A2.2NP1Environmental Practical 1 INTERPRETATION OFAERIAL PHOTOGRAPHS
Summary • The aerial photograph • Outline of aerial surveys • Thematic interpretation • Thematic classes • Spatial patterns • Practical interpretation • Image enhancement
The Aerial Photograph • Aerial photographs are a very useful source of data for environmental surveys. They have applications in: • Topographic surveying • Geomorphology • Geology and resource surveys • Soil surveys • Land use and crop management • Urban and/or industrial development • Archaeology • to name but some!
The Aerial Photograph • There are three types of film in common use: • Black and white panchromatic • True colour • False colour infra-red • Each of these has their particular advantages and drawbacks. • They are complementary - not alternatives.
The Aerial Photograph Scale and Distortion • The scale of a vertical photo is the ratio of the flying height to the focal length of the lens. If the ground height varies, the scale will thus change. • This is seen as a displacement of the image point along a line radially outwards from the centre of the photograph. • The amount of displacement depends on the height of the image point and increases towards the edge of the photograph. • Thus an aerial photograph is NOT a map.
The Aerial Photograph Photographic Tone • Tone is the brightness of a black and white image. It is determined by the intensity of light falling on the photographic emulsion for a given duration. • Over a given range the tone is linearly related to the light intensity - the gradient of this relationship is the sensitivity of the film. • The range of tones in the image is its contrast.
The Aerial Photograph Photographic Colour • True colour photographs record the intensity in several wavelengths using the tone range of dyes, usually red, green and blue.
The Aerial Photograph Photographic Colour • False colour photographs use one colour to record the tone range in another wavelength, e.g a red colour is used to show infra-red radiation.
Outline of aerial surveys • Vertical aerial photos are taken along the flight line at intervals which allow for a 60% overlap of each photo. Successive flight lines side-lap by ca. 30%. • Survey cameras are of 150-200mm focal length and use large-format film from which standard sized prints (250mm square) are made. • The cameras are mounted on gimbols which ensure the optical axis remains (nearly) vertical in flight.
Outline of aerial surveys • The overlap allows pairs of photographs to be viewed stereoscopically - each part of the ground is photographed from two positions. • The use of a 60% overlap allows every alternate photo to be removed to leave a photomosaicwhich still has 10% overlap for matching.
Outline of aerial surveys • Oblique aerial photographs are normally used for visual impact and perspective. • They sometimes give a better impression of geomorphology within a single photograph. • However, they can be more difficult to interpret and cannot easily be used for map-making.
Outline of aerial surveys • In some surveys a set of cameras may be used, each carrying different films or using different lenses. • Thus a set of vertical cameras might use black & white, colour and infra-red film and be supplemented by a wider angle or an oblique camera.
Outline of aerial surveys • Each photograph carries the following information: • Survey name or reference number • Individual photograph serial number • Date and time • Geometric reference marks • Plus possibly also: • Flying height • Camera details
Thematic Interpretation • Aerial photographs require interpretation. • Two quite different forms of interpretation are in use: • Thematic interpretation • Photogrammetric interpretation
Thematic Interpretation • The aim of thematic interpretation is to use relative brightness and/or colour plus spatial pattern to map the distribution of some aspect such as soil or vegetation. • The aim of photogrammetric interpretation is to use relative geometric position to provide information for topographic map making.
Thematic Interpretation • Thematic interpretation is based on the subdivision of the photograph into areas that are visually distinct. These are termed thematic classes. • Some such interpretation may require intelligent guesswork, based on experience and the collation of evidence. • Most thematic interpretation is thus subjective to some extent.
Thematic Interpretation • Each thematic class is then interpreted in turn. • Ground truth is essential for accurate interpretation, since many tones or textures can appear identical on a photograph yet can arise from quite different causes. • It is therefore important to visit an area if at all possible in order to establish what the classes actually are in any particular case.
Thematic Interpretation • The evidence from a single photo is usually quite limited. At the very least a stereo pair will give surface relief and texture. • Ideally different types of photo should be used together - e.g. black & white plus colour or infra-red. This a simple example of so-called multispectral sensing
Thematic Interpretation • Quite often no unambiguous answer can be obtained from photographic evidence alone. • The best outcome may be a set of hypotheses that require more (or different) evidence to resolve them. • Even experienced workers may disagree on the correct interpretion of some images.
Thematic classes • Thematic classes can be defined using a variety of features. • The principal requirement is that the classes must be consistent and related to the factor(s) upon which the study is based. • The following can often be used to define thematic classes:
2 1 Thematic classes: (1) cultivated ground; (2) woodland
1 2 Mixed farmland: (1) grain; (2) scrubland
Shingle ridges Mudflats Beach SEA
Bare shingle beach Vegetated shingle ridges Mudflats and channels Scrubland Woodland Cliff-line Cultivated land (mottled)
Thematic classes • The tone is determined both by the albedo (reflectivity) of the subject and by the strength and direction of the illumination. • Thus the tone of a area of ground can appear different at different times of day or of the year. • If the image is available in digital form it may be an advantage to use image processing techniques to emphasis the classes.
Thematic classes • In general, comparisons are best made within a single photo or else within a given run of photos. • Comparisons between runs can be misleading if they were flown at different times under different illumination conditions.
Spatial patterns • The interpretation of spatial pattern is a central objective photographic interpretation as a whole. • The assumption is that the visible spatial pattern is caused by variation in some underlying parameter of interest to the study. • The objective is to extract and explain this variation.
Spatial patterns • Geomorphological patternsmay be erosional, and so related to the underlying bedrock geology, or they may be constructional and so related to drift deposits. • They can be used to define areas of historical or geological change, natural hazards, mineral resources, potential land uses, limits to development and so forth.
Spatial patterns • Drainage patterns are defined by network shape and by areal density. • They are controlled by bedrock type, structure and climatic regime. • Vegetation patterns may reveal soil types, water content, pollution, old settlements, archaeological remains and relict ground structures (frost disturbance, landslides, subsidence) etc.