Types of aerial photographs. Types of aerial photographs. Introduction to aerial photos. High angle oblique OBLIQUE AERIAL PHOTOGRAPHS

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1 Types of aerial photographs Introduction to aerial photos Related to camera angle Vertical Oblique Relate to film b/w panchromatic b/w infrared Color Color infrared Related to scale Types of aerial photographs Vertical Oblique Low angle OBLIQUE AERIAL PHOTOGRAPHS low angle oblique horizon not visible Oblique High angle high angle oblique horizon visible LOW ANGLE OBLIQUE AERIAL PHOTOGRAPH High angle oblique 1

2 Vertical aerial photographs National Aerial Photography Program (NAPP) 9 x 9 in 1:40,000 National High Altitude Photography (NHAP) 9 x 9 in 1:80,000 ELEMENTS OF A VERTICAL AERIAL PHOTOGRAPH Fiducial marks x Principal point NADIR 9 inches = 22 cm TAKING VERTICAL AERIAL PHOTOGRAPHS RELATIVE POSITION OF FLIGHT LINES 2

3 Stereotriplet set of infrared vertical aerial photographs of trees COMPARATIVE GEOMETRY OF A MAP V.S. AN AERIAL PHOTOGRAPH OF THE SAME AREA WHAT IS A PHOTOGRAPH? Photographic film A visual record of the response of many small detectors to energy incident upon them Silver halide grains in the film s emulsion respond to energy = film s exposure B/W FILMS - 1 SILVER HALIDE GRAINS between 0.1 and 5µm within the emulsion absorb photons that strike their surfaces: note that pure silver halide grains are only sensitive to UV and blue radiation. By adding dyes to the surface of the Silver grains, halide grains they can become sensitive to green, red, and IR. B/W FILMS - 2 THE BASE provides support for the emulsion and backing. The base must be: transparent (optically) resistant to moisture must be flexible Silver halide grains 3

4 B/W FILMS - 3 THE BACKING contains a light absorbing dye. This dye must absorb all light to prevent it from moving back into the emulsion layer What happens when a photograph is taken (exposure)? Photons of radiation reflected from a scene strikes the emulsion brighter scene produce more photons Silver halide grains 2. The silver halide grains undergo a photochemical reaction which forms an invisible latent image in the emulsion PROCESSING B/W FILMS B/W film yields negative images, both tonally and geometrically The latent image is made visible through film development Film is exposed to an alkaline solution which causes silver halide which was exposed to light to turn to metallic silver which is black Silver halide which was not exposed to light remains unchanged and is transparent Positive images are made by passing white light through the negative onto light sensitive materials. The more light passing through the negative, the darker the image 4

5 SPECTRAL SENSITIVITY OF B/W FILMS Most B/W films can record wavelengths as short as 0.25µm, below our scope of vision. However, these shorter wavelengths will not travel effectively through glass, therefore only energy from the 0.35µm and up zone is recorded captured THE ELECTROMAGNETIC SPECTRUM Remember: the major difference between these film types is the dye which coats the silver halide grains of films B/W PANCHROMATIC FILM Courtesy of: B/W INFRARED FILM Deciduous trees have a higher reflection than coniferous trees in the infrared zone of the electromagnetic spectrum 5

6 Silver halide grains Emulsion layers NORMAL COLOR FILM Each color-sensitive layer has silver halide crystals coated with color-sensitive dyes (principle same as B/W film) The yellow filter prevents blue and UV wavelengths from penetrating into lower layers silver halide is naturally sensitive to blue and UV UV filter best to use this on any camera Blue-sensitive layer Yellow filter Green-sensitive layer Red-sensitive layer Base Backing NORMAL COLOR HIGH-ANGLE OBLIQUE AERIAL PHOTOGRAPH UNIVERSITY OF WISCONSIN, MADISON Each color-sensitive layer has silver halide crystals coated with color-sensitive dyes (principle same as B/W film) A yellow filter must be used on the camera to prevent blue and UV wavelengths from penetrating into lower layers Some films have a yellow gelatin layer on top of the other emulsion layers Yellow filter Silver halide grains COLOR INFRARED FILM Emulsion layers Infrared-sensitive layer Green-sensitive layer Red-sensitive layer Base Backing Color infrared film is also referred to as CAMOUFLAGE FILM First developed by the military to detect camouflaged objects. Those painted or died green, or trees which had been cut appears blue or gray against natural vegetation which appears red FALSE COLOR FILM colors seem false as they do not look like colors we are familiar with for particular objects Color vs. color infrared film COLOR INFRARED HIGH-ANGLE OBLIQUE AERIAL PHOTOGRAPH Courtesy of: UNIVERSITY OF WISCONSIN, MADISON 6

7 LOW ANGLE COLOR INFRARED AERIAL PHOTOGRAPHS June 30 July 1 July 2 Man-made turf August 11 September 17 October 8 More examples 7