26 2 Geological Mapping in Exploration CLICK 8 9 10 11 12 %side lop 、BUN QNE, 6 4 3 2 60%forward lap Fig.2.3 Typical specification of an air photo survey designed to obtain full stereo coverage 2.2.2 Acquiring Air Photographs Many governments (including all first world countries)have acquired air photo coverage of their territories and these can usually be purchased from the relevant government agency.Needless to say,the quality and coverage of this product varies enormously,but since it is a cheap resource,it is always worth checking to see what is available.In areas where there has been a high level of mineral explo ration,sur veys fown by previous explorer nay also be available.If none of these avenues y a useful product,it is p ssible to commi on your own sompable in cost to purchingh reolution ste maey for em area and gives the opportunity to specify a scale and coverage that will suit your project. 2.2.3 Geological Interpretation Air photographs(along with other similar remote sensed products such as satellite and radar imagery)provide both a mapping base on which to record field observa- tions and an integrated view of landscape on which map-scale patterns of lithology and structure can be directly observed or interpreted.Where available at a suitable scale and resolution,they are the pre-eminent medium upon which to construct a geological map. For any geological mapping po makings imagery. image interpre e erating. ning phases of that progrmme e,co .The initial rol and pla provide:
26 2 Geological Mapping in Exploration CLICK ! 3 1 2 5 4 6 11 12 9 10 8 7 20% side lap 60% forward lap RUN ONE RUN TWO Fig. 2.3 Typical specification of an air photo survey designed to obtain full stereo coverage 2.2.2 Acquiring Air Photographs Many governments (including all first world countries) have acquired air photo coverage of their territories and these can usually be purchased from the relevant government agency. Needless to say, the quality and coverage of this product varies enormously, but since it is a cheap resource, it is always worth checking to see what is available. In areas where there has been a high level of mineral exploration, surveys flown by previous explorers may also be available. If none of these avenues yields a useful product, it is possible to commission your own survey. This is comparable in cost to purchasing high resolution satellite imagery for the same area and gives the opportunity to specify a scale and coverage that will suit your project. 2.2.3 Geological Interpretation Air photographs (along with other similar remote sensed products such as satellite and radar imagery) provide both a mapping base on which to record field observations and an integrated view of landscape on which map-scale patterns of lithology and structure can be directly observed or interpreted. Where available at a suitable scale and resolution, they are the pre-eminent medium upon which to construct a geological map. For any geological mapping programme making use of remote sensed imagery, image interpretation represents the idea-generating, integrative, control and planning phases of that programme. The initial interpretation made from the images will provide:
2.2 Mapping Using Reflectance Imagery as a Map Base 之 definition of areas of outcrop and areas of superficial cover: preliminary geological interpretation based on topographic features,drainage geological hypotheses for field checking: selection of the best areas to test these hypotheses; .familiarity with the topography and access routes to assist in logistic planning of the field programme-access roads and tracks,fording points for streams and gullies,potential helicopter landing sites,etc. Air photo or satellite image interpretation needs to be carried out before,during and after the field phases of the mapping process.Obviously,detailed interpretation making use of stereo viewing can be most conveniently done at an office desk,but. the fclne Thein"elation of p咖ooa0ics心etobe attemple ability to use a pocket stereoscope on the outcrop is ar es tial skill to acquire. Since makingand interpreting th photo and ev p te om Geological interpretation of remote sensed imagery complements field mapping and should never be regarded as an adequate substitute for it. Skills required for the geological interpretation of remote sensed imagery are very much the same as those needed for field mapping.However.some geologica maps.The next section describes some of these techniques 2.2.4 Determining Scale The scale of an air pho tograph is deter mined by the height above the ground of the airplane taking the photograph,divided by the focal length of the camera used (Fig.2.4).Thus: Photo height above ground Focal length of camera A scale is generally printed onto the edge of an air photograph but this is a nomi- nal scale only and should always be checked for a number of scenes across the area of the air photo survey. iigyskiledans experienced geologists are availabl e who specialize in the field of air phote ms in the a possibility of any substantial field access to the region
2.2 Mapping Using Reflectance Imagery as a Map Base 27 • definition of areas of outcrop and areas of superficial cover; • preliminary geological interpretation based on topographic features, drainage patterns, colours and textures of rocks, soils and vegetation, trend lines of linear features, etc.; • geological hypotheses for field checking; • selection of the best areas to test these hypotheses; • familiarity with the topography and access routes to assist in logistic planning of the field programme – access roads and tracks, fording points for streams and gullies, potential helicopter landing sites, etc. Air photo or satellite image interpretation needs to be carried out before, during and after the field phases of the mapping process. Obviously, detailed interpretation making use of stereo viewing can be most conveniently done at an office desk, but, as ideas change or evolve, interpretation of photo features will have to be attempted in the field as well. The ability to use a pocket stereoscope on the outcrop is an essential skill to acquire. Since making and interpreting geological observations on the photo and outcrop are two aspects of the same process, they should ideally be carried out by the same person.13 Whenever possible, the field geologist should do his own interpretation. Geological interpretation of remote sensed imagery complements field mapping and should never be regarded as an adequate substitute for it. Skills required for the geological interpretation of remote sensed imagery are very much the same as those needed for field mapping. However, some practical techniques need to be learned in order to turn air photo observations into usable geological maps. The next section describes some of these techniques. 2.2.4 Determining Scale The scale of an air photograph is determined by the height above the ground of the airplane taking the photograph, divided by the focal length of the camera used (Fig. 2.4). Thus: Photo scale = 1: Airplane height above ground Focal length of camera A scale is generally printed onto the edge of an air photograph but this is a nominal scale only and should always be checked for a number of scenes across the area of the air photo survey. 13Highly skilled and experienced geologists are available who specialize in the field of air photo and satellite image interpretation. Their use is indicated for training purposes; where they have particular knowledge of the geology or landforms in the area to be mapped; or where there is little possibility of any substantial field access to the region
28 2 Geological Mapping in Exploration Photo image f:focal length of camera Camera lens Scene Scale of photo= Fig.2.4 How to calculate the scale of an air photograph The airplane altimeter height(i.e.height above sea level)and camera focal length are also n of an air phot h and provide a the e exact scale. ing the above form of the ground above sea level is known for that scene.Cameras designed for photo graphic surveys have very long focal lengths to enable them to fly at greater heights for a given scale. Even though the pilot of the plane tries to maintain a constant ground height whilst flying a photographic survey.this is not always possible.The scale can thus vary from image to image.The variation in scale from this cause is usually small. but is greatest for large scale photographs and in areas of strong relief. Another way of checking the scale is to the ler in the gth of a known feature central portio of the such ction of road or stre ）and compar Make several such measurements along different bearings on each image and take the average to get the true scale for the scene. In addition to these scale variations,the stated photo scale is correct only for the central area of the photograph and is progressively distorted towards its edges. Since the distortion increases exponentially outwards,the central 60%of the scene has only minimal distortion which can generally be ignored.The radial distortion ecteltionships.For this reason,if at all possible,interpreta should not be ied to the edge of a ph This is of photos along the flight line (the forward lap)where a60%overlap is usually
28 2 Geological Mapping in Exploration Scale of photo = h f f:focal length of camera h:height of camera above ground Camera lens Photo image Scene Fig. 2.4 How to calculate the scale of an air photograph The airplane altimeter height (i.e. height above sea level) and camera focal length are also normally marked on to the edges of an air photograph and provide a means of calculating the exact scale, using the above formula, provided the average height of the ground above sea level is known for that scene. Cameras designed for photographic surveys have very long focal lengths to enable them to fly at greater heights for a given scale. Even though the pilot of the plane tries to maintain a constant ground height whilst flying a photographic survey, this is not always possible. The scale can thus vary from image to image. The variation in scale from this cause is usually small, but is greatest for large-scale photographs and in areas of strong relief. Another way of checking the scale is to measure the length of a known feature in the central portion of the photo (such as a section of road or stream) and compare it with the same section identified on a detailed topographic map of the same area. Make several such measurements along different bearings on each image and take the average to get the true scale for the scene. In addition to these scale variations, the stated photo scale is correct only for the central area of the photograph and is progressively distorted towards its edges. Since the distortion increases exponentially outwards, the central 60% of the scene has only minimal distortion which can generally be ignored. The radial distortion also affects angular relationships. For this reason, if at all possible, interpretation should not be carried to the edges of a photograph. This is easy to do on the edges of photos along the flight line (the forward lap) where a 60% overlap is usually
2.2 Mapping Using Reflectance Imagery as a Map Base available with adjacent frames,but more difficult on the photo edges across the flight line (the side lap)where the overlap with adjacent runs is generally only 20%14 or less Air photo necessarily be en as accurat affect only a few photographs and can be picked up and corrected when adjacent photos are compared during the initial interpretation period.It is a good idea to compare each photo with a topographic base map and,where necessary,correct the north arrow marked on the photograph. 2.2.5 Stereoscopic Image Pairs When images of a feature from two different angles are taken,there is a relative shift in the on the Parallax.Thewo of the e images.This effect is known a s form aleft and ri stere pair and,takento gether.cont onal information about the feature images must be arranged for viewing so that the left eye sees the left image and the right eye sees the right image.The brain then combines the two views to create a three-dimensional impression of the feature,in exactly the same way as the eye brain combination would have created three-dimensional information if they had directly viewing the feature in the real world. Images are normally placed for viewing at a distance of around 200 mm from the (Hhei)e pproimately 55-65 mm apart (Bbase distance) oscop ic in ge pairs,the ratio of hei nd 3.If the same ratic distortion on viewing.For example,to take scale-correct stereoscopic photographs of an outcrop in the field,follow the procedure photograph the outc 0 -this beco omes the left im estimate the distar H)to the outc op say,12 m) 3.step a distance of H/3(B)to the right(4 m in our example)and take a second photograph of the outcrop-this becomes the right image. When subsequently mounted side by side on the page of a notebook or the tw 0 ges,when viewed so and the righ eye the righ onal view fthe outcrop Figure 2.5 illustrates an air photo survey with a plane fying at 200m taking pho tographs of the ground beneath it every 65 m.For this survey the ratio of H to B is 3
2.2 Mapping Using Reflectance Imagery as a Map Base 29 available with adjacent frames, but more difficult on the photo edges across the flight line (the side lap) where the overlap with adjacent runs is generally only 20%14 or less. Air photos usually have a north arrow plotted on their edge but this arrow cannot necessarily be taken as accurate – any yawing of the plane at the moment when the photo was taken can make this considerably in error. This problem will usually affect only a few photographs and can be picked up and corrected when adjacent photos are compared during the initial interpretation period. It is a good idea to compare each photo with a topographic base map and, where necessary, correct the north arrow marked on the photograph. 2.2.5 Stereoscopic Image Pairs When images of a feature from two different angles are taken, there is a relative shift in the apparent position of the feature on the images. This effect is known as parallax. The two images form a left and right stereo pair and, taken together, contain three dimensional information about the feature. To recover this information, the images must be arranged for viewing so that the left eye sees the left image and the right eye sees the right image. The brain then combines the two views to create a three-dimensional impression of the feature, in exactly the same way as the eyebrain combination would have created three-dimensional information if they had directly viewing the feature in the real world. Images are normally placed for viewing at a distance of around 200 mm from the eyes (H – height), and eyes are approximately 55–65 mm apart (B – base distance). On viewing stereoscopic image pairs, the ratio of height to base (H:D) is therefore around 3. If the same ratio is used during acquisition of the images there will be no scale distortion on viewing. For example, to take scale-correct stereoscopic photographs of an outcrop in the field, follow the procedure: 1. photograph the outcrop – this becomes the left image; 2. estimate the distance (H) to the outcrop (say, 12 m); 3. step a distance of H/3 (B) to the right (4 m in our example) and take a second photograph of the outcrop – this becomes the right image. When subsequently mounted side by side on the page of a notebook or report, the two images, when viewed so that the left eye sees the left image and the right eye the right image; will give a distortion-free 3-dimentional view of the outcrop. Figure 2.5 illustrates an air photo survey with a plane flying at 200 m taking photographs of the ground beneath it every 65 m. For this survey the ratio of H to B is 3 14Photography for mountainous areas, where flying predetermined flight lines may be difficult, needs a wider side lap of 25% or more
30 2 Geological Mapping in Exploration D=Distance between adjac 65meters In the area of H:Height of camera above SU n as a tree.is bu om TREE Area of Image 1 Area of Image 2 OBSERVER D:(eye spacing)=65mms H=200 mms Image 1 Image 2 Fig.2.5 How the perception of depth in a stereoscopic photo pair is determined.When viewing th oof the distance between the eyes(D)and the distan e(H)is H will be no ye If the ratio of HP ter than 3 (which w occur if the photos were taken closer together or from a greater height above ground)viewing the stereo pair will exaggerate the apparent height of object so there will be no vertical exaggeration when viewing adjacent images stereoscop ically.However,with almost all air photo surveys,a height to base ratio of 3 would entail flying very low and require an unacceptably large number of photographs to cover any significant area.Bec s are flown with an H:B ratic greater than 3.leading t eeofthis,motsurCfnmeasiom an exaggeration of the
30 2 Geological Mapping in Exploration OBSERVER D: (eye spacing) = 65 mms H = 200 mms Image 1 Image 2 D=Distance between adjacent image centres 65 meters H : Height of camera above ground 200 meters Click ! Click ! Area of Image 1 Area of Image 2 TREE In the area of image overlap, each object, such as a tree, is photographed twice, but from a slightly different angle Fig. 2.5 How the perception of depth in a stereoscopic photo pair is determined. When viewing images, the ratio of the distance between the eyes (D) and the distance to the image (H) is approximately 3. If the images have been collected using the same ratio of D to H – as illustrated – there will be no vertical scale distortion on viewing. If the ratio of H/D is greater than 3 (which would occur if the photos were taken closer together or from a greater height above ground) viewing the stereo pair will exaggerate the apparent height of object so there will be no vertical exaggeration when viewing adjacent images stereoscopically. However, with almost all air photo surveys, a height to base ratio of 3 would entail flying very low and require an unacceptably large number of photographs to cover any significant area. Because of this, most surveys are flown with an H:B ratio greater than 3, leading to an exaggeration of the vertical dimension on stereoscopic