Photogrammetry

PhotogrammetryPhotogrammetryScience or art of obtaining reliable measurements (and making maps) by means of photographs.sciences of geometry, mathematics and physics that use the image of a 3-D scene on a 2-D photo to reconstruct an accurate model of the original 3-D scene.Principles of photo-grammetry are important to photo interpretation because they enable quantifying interpreted terrain features.Interpretation has mostly to do with what is present in the imagery; photo-grammetry has to do with where interpreted objects are on the ground and their size.

Basic InformationUses: Aerial photosAid: geological investigations, soil surveys, land surveys, tax mapping, reconnaissance and military intelligence, urban and regional development, transportation system investigations, quantity estimates, shore erosion, etc.Mathematical methods have been developed to make precise 3-dimensional measurements from photos.Phototriangulation: 3-dimensional positioning of survey stations.

Photogrammetry can not be used successfully over the following types of terrain.Desert or plains areas, sandy beaches, and snow the photograph as uniform shades with little texture.Deep canyons or high buildings that conceal ground surface.Areas covered by dense forest.

Basic InformationBasic Categories Metrical photogrammetry obtaining measurements from photos from which ground positions, elevations, distances, areas, and volumes can be computed and topographic or planimetric maps can be made.

Photo interpretation evaluation of existing features in a qualitative manner.Types of PhotogrammetryAerial series of photographs of an area of terrain in sequence using a precision camera.Terrestrial photos taken from a fixed and usually known position on or near the ground with the camera axis horizontal or nearly so.Close range camera close to object being observed. Most often used when direct measurement is impractical.HistoryThe first use of photogrammetry was by Arago, a French geodesist, in 1840. This included topographic and terrestrial.The first aerial photogrammetry was by the French in 1849 using kites and balloons.Laussedat (French) father of photogrammetry.1st in N. America Deville, Surveyor General of Canada.U.S.G.S. adopted photogrammetry as mapping process in 1894 mapping border between Canada and Alaska.Airplanes brought great change to photogrammetry.1st used in 1913.Used extensively for photo interpretation.After mid 60s advent of computer and plotting has made photogrammetric mapping accurate and affordable.

Geometry of PhotographsOrthographic projection each point projected normal to reference plane. Orthographic projection is a means of representing a three-dimensional object in two dimensions. Perspective projection each point projected through a central point, due to points being at different elevations, they look 3 dimensional. Approximate representation, on a flat surface (such as paper), of an image as it is perceived by the eye. Principal point (center of photo) located at the intersection of lines joining the Fiducial points.

Orthographic projectionPerspective projectionElements of photogrammetryIncludeH = height above datum from which photos taken or flying heightf = focal length of camera lens either in inches or mm.Scale of photograph S = f/(H-h)f = focal length 6 or 152.4 mm is commonh = height (elevation) of groundH = Flying height of place above datum

Relief DisplacementRelief Displacement exists because photos are a perspective projection.Use this to determine the height of object:h=

h = height of objectd = radial distance to top of object-radial distance to bottom of object.r = radial distance to top of object.d (H) r

Pocket stereoscope

Mirror stereoscope

Photo-interpretation keysMain objective is interpretation of data.Primary method of doing is by Visual interpretation.Based on 7 elements of Visual interpretation called interpretation keys.Tone: relative brightness or colour of objects in an image.

Visual interpretation (Continued)

Shape: Refers to structure or outline of individual objects.

SizeRefers to spatial dimension of the object on ground.

Visual interpretation (Continued)

PatternRefers to the spatial arrangement of visible objects.

Visual interpretation (Continued)

TextureRefers to the arrangement and frequency of tonal variation in particular areas of an image.

Visual interpretation (Continued)

ShadowProvide an idea of the profile and relative height of a target or targets.

Visual interpretation (Continued)

AssociationSome objects are always found in association with other objects.

Visual interpretation (Continued)

Siterefers to topographic or geographic location.

Visual interpretation (Continued)