Geological Survey of Ireland Information Circular 81/1

II DF000 192 .4.' .1 lI! I tlit. (hts] [IX!i i 11! I !L4'I') I IT i it Information Circular 81/1 Aerial Photography AERIAL PHOTOGRAPHY Information on the 1:30,000 aerial photographic survey of Ireland Prepared by The Aerial Photography Unit GEOLOGICAL SURVEY OF IRELAND INFORMATION CIRCULAR 81/1 Published by authority of the Minister for Industry and Energy. Printed at the Geological Survey Office by John Duffy. 1981 Bray, County Wicklow, (photo number 0 176). The cover illustration shows a portion of the 1 :3OOOO print of FOREWORD A vertical aerial photographic survey of Ireland was originally commissioned by the Geo- logical Survey of Ireland for use in the construction of geological maps. During the four years since the completion of the survey, however, there has been increasing demand for the photographs from outside the Geological Survey, mostly from institutes and individuals involved in various professional pursuits, but also from the general public. The aerial photographic survey was the,first complete survey of its kind of the whole of the State and consequently many parts of the country are covered by vertical aerial photographs for the first time. The photographs are therefore relevant to a far wider public than seemed possible In the past and the numerous enquiries which the Geological Survey receives from all parts of the country clearly demonstrates a growing interest in their existence. This booklet has been designed primarily to introduce non-professionals to the photographs and to their potential uses, as well as to their limitations, but the section entitled 'Selecting and Ordering' should be of particular assistance to all users of the photography. CYRIL E. WIWAMS Director. © Geological Survey of Ireland 1981 INTRODUCTION TO THE PHOTOGRAPHY Between 1973 and 1977 the Geological Survey of Ireland organised the first complete aerial photographic survey of the Republic of Ireland. The photography was undertaken by the Institut Gdographique National, of France and the negatives obtained under this contract have now been returned to Ireland. High quality prints, enlargements and diapositives are available from the Ordnance Survey which is storing and processing the negatives on the Geological Survey's behalf, The object of this brochure is to explain the photography, its uses and its availability to the non- professional user. Readers familiar with aerial photographs may wish to turn directly to the section on 'Selection and Ordering'. VERTICAL AERIAL PHOTOGRAPHY Vertical aerial photographs are taken with a large and highly sophisticated camera mounted us the floor of an aircraft so that the lens is polnting vertically down during flight. Photographs are taken at successive intervals so that they overlap each other in the direction of flight. Each flight Is flown parallel to the next so that the photographs also overlap at the sides and in this way a cõntlnuous sheet of overlapping photographs is formed. Fields, roads, rivers, towns and forests can be clearly identified on the photographs together with a wealth of other detail. A photograph is not a map Though an aerial photograph looks like a map, distances on the photographs are distorted. This is particularly true towards the edge of the photograph where the camera Is, as it were, looking sideways. Distances are also distorted by the effects of topography below the camera. For this reason the photographs can only illustrate and must not be used, for example in court proceedings, to measure distances. The definition of the photographs The detail to be seen on aerial photogriphs depends mainly on the scale of the photography which, in turn, depends on the flying height of the aircraft. The Geological Survey photography was flown at a height of 15,000ft to give a nominal scale of 1:30,000 or about two inches to one mile. A small field measuring lOOm x lOOm (about 2 acres) will, at this scale, appear as a rectangle a little over 3mm square. Houses therefore appear as very small dots. What you can see depends very much on an object's size, how sharply the object is defined and, to a certain extent, on your ability to know what you are looking for. Some Uses for aerial photographs The Geological Survey conunlssloned the photography primarily for photo-geological interpretation but the photographs are also suited to a number of other purposes. They may be used, for instance, by soil scientists to trace the extent of particular soil types, through changes in vegetation, or by foresters to study species distribution and to examine ground conditions in advance of afTorestatlon. In the educational sector the photographs can be Utlilsed In geographical and environmental studies as well as providing aids to the archaeologist. In planning, the photographs are suitable for projects ranging in scale from individual farm development to desk studies for major engineering projects. It requires expert knowledge to interpret aerial photographs over wide areas, but for the farmer, or teacher the visual impact of the aerial photograph, particularly when enlarged, can be more valuable than a map. Anyone viewing an aerial photograph of an area known to them will find that their personal knowledge of the area will come to their assistance in interpreting the photograph. eyes were 13 miles apart, the distance flown by the aircraft between Prints and enlargements (some vital statistics) stereoscope, it is as if your the relief of the ground and objects is greatly exaggerated. Hills rise up A contact print made from an aerial photograph negative measures 9" x 9" (23 x 23cm). At shots. Because of this, viewer as do tall buildings and stands of trees. Viewed in this way the photo- 1:30,000 the vital statistics are as follows: sharply towards the graphs are especially valuable for interpretation. They can also be used now to make maps. Flying Height Scale Length of side Area of Photograph Jnstruments (m) (ft) (nominal) (kin) (miles) (sq km sq miles) A simple stereoscopic viewer may be bought for a few pounds. The best results, however, are obtained using a standard mirror stereoscope which enables the whole overlapping area to be 4,572 15,000 1:30,000 6.87 4.3 47.2 18.5 viewed at once and enlarged with the aid of inbuilt binoculars. These stereoscopes axe more a matter of hundreds of pounds. (Note: None of the measurements can be completely accurate; remember a photograph Is not expensive; simple measurements of height may be made with the mirror stereoscope the actual a map) While making of maps is done with highly sophisticated plotting instruments costing tens of thousands owned and operated by commercial companies, which TABLE 1. Some basic data on the photography of pounds. Such instruments are usually can produce maps from the Geological Survey photography. What you can see may be greatly enhanced by enlargement. Up to 1:7,500 there Is no visible the loss in quality. The grain of photograph is noticeable at 1:2,500 so we do not recommend p&.ki1 maps from photographs (accuracies expected) enlargement this scale. You can have the whole or part negative enlarged. assist beyond of the To The accuracy of maps constructed from aerial photographs depends on the scale of the you in choosing an appropriate enlargement, some enlargement factors for selected scales are photography and its quality, as well as the instrument and the skill of its operator. The instrument, given the on in Table 2 together with distance the print of lOOm on the ground. when set up, will remove from the photograph all the inaccuracies mentioned earlier. The photo- grammetist, viewing the stereoscopic landscape below, is able then to plot roads, houses and rivers Scale Negative Gives Distance on print of Multiplied by a In their correct positions. He is also able to plot contours and measure spot heights from the (enlargement factor) scale of of lOOm on ground photographs. (mm) Accuracies down to lm in plan and in in elevation, for carefully determined spot heights, Survey's photographs and contouring from them at 5m 1:30,000 1 1:30,000 3.3 have been obtained from the Geological practise. 12 125,000 40 intervals is standard 66 20 115,000 Diapositives (prints on clear film) 284 110,560 94 Ordinary prints are not suitable for accurate map-making on sophisticated plotters. Tran- ordering photographs for 3 110,000 100 sparent prints, or diapositives are required for this purpose. When photogrammetry it is wise to get both prints and diapositives of the area remembering also that ,, 4 1 7,500 133 you will require stereoscopic cover such that the whole area to be mapped occurs on at least 6 1 5,500 200 two photographs. 12 1: 2,500 40.0 SELECTING AND ORDERJNG The enlargement factor = Scale of negative Choosing photographs (flight indexes) desired scale The photography is indexed on seventeen map sheets, one sheet for each onshore zone of Note: (As the scale of the negative is nominal, other scales and measurements are also the National Grid (Fig. 1). The sheets axe on display in the Geological Survey and a portion of the nominal) B zone Is shown in Fig. 2. On this, as on all the index sheets, the outline to every second photo- frame is shown against a background from the half-inch Ordnance Survey map. it is easy to TABLE 2. Some enlargement factors for given scales interpolate the intervening photographs from the index if necessary. To obtain continuous ground cover you will require only every second print, as these overlap each other by 10%. This is referred How the photographs fit together to as normal cover.

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