Introduction to Photogrammetry

Introduction to Photogrammetry

• Photogrammetry involves obtaining information about physical objects through the use of photographs
• It includes taking photographs of objects, processing the photographs, and measuring the photographs to produce end results like topographic maps.

Branches of Photogrammetry

• Aerial photogrammetry involves taking photographs of an area with a camera mounted in an aircraft.
• Terrestrial photogrammetry involves taking photographs of an area with a camera fixed on or near the ground.
• Space photogrammetry involves taking photographs with cameras fixed in space, such as in an artificial satellite or on the moon.
• Close-range photogrammetry involves keeping the camera close to the object to obtain detailed information. It is used in surgery, architecture, and laboratory investigations

Categories of Photogrammetry Work

• Metric or quantitative work involves determining ground positions, distances, elevations, areas, volumes, and various types of maps.
• Photo interpretation or qualitative work involves analyzing photographs to identify objects and assess their significance.
• Remote sensing is a branch of photo interpretation that obtains photographs and data from infrared sensors, thermal scanners, and satellites.

Uses of Aerial Photogrammetry

• Preparation of topographic maps
• Classification of soils for agriculture and forest development
• Geological investigations to identify and interpret rocks, faults, dip, etc.
• Used for military intelligence for strategic planning
• Mining
• Archaeology
• Monitoring wildlife and forest cover
• Land use classification for residential, industrial, agricultural, commercial, etc.

Limitations of Aerial Photogrammetry

• Unsuitable for dense forests and flat sands due to difficulty identifying points
• Unsuitable for flat terrain where contour plans are required
• Requires skilled and experienced personnel
• Expensive for surveying small areas

Aerial Camera

• The primary function of terrestrial and aerial cameras is the same: taking pictures
• Aerial cameras are mounted on fast-moving airplanes, so the requirements for aerial cameras are quite different than for terrestrial ones.
• Aerial cameras require a high-speed and efficient shutter, a fast lens, and a high-speed emulsion for the film.
• They also require a magazine to hold large rolls of film
• Aerial cameras are surveying instruments of high precision.

Aerial Camera Components

• Lens assembly (lens, diaphragm, shutter, filter)
• Camera cone
• Focal plane
• Camera body
• Drive mechanism
• Magazine

Lens Assembly

• High-quality compound lenses are used in aerial cameras
• The accuracy of the work depends on the quality of the lens, which gathers light and focuses it on the focal plane.
• A filter is placed in front of the lens for protection from dust, to reduce the effect of atmospheric haze, and to provide uniform light distribution.

Shutter and Diaphragm

• The shutter controls the exposure time
• Two types of shutters are in use: between-the-lens shutters and focal plane shutters
• Between-the-lens shutters are more popular in mapping cameras
• The diaphragm controls the amount of light passing through the lens by varying the aperture size.
• For a normal 6" focal length camera, the aperture size varies from 7 mm to 38 mm.

Camera Cone and Focal Plane

• The camera cone supports the entire lens assembly
• It has collimation marks defining the focal plane
• The cone is made of material with a low coefficient of thermal expansion
• The focal plane is located exactly above the collimation marks
• The collimation marks maintained at a distance for the best possible image

Camera Body, Drive Mechanism, and Magazine

• Camera bodies house the drive mechanism
• The camera body is usually one-piece casting
• Drive mechanisms is powered by an electric motor
• Cameras are provided with carrying handles
• Camera magazine holds the film, holding 60 - 120m of film
• Some aerial cameras use glass plates instead of roll films, glass plates provide high accuracy, but are costly.

Types of Photographs

• Terrestrial photographs are taken with a phototheodolite having the camera station on the ground and the camera axis horizontal or nearly horizontal
• These photos present front views of objects, generally used for surveying structures, monuments, or archaeological sites
• Aerial photographs are taken with an aerial camera having the camera station in the air and the camera axis vertical or nearly vertical.

Types of Aerial Photographs

• Vertical photographs are taken when the camera axis is vertical and the ground is perfectly flat
• When the camera axis is vertical, the photo plane is parallel to the datum plane
• Tilted photographs are those with a camera axis slightly tilted from vertical (less than 3°)
• Vertical photographs are used in topographical and engineering surveys

Oblique Photographs

• Oblique photographs are taken when the camera axis is considerably inclined to the vertical (more than 3°)
• The camera axis is intentionally kept oblique because the view in a vertical photograph is unfamiliar to the human eye
• Low oblique photographs do not show the horizon and are used to compile reconnaissance maps of inaccessible areas
• High oblique photographs show the horizon and are used for military intelligence

Convergent and Trimetrogen Photographs

• Convergent photographs are oblique photographs taken with a pair of cameras simultaneously at successive exposure stations, with their axes tilted at a fixed inclination
• Useful to create stereopairs and 3-D views of terrain
• Trimetrogen photographs are a combination of vertical and low oblique photographs exposed simultaneously from two air cameras

Definitions in Aerial Photogrammetry

• Exposure station (O) is a point in the air occupied by the camera lens at the moment of exposure
• Perspective projection is a projection said to be perspective when the straight rays radiate from a common point and pass through points on the sphere to the plane of projection
• A photograph is a perspective projection
• Perspective center is the real or imaginary point of origin of bundles of perspective rays
• Flying height (H) is the elevation of the exposure station above sea level

Definitions Continued

• Flight line is the line drawn on a map to represent the track of the aircraft
• Altitude is the height of the aircraft above the ground
• Focal length (f) is the distance from the front nodal point of the lens to the plane of the photograph
• Principal point (p or P) is the point where the optical axis of the camera strikes the photograph and coincides with the intersection of the x and y axes
• Ground principal point (P) is where the camera axis strikes the ground

More Definitions

• Nadir point (n or N) also known as the plumb point, the point where a plumb line dropped from the front nodal point strikes the photograph
• Ground nadir point (N) is the point where a plumb line dropped from the front nodal point strikes the ground
• Principal plane is the plane passing through O, N, and P
• Principal line is the line np of intersection of the principal plane with the plane of the photograph

Angles and Planes

• Tilt (t) is the angle which the optical axis makes with the plumb line
• Swing (s) is the horizontal angle measured clockwise from the positive y-axis to the photo nadir point
• Isocenter (i) is the point at which the bisector Oi of the angle of tilt meets the photograph
• ni = ip = f tan(t/2)
• Azimuth of the principal plane is the clockwise horizontal angle (A) from the ground north meridian to the principal plane
• Picture plane is the plane containing the image at the time of camera exposure
• Air base is the distance between two consecutive exposure stations

Scale of a Vertical Photograph

• The scale of a photograph is the ratio of a distance on the photograph to the corresponding distance on the ground
• Maps have a uniform scale because of orthographic projection
• Photographs are perspective projections, where the scale varies with terrain elevation

Scale Equations

• For horizontal ground: Scale of photograph = S = (map distance) / (ground distance) S = f / (H - h)
• The scale of the photograph varies with changes in ground elevation
• When the ground elevation is variable: S = f / (H - ha)
• For variable ground elevation: R.F. = 1 / ((H-h) / f)

Scale Types

• Datum scale : If all ground points are projected vertically onto mean sea level
• Average scale : if all points vertically projected upward/downward onto plane representing avg terrain elevation
• Scale found by ground distance: Scale = photo distance / ground distance
• The photo scale can be found by comparing the photo distance and the map distance between two well-defined points at the same elevation where reliable map of the area is available

Relief Displacement

• When the ground is not horizontal, the photographic scale varies from point to point
• Points on a photo are displaced because of the relief, this is the relief displacement.
• Relief displacement Equation: r = (R * f) / (H - h)
• d = R.F * [1/(H-h) - 1/H]

Relief Displacement Conclusions

• Relief displacement increases with the distance from the principal point
• Relief displacement decreases with an increase in flying height (H)
• Relief displacement is zero for the point vertically below the exposure station
• Displacements above datum is positive radially outward, below datum it is negative radially inward

Tilted Photographs

• In vertical photographs, the scale is constant if the ground is flat
• If a tilted photograph is taken, the scale isn't uniform
• The lower half of the photo has a larger scale than the upper half
• m'a/NA = Om'/ON
• Scale of the photograph = ( f sect - ma sint ) / H-ha

Co-ordinates

• For the calculation of co-ordinates:
• Scale for point A = f / H-Ha
• Scale for point B = f/ H-Hb
• The co-ordinates of A and B as follows:
• Xa = Xa / Scale at point A etc

More Co-ordinate Info

• Ground points must be linked to known locations
• Ground control is used to link ground points
• It is also needed to orient the photographs
• Ground control is divided into horizontal and vertical controls
• Horizontal control helps specify positions on a horizontal place
• Vertical controls establish elevation

Types of Controls

• Basic control consists of triangulation/ traverse stations, azimuth marks, bench marks
• Photo control focuses on the images of identified points on the photographs, established with respect to basic control
• Position of ground control can be located two ways: post and pre-pointing methods

Overlapping Photographs

• Vertical photographs are usually taken along flight strips in aerial surveys
• Longitudinal overlap is also know as forward overlap
• Preferred overlap % is 55% - 65% with average of 60%
• Overlap between adjacent flight lines is lateral or side overlap (15 - 35%)

Reasons for Overlaps

• Allows overlap % of at least 50% for viewing pairs of photographs stereoscopically
• End lap provides for accurate registration of different prints
• Overlap allows the edges of images to be rejected and central portions examined
• Gaps between strips are avoided through side lap
• Provided better view of images for stereoscopic examination
• Each portion of the territory is photographed 2 to 3 times, can reject distorted images

Flight Planning

• Flying height depends on the aircraft and contour interval in topographic mapping
• Flying height also depends on the mapping scales
• Accuracy increases with decreased flying height
• Data for planning is as follows: focal length, aircraft/ area size

Equations for Planning

• Flying height = c factor or contour interval
• c factor ranges from 500 - 1500 depending on the surrounding mapping environment
• N = A / a total / area single
• l = photo length in flight
• W = photo width normal to direction

Area Equations

• l = le percentage end lap, same with W
• L and W are in ground distance to l and w
• Scale = f / H
• a = NL * Wl
• A/a

Number of Photographs Required

• Length for Area = Lo/Wo, the equation is as follows:
• N1 = L0 + 1
• L
• The number of strips (N2) are as follows":
• N2 = Wo+1
• W
• Actual spacing between the strips
• W
• d
• Exposure Interval (T)
• T = L /V

Crab and Drift

• Crab is angle between flight line and photograph edge
• Crab occurs if the camera focal places tilts
• Should be fixed for accurate image
• Drift is the lateral shifting , which is caused by wind and other natural forces
• Drift causes gaps

Planning of Flight

• Flight data
• 1. focal
• 2 .aircraft
• 3 size

Photomaps and Mosaics

• Aerial images/maps are replaced by photo maps (mosaics
• The Mosaic is a combination of multiple photographs, taken to make a larger range
• Mosaic are required for enlarged scales than can be displayed by a single image

Advantages/ Disadvantages

• Cheaper set up costs
• Timesaving in prep
• Objects are easily recognizable and mosaics interpreted/understood without advanced knowledge
• They are not planemetric and suffer from displacement/scale variations
• Cannot be used quantitative testing

Stereoscopic Vision

• Normal eyesight is a binouclar vision, that can perecive with both eyes simultaneously
• Those who only see out of eye the monocular
• A Stereosopic view will combine images for two in order to create 3-d
• The Parallax Angle is how angles connect images and is typically 66 mm on average for eyesight

Stereoscopic Fusion

• Stereoscopic fusion used two images slightly different from eachother to give accurate view of distance
• Pairs of those photo are know as steropair and use steroscopic/ anaglypy devices to view them
• Stereoscopes are what they are known
• This allows the left eye to se the left and right to viewright side, the brain combines for the accurate results