1 Photointerpretation

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3 Elements of Photography Photographic Films an undeformable plastic film coated with a layer of light sensitive material (emulsion) the emulsion consists of a layer of light sensitive silver halide crystals, or "grains", held in place by a gelatin when the emulsion is exposed to light, the grains undergo a photochemical reaction which leads to forming the so-called latent image, invisible in the development process, the exposed silver salts are reduced to silver grains, that appear black, thus forming a visible image

4 Elements of Photography

5 Photographic Films - Resolution Because of the grain of a photographic image, there is a physical limit to the size of an object that can be recorded; this limit is called resolving power of a film

6 Elements of Photography Photographic Films – Geometric precision –is influenced by: the capability of recording smallest details, which is a function of the relative contrast between objects to be represented, the optical characteristics of the camera and the lens used, and of the resolving power of the film. As an indication, some average resolution values for b/w panchromatic films are the followings: Average scale of the flightResolution (m) dimensional stability of the support; planarity of the support.

7 Elements of Photography Black and white films –orthochromatic: seldom used, with sensitivity range from ultraviolet to 0.55 mm (colours: violet, blue, green), corresponding to natural sensitivity of silver salts –panchromatic: to cover the whole range of the human eye's sensitivity, the emulsion is added with dyes, obtaining a sensitivity range from 0.4 to 0.7 mm, the complete visible spectrum –infrared: with appropriate dyes, emulsion sensitivity can be extended up to 0.95 mm, so covering the whole visible spectrum and a portion of the near infrared; farther extensions of sensitivity, even if technically feasible, do not have practical interest because the resulting emulsions are rather unstable

8 Elements of Photography Colour Films A colour film consists of three different emulsions, each of them being sensitive to one third of the visible spectrum. As the natural sensitivity of silver salts is in the region of blue, between the blue and the green layer an yellow layer is added to function as a filter; it absorbs the blue light thus impeding to expose the underlying layers, which are also partly sensitive to blue –Blue –Yellow filter –Blue + green –Blue + red –Support

9 Elements of Photography Colour Films – Infrared False Colour

10 Elements of Photography Films Types – Summary

11 Elements of Photography Filters to improve radiometric resolution or contrast –ultra-violet filters –yellow filters to improve homogeneity of exposure –antivignetting filters required by the film type –yellow filters with infrared false colour film

12 Aerial Photographs Photogrammetric cameras

13 Aerial Photographs Photogrammetric cameras

14 Aerial Photographs Focal length –The focal length is very important because it determines the field of view, hence the amount of earth's surface that can be framed by a single photograph. –In aerial photography, a "normal" lens has f=300 mm, a "wide angle" has f=150 mm and "super wide angle" has f=90 mm. Lenses with longer values of f are termed "telephoto".

15 Aerial Photographs Photo Scale

16 Aerial Photographs Taking Aerial Photographs

17 Aerial Photographs Taking Aerial Photographs

18 Aerial Photographs Taking Aerial Photographs

19 Aerial Photographs Taking Aerial Photographs

20 Aerial Photographs Effect of Relief on Scale

21 Aerial Photographs Stereoscopy

22 Aerial Photographs Stereoscopy

23 Aerial Photographs Some formulas... –1) flight base B (ground separation between two consecutive photos): B = Lf · (1 - p/100) where: Lf = ground distance corresponding to the side of the photo P = percent of endlap –2) spacing of two adjacent flight lines A: A = Lf · (1 - q/100) where: q = percent of sidelap –3) ground stereoscopic zone Sz: Sz = A · B = 2Lf · (1 - p/100) · (1 - q/100) –4) number of photograms N needed to cover a given area: N = St / Sz where: St = ground area

24 Aerial Photographs Characteristic elements of an aerial photograph Frame number Altimeter Spirit level Fiducial mark Date and time Client or Company name

25 Airborne Digital Sensor Example: Leica ADS40 –Three sensors in one — black and white, color and NIR false color –High quality DTMs from three-line stereo sensor data –Reduced ground control requirements –End-to-end digital flow — no film processing or scanning –Seamless strip imagery along each flight line

26 Photointerpretation Principles Photoidentification: recognition of objects in a photogram Photointerpretation: photographic images analysis for object identification and their meaningfulness evaluation, to deduct information that can not be observed directly on the photograph. Of course, as photointerpretation cannot be done without photoidentification, the term photointerpretation is commonly used to include both steps

27 Photointerpretation Principles The elements by which object recognition is possible are: –colour and tone –shape and size –texture –shadow –distribution –localisation –association

28 Photointerpretation Colour and tone More than tone, we use tone contrast; it is strongly affected by atmospheric transparency Tone is affected by sun orientation, position in the photogram, surface characteristics Tone is not a fixed property of a given object The use of colour greatly increases the possibility of detecting objects

29 Photointerpretation Shape and size Many objects can be identified also on a single photo, but the three- dimensional model obtained by a stereoscopic couple provides much more information The minimal size for an object in the image be identified is mm, if there is enough tone contrast with surrounding objects or background The stereoscopic threshold, i.e. the minimum height that an object must reach to be perceived in relief when observed in stereoscopy, is a critical parameter in photointerpretation. The general formula to calculate height h of a given object is: h = H · dp / P where: H= flight height above ground dp= difference of parallax between the top and base of the object P= absolute parallax of the base of the object

30 Photointerpretation Parallax

31 Photointerpretation Shape and size h/(H-h) = dP/P h = HdP/(P+dP)

32 Photointerpretation Photointerpretation keys Photointerpretation keys are a valid helping tool for image analysis –Selective keys: a series of descriptions and illustrations, sometimes also with sample stereoscopic pairs. The photointerpreter selects, based upon his synthetic opinion, the description that fits better with the object on the image to be identified –Dichotomous keys: the photointerpreter is driven through successive choices, from the general down to the detailed level, using the presence or the absence of elements that are identifiable in the image A complete dichotomous photointerpretation key is very difficult to prepare

33 Photointerpretation Photointerpretation keys

34 Photointerpretation Photointerpretation methodology Systematic Free

35 Photointerpretation The Minimum Mapping Unit 1: m 1 cm 1 cm 2 on the map = 1 ha on the ground

36 Photointerpretation The Minimum Mapping Unit 1: m 1 cm 1 cm 2 on the map = 25 ha on the ground

37 Photointerpretation The Minimum Mapping Unit thematic cartography always implies a certain amount of generalisation to produce an information that is coherent with final user's demand ( ---> the scale) given a final scale of the work, there is a limit to the minimum size of an object that can be represented on a map –theoretical limit, beyond which the human eye is no more able to distinguish an area from a point –technical limit, relating to cost and precision of the printing process –practical limit at IAO we adopt as minimal mapping unit: – an area of 4 x 4 mm at the final scale for objects more or less polygonal in shape –for linear objects, the limit is a minimum width of 2 mm, with at least 7-8 mm of length

38 Photointerpretation The Minimum Mapping Unit 1: m 0.4 cm 1 cm 2 on the map = 6.25 ha on the ground

39 Photointerpretation Transfer –An aerial photograph cannot be overlaid onto a topographic map, even after enlargement or reduction to the same scale Orthographic vs. Perspective projection

40 Photointerpretation Transfer Radial distortion

41 Photointerpretation Transfer Vertical displacement

42 Photointerpretation Transfer Tilt displacement

43 Photointerpretation Transfer - orthophotos

44 Photointerpretation Transfer – other methods At sight, –on a map –on a photomosaic –on a satellite image Using optycal systems Using photogrammetric devices

45 Photointerpretation Satellite images –stereoscopic viewing (with some exceptions) is not possible: relief is "interpreted" rather than viewed, and this asks for a certain degree of experience –variation of scale and relief deformation are almost null in satellite image, because of the very high flight height and the narrow visual angle –so, if the satellite image has been georeferenced, interpretation and transfer are a single operation –the role of colour becomes very important: satellite images very often have non-real colours (FCC, False Colour Composit), each of them with its own significance in term of reflected wavelength –all those elements that indirectly help in object identification, such as texture, association, site etc., become more important –image quality for photointerpretation purposes is strongly influenced by the processing procedures, especially filtering and contrast enhancing: the photointerpreter should always take part to the processing of the images that he must interpret

46 Photointerpretation Satellite images –linear features and limits between objects, are almost never clearly defined, at least at scale between 1: and 1: ; only at this latter a Landsat TM 30 x 30 m pixel shrinks to 0.1 mm in size, so that it is no longer perceived by the human eye as a separate bi-dimensional object –satellite images allow synoptic vision of large portions of land, so recognising many small scale elements that would be otherwise too fragmented in a set of aerial photos: due to this reason, for example, satellite images are very appreciated by geologists (identification of faults, structures, etc.) –satellite images can provide several spectral bands, and observations can be repeated after a short delay; images can be taken in different moments of the year, thus providing multi-temporal information –using satellite images, comparisons are now possible with a rich series of historical data (first MSS images date back to 1972); this give valuable information about phenomena that are continuous and very extended in space and time, such as desertification or deforestation (global changes)