Presentation on theme: "Aerial Photography and Photogrammetry"— Presentation transcript:
1 Aerial Photography and Photogrammetry Interpretation and Measurement
2 Aerial PhotographsPhotographs taken from a platform, usually an airplane, flying above the earth’s surface.Can be taken from space (by astronauts) but usually taken from within the atmosphere
3 Types of aerial photos Vertical air photos Oblique air photos Vertical (on nadir) or nearly vertical angle to the local ground surface (90° ± 3°)Oblique air photosTilted away from verticalHigh-obliqueShows the surface, the horizon, and a portion of skyLow-obliqueShows only the surface
11 Analog vs. Digital Analog: Film Digital: Electronic storing device Emulsions react with lightB&W, Color, Color infraredPrints, negativesDigital: Electronic storing deviceCalibrated sensorsDigital values
12 Film vs. DigitalFilm uses grains of silver chloride embedded in gel rather than pixels.Silver chloride turns to silver (opaque) when exposed to light. Creates a negative.Must pass light through negative to create a positive (print).Typical B&W film sensitive to UV through red wavelengths (panchromatic)Some films sensitive to infrared (IR film)
13 Characteristic CurveFilm records radiance as tone – the more light (radiance) that hits the film, the more grains of silver chloride are converted to silverThe relationship between radiance and tone is captured in the characteristic curve of the film.
14 The Characteristic Curve Image courtesy Sprawls Educational Foundation
32 Photointerpretation Keys can be used to standardize interp. Photointerpretation is both a science and an art
33 PhotogrammetryTechnique of obtaining reliable measurements of objects from their photographic imagesHeights of objectsAreasLengthsDensityEtc.
34 Photographic scaleRelationship between the linear distance on a vertical photograph and the corresponding actual distance on the groundScale is expressed as ‘representative fraction (RF)’ between linear measurements on photo (the numerator) and corresponding distance on the ground (the denominator)
35 Photo scale –> ground distance Example1/24,000 or 1:24,0001 unit on photo = 24,000 units on ground1 cm = 24,000 cm1 mm = 24,000 mm1 inch = 24,000 inches1 inch = 24,000 in / 12 in/ft = 2,000 ft
36 ScaleScale is the ratio of the measured length of an object on an image to its real length on the groundAlways expressed as a ratio (e.g. 1:24,000)Small scale photo covers large area on groundSmall scale photo has less detailLarge scale photo cover small area on groundLarge scale photo has considerable detail
38 Scale: Photo-Ground distance Scale (RF) when given a photo measurement (PD) and the corresponding ground measurement (GD)
39 Scale: Photo-Ground distance Distance between two pointsOn the ground = 1200 mIn the photo = 5 cm
40 Important facts Scale is not uniform within a photo Pitch, roll, yawTerrainUsed for vertical airphotos onlyAverage or Nominal scale
41 Photogrammetry Summary You can calculate many characteristics of ground properties (e.g., building heights, shrub density) from aerial photographs if you know the scale and can use simple geometry and logic.