Geological Survey of Ireland Information Circular 81/1

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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.

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. For stereoscopic cover every print is required. The ifight lines, with the exception of those over some offshore Islands, were orientated The index on display in the Geological Survey may be consulted during nonnal office hours. east-west. Each photograph overlaps its neighbour by 60% in this direction and overlaps its neigh- Dyeline copies of the index sheets are available for a small charge (see current price list). bouring flight line by 25% on its northern and southern margins. This means thatevery point on if you are interested in procuring photographs of a small area only, we are prepared to try the ground appears on at least two photographs. The 60% overlap is essential if the photographs to Identify the photographs for you if you send us a tracing or copy of the half-inch Ordnance are to be viewed stereoscopicafly. Survey Sheet giving the minimum area which you require. The tracing should include plenty of detail so that the area may be identified with ease. We would prefer you to select your own to interpret your requirements correctly. Table 3 gives the Stereoscopic viewing (viewing In three dimensions) photography as we cannot guarantee and east-west for normal coverage of areas up to 30 miles x A three dimensional view of the ground below may be achieved by using two photographs number of photographs north-south area measuring 14 miles from north to south and 5 miles from east to of the same area taken from different points. When the two images are fused together, using a 30 miles. For example an 7 6 - NATIONAL GRID ii/if: ðId:l I , I §00000 . ,1IlIL-;; A B D E . I I . i.:iji ...... 400000 B - _!_ _ ---- I . I I I 7jl.rl,hi.,1.,l I I FHK \ 9 300000 - S: ,,. J ðk :" - ' c; ) m hlInI$ flj \ .. I N P c LM . . '\ ,::ir.+m = 1 .Ç4 200000

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Figure 1. The zones of the National Grid. ii r,ah(,r/, 'N__ ' &ì_ The photo index sheets have been plotted on the basis of the zones of the National Grid. Every - _/I_1 :. ¡ßiiiiI'ic, I I «' ___i_ . F)1 - IðII?!. . photograph number is preceded by a letter which represents the grid zone in which the photo- . eh.,ru' *:..J graph lies. Without its identifying letter the number of a photograph is meaningless. L1_.L.4thU - V . iiai.i Based on the Ordnance Survey permission of the Government (Permit No. 3275) - by . fl, -..... (1 ß ' I 4 1, } IqAl--a,tv west will require 6 x 3 = 18 photographs. For stereoscopic coverage this figure is simply multiplied %.,'

by 2 = 36 photographs. You may be able to get away with fewer photographs; an estimate of the LLagha - .,. -. absolute minimum, may be found by reducing the number of photographs, read from the table - .

east-west and north-south, by one (6 x 3 becomes 5 x 2= 10 as opposed to 18 photographs). ;. . -- -- D ... .

Tieve hid - - NORTH - SOUTH EAST - WEST Leithid I 0 . Taobh an \ .3 LAlzuyf'* vI,ddh,uwn I j Miles Photos Miles Photos Miles Photos Miles Photos Miles Photos Miles Photos th,ft j VFi(I/Uijht LA OUijiII LA..m,,iri 1 1 11 5 21 8 1 1 11 4 21 7 ¡ ( 1$16 £ : -. 2 2 12 5 22 8 2 2 12 4 22 7 AitaìzL. 3 2 13 5 23 8 3 2 13 5 23 7 ÇB ... I SM ,. 4 2 14 6 24 9 4 2 14 5 24 7 ' bhairi_A&.n. / ) 5 3 15 6 25 9 5 3 15.5 25 8 6 3 16 6 26 9 6 3, 16 5 26 8 S 7 3 17 6 27 9 7 3 17 627 8 !ÇT" &33RI(AL _____ 8 4 18 7 28 10 8 3 18 6 28 8 9 4 19 7 29 10 9 4 19 6 29 9 10 4 20 7 30 10 10 4 20 6 30 9

FIgure 2. A portion of the photo index map Grid, is shown to sca'e. Detail TABLE 3. Photographs required to cover given areas A portion of the photo index map for the B zone, of the National may be seen beneath the over- from the half-inch Ordnance Survey Sheet 1 (northwestern Donegal) For normal coverage of the lapping frames marking the outlines of every second photograph. be required. For stereoscopic Viewing the photography coastal strip shown, photographs B97, B99, B101 and B103 would B99, B100, B101 B102 and B103 would Copies of the photographs may be examined, with a view to purchase, at the Geological coverage of the same strip, photographs B96, B97. B98, be required. Survey (address given below) when the sales area Is open. When you lay the photographs out you photographs The photo index sheets may be purchased separately, or as a set. The frames of the the that these may find some of the prints are apparently upside down. This is due to fact that the aircraft from adjacent sheets are often shown on the sheet margins in dotted outline. Remember flew In opposite directions on adjacent flight lines. The way up may be determined from the are designated by the letter of the adjoining sheet. flight indexes. Based on the Ordnance Survey by permission of the Government (Permit No. 3274)

8 9

Is. APPENDIX Ordering prints and diapositives Once you are satistled that you know the zones and the numbers of the photographs which Survey Photography you require, an order may be placed with the Geological or with the Ordnance Survey using the Geological proper form with copyright declaration. Details of the prices etc. are given on the current price list. Data Sheet Ordering enlargements The cost of enlargements is based on the size of paper which has to be used. The paper sizes required for whole-negative enlargements at scales ranging up to 1:7,500 are given in table 4. 1:30,000 (nominal) 2.11 inches to 1 mile. After choosing the appropriate paper size for the scale required, the cost can be obtained from the SCALE OF pHOTOGRAPHY: current price list. It should be noted that partial enlargements are more expensive than enlargements of whole negatives, because they cost more to produce. 15,000 ft or 4,560m with block adjust- ALTITUDE AT WHICH FLOWN: ments for mountainous areas. Paper Size Range of scales to fit paper sizes

24 x 24 Contact print only 1:30,000 Wild RC8 and RC1O with 6" (152mm) SO x 50 1:30,000 to 1:15,000 CAMERAS: 65x65 1:13,000 to 1:11,000 focal length. 90x90 1:10,560 to 1:7,500 120 x 120 Partial enlargement recommended over 1:7,500 April-September inclusive 1973-1977. DATES: Note: It is worth asking yourself how exact an enlargement you do in fact need, given the scale effects of the photograph. For instance, would the maximum enlargement on to Details of dates, altitudes and photographic 65 x 65cm paper be sufficiently DETAILS AVAILABLE: close to 1:10,560 to suit your purposes? operations, available on request. TABLE 4. Whole negative enlargements: scales to fit standard paper sizes. quality prints available. If you do require a partial enlargement it is desirable that you indicate the exact area QUALITY: High definition, high PHOTOGRAPHIC 1973 photography. which you require to be enlarged on a tracing or xerox copy of the contact print. If you measure Contrast a little high in the longest side of the area and multiply this by the enlargement factor given in Table 3 you can select the correct paper size and cost from the current price list. For example an area 12 x 8cm quality. Accuracies of on the contact print enlarged to 1:7,500 will have its long axis multiplied by four (12 x 4=48cm). Full photogrammetlic PHOTOGRAMMETRY and planar location This will fit nicely onto 50 x 50cm paper. When ordering, please give the scale and/or the en- as low as im for vertical largement factor. of spot heights recorded. Expect t im for spot heights in areas of normal. Paying for photography clear detail. Contouring to Sm Orders should be accompanied by a remittance for the whole cost of photography etc. Please note that all cheques and orders must be made out to THE ASSISTANT DIRECTOR 60% forward and ORDNANCE SURVEY. A current price list will be supplied at any time on request. This gives a Navigation excellent; FLYING: no gaps in either list of the products normally available. 25% sidelap; there are normal or stereoscopic coverage. Flight for offshore Copyright and royalty line orientation E-W except A small royalty is included in all sales, and copyright is reserved on all photography sold. islands. If it is your intention to reproduce for any of the photography or to use Ordnance Survey maps x 23cm) inside frame. for photogrammetric purposes, please enquire about your position with respect to copyright and CONTACT PRINTS: 9" x 9" (23 possible royalty obligations. Equivalent to 4.3 miles (6.87km) on the ground or area of 18.5 sq. miles (47.2 sq. kin).

1:2,500 recommended. ENLARGEMENTS: Enlargement up to Geological Survey of Ireland, 14 Hwne Street, Dublin 2.