has been done except in `Iraq, Palestine, Transjordan and Egypt. In Palestine many sites have been photo graphed by the Royal Air Force, at the suggestion of the Depart ment of Antiquities. A photograph of Masada on the Dead sea is described in Antiquity. Remarkable remains were revealed in the Amman region (Transjordan) by R.A.F. air-photographs. These show an elaborate system of stone walls and enclosures designed probably for the rapid herding of animals when threat ened by a raiding party. A preliminary report on these was pub lished by Flight-lieut. P. E. Maitland in 1927; further results have been obtained in the same area by Group-captain L. W. 13. Rees, V.C. In Egypt valuable archaeological results were secured as a by-product of the third Nile aerial survey (1922). Oblique photo graphs have been taken of Numantia in Spain, but obliques are always inferior to verticals in archaeological value.
It is usually imagined that the camera, when fixed in an aero plane, records marks on the ground which are invisible to the eye of an observer. That is not so. The observer can see these marks more plainly than the camera records them, for he sees them in colour. The most remarkable discoveries that have been made are due to plants, which are sensitive to slight differences of soil and moisture. For example, if a ditch has been dug on a chalk down and the down has afterwards been ploughed flat and sown with corn, for ever afterwards the subsoil filling (or silt) of that ditch differs from the adjacent never-disturbed soil. Nothing can ever restore chalk once dug to its former state. Archaeologists have long known this, for one of the principal needs in excavation is to distinguish between disturbed and undisturbed soil. But one cannot dig up a whole field or several fields to find a ditch which, after all, may not exist. Here it is that a vertical view helps; for the effect of this moister and more fertile silt upon a crop of corn is to promote its growth and deepen its colour. Thus from above one sees, and can photograph, a belt of darker green corn fol lowing the line of the vanished ditch. These lines are sometimes visible on the ground, from across a valley, or even at closer quarters. Sometimes (as in parts of the Stonehenge avenue) they are quite invisible. But always, when more than a single ditch is concerned, the distant view is necessary to convert chaos into order. The reason for this necessity can best be explained by means of a comparison. If one looks through a magnifying-glass at a halftone illustration made through a coarse screen, it ceases to be seen as a picture and becomes a meaningless maze of blurred dots. If one holds it some distance off and looks at it with the naked eye it becomes a picture again. The observer on the ground is like the user of the magnifying-glass; the observer (or camera) in the air resembles him who looks at the picture from a distance. (See SURVEYING.) The majority of British prehistoric sites, and many later ones, were a maze of ditches and pits, dug for drainage, storage, habita tion, defence or boundary purposes. Many still exist on the downs, undisturbed and turf covered; many more have been flattened by cultivation. All of the latter can be re-discovered by air-photog raphy, provided only that the arable has not been allowed to revert to grass. Even then traces of the ditches are sometimes visible, especially on poor soils and in dry summers, by a belt of darker green. Air observation, however, is most fruitful when young crops are growing; then discovery is easy and rapid, and every flight is productive. Such sites may afterwards be seen to exist by an observer on the ground ; but few of them could ever have been discovered except from the air. Chalk is not the only soil that produces these streak-sites ; they have been observed on oolitic limestone near Bath and gravel near Exbury.
Other factors enable air-photography to record ancient sites. Prehistoric cultivation-banks are revealed because they either throw slight shadows or because when ploughed they appear as belts of lighter soil, from the chalk grains mixed with them (see plate). From photographs the prehistoric field-system of a dis trict can be accurately mapped. Again, rabbits work in the looser silt or filled-up ditches (as well as in the soil of the lynchets), and if there are many rabbits a white line, or row of white patches, is visible from the air. Daisies and poppies grow from choice above these ditches, and barrows and hill-top camps have thus been revealed by white and scarlet circles. Lastly, the low shadows at sunrise and sunset etch the slopes of low banks in deep black. That is the time to photograph lynchets. On a June morning before breakfast the greater part of Salisbury plain is seen to be covered with the banks of abandoned Celtic fields ; but afterwards they "fade into the common light of day." The great ramparts of hill-top camps throw a shadow even at midday, but are best photographed when the sun is low, for then not only do the ramparts stand out best, but also the banks and pits of the habitations within.
Hambledon Hill, Dorset, England (see fig. I on plate), one of the finest hill-top camps, is on an isolated hill about 3ooft. above the surrounding country. The entrance of the camp is protected by a hook-like prolongation of an outer rampart; from it led a pathway within the camp, passing between oblong and circular pits, the site of huts. Those, especially in the middle portion of the camp, are very clearly seen, and beside them is a long barrow, far older of course than the camp, which was probably made in the early iron age. Across the narrow ridge connecting the hill of the camp with the next hill, was built a formidable double scarp to-scarp rampart ; and beyond this can be seen the faint outline of a probably older camp (only partially included). The hill must have been permanently inhabited. A scattered flock of sheep in the inner portion of the camp will give some idea of the scale of the photograph.
Oakley Down, Dorset, England, another noted site, is between Salisbury and Blandford, on chalk soil now covered with turf. The principal features are: (I) The raised causeway of the Roman road running diagonally across the district ; a group of round barrows, or burial mounds of the early bronze age, older than the Roman road, which cuts its way through two disc-barrows; (3) a network of Celtic fields, particularly noticeable in one corner of the photograph. In the valley below is the remains of a contem porary pond (very large, but now dry), and there are signs of Celtic ploughing on both sides of the Roman road.