PHOTOGRAMMETRY

WHAT IS PHOTOGRAMMETRYThe science of quantitative analysis of measurements from photographs

Photos - lightGramma - to drawMetron - to measure

DEFINITIONPhotogrammetry can be defined as the art, science and technology of obtaining reliable information about physical objects and the environment through process of recording, measuring and interpreting photographic images and patterns of recorded radiant electromagnetic energy and other phenomena.

As implied by its name, the science originally consisted of analyzing photographs.

What does it do?

Measures the 3-D location of points or features at one instant using between one and many cameras at the same time, or over a period of time using one camera It provides high accuracy results between 1 part in 5000 to 1 part in 1,000,000 of the largest dimension of the object being measured The method can be applied to objects ranging from mm to kilometres in size Statistical self checking is available and estimates of precision can be produced as part of the measurement process

Photogrammetry has two distinct areas of working:-

Metric photogrammetry Interpretative photogrammetry

Metric photogrammetry which involves precise measurements and computations to determine shapes and sizes of objects and Interpretative photogrammetry which deals with the identification and recognition of objects from photographs and satellite images.

In metric photogrammetry, the concept of measurement is implicit and is well understood simply by the term Photogrammetry, whereas the interpretative photogrammetry includes photo-interpretation as well as the new scientific tool remote sensing. Photographic interpretation involves systematic study of photographic images for the purpose of identifying objects and deducing their significance, and may be considered to be limited to photographs only.

BASIC CONCEPTThe primary objective of the technique is to derive precise coordinates of a point.This is done by viewing the area from two different angles, thereby recreating the same conditions as it existed at the time of photography.

GRAPHIC DESCRIPTION OF AERIAL PHOTOGRAPHY

WHY PHOTOGRAMMETRYVery preciseTime effectiveCost effectiveBased on well established and tested algorithms.Less manual effortMore geographic fidelity

Cont.Corrects all sorts of distortions.provide a reasonable geometric modeling alternative when little is known about the geometric nature of the image data.provide an integrated solution for multiple images or photographs simultaneously achieve a reasonable accuracy without a great number of GCPs create a three-dimensional stereo model or to extract the elevation information

What does photogrammetry get used for?

Mapping Shipbuilding Architectural models of buildings or facades, Archaeology surveys Medical uses e.g. human body scans for back problem or gait analysis Missile or plane tracking Antenna measurement Verification of the design of manufactured structures. Virtual reality Entertainment

How does photogrammetry work?Take images Get the camera parametersFind the locations of features (targets, edges, corners, etc.) on groundRecreate conditions as were at time of photographyCorrespond features between imagesCompute the 3-D location of corresponding points using triangulation Generate ortho imagesMapping

Photogrammetric MeasurementPros and Cons

Advantages:non-contact process2D or 3D options for informationhigh density of measurement possiblephotographs are a permanent recordmeasurements are made in the laboratoryre-measurement is possibleDisadvantages:photographic coverage is limitedphotographic processing delaysmeasurement and analysis delaysefficient only for large data sets

Types of photogrammetryTerrestrial AerialSatellite

CLASSIFICATION OF AERIAL PHOTOGRAPHS

Aerial photographic can be classified based on the type of camera as - frame, - continuous strip, and - panoramic photographs

Considering only the frame photographs due to their frequent use in photogrammetry, classification can be made according to

(i) Camera orientation, (ii) Format size, (iii) Angular coverage, and (iv) Type of emulsion.

CAMERA ORIENTATION A vertical photograph is taken with the optical axis of the camera held in a vertical or nearly vertical position [Fig.1(a)] .

A high oblique is a photograph taken with the optical axis deliberately tilted far enough from the vertical to show the earths [Fig. 1(c)].

A low oblique is an oblique in which the camera axis is strongly tilted but not enough to show the earths horizon [Fig. 1 (b)].

Convergent photograph is a sequential pair of low oblique photo where the camera optical axes converge toward one another. Each photograph covers essentially the same portion of the ground [Fig.1 (d)]. (D) Convergent pair of photographs.

Usually a vertical photograph is taken with the optical axis unintentionally tilted by a slight amount from the vertical. This is referred to as a tilted photograph. The unintentional tilt angle is usually less than 30.

ANGULAR COVERAGE (It is a function of focal length and format size)

Descriptions Angular Coverage Focal Length UsesNarrow angle100 200 610 & 915 mmUsed for intelligence, general interpretation, and mosaics.Normal angle500 - 750210 & 300 mmUsed for interpretation mapping, color photography, mosaics, and ortho-photography. Wide angle 850 - 950153 mm This is the most commonly used camera for mapping.Ultra-or super-wide angle1100-130088 mmUsed for mapping of areas with very little relief.

EMULSION TYPE

Emulsion UsesPanchromatic black and white.This is the most widely used type of emulsion for photogrammetric mapping and interpretation.Color.This emulsion is used for interpretation and to a limited extent for mapping. It differentiates between colour hues to greater extent than black and white photography.Infrared black and white.This emulsion is used for interpretation and intelligence. It penetrates haze more completely than panchromatic emulsion because of the longer wavelength. It is used to detect camouflage. It is seldom used for mapping.Infrared color (false color).This emulsion is used for interpretation, particularly in the analysis of plant and crop diseases, soil analysis, and water pollution.

Types of aerial photographs

ON THE BASIS OF SENSOR SYSTEM USED A classification of photogrammetry is done based on the type of sensor system used for getting the photographs or imagery : i)Radargrammetry radar.X-ray photogrammetry X-ray. Hologrammetry holographs. Infrared or color infrared/Colour photogrammetry camera

GEOMETRY OF RECREATION When single photographs are used with the stereoscopic effect, if any, produced by reflected mirror images, it is called monoscopic photogrammetry.

When two overlapping photographs are observed and measured or interpreted with a stereoscopic viewing device, which gives a three dimensional view and creates a mental impression of a relief model of the terrain, it is called stereo photogrammetry.

DATA REDUCTION

Under this classification there are three processes know as (i) Instrumental or Analog, (ii) Analytical(iii) Digital

ANALOGUE PHOTOGRAMMETRY In analog photogrammetry, optical or mechanical instruments were used to reconstruct 3D geometry from two overlapping photographs. The main product is a topographical maps.

ANALYTICAL PHOTOGRAMMETRYThe computer replaces some expensive optical and mechanical components. The devices are analog/digital hybrids. Outputs of analytical photogrammetry can be topographic maps, but can also be digital products, such as digital maps and DEMs

DIGITAL PHOTOGRAMMETRYDigital photogrammetry is applied to digital images that are stored and processed on a computer. Digital photogrammetry is sometimes called softcopy photogrammetry. The output products are in digital form, such as digital maps, DEMs, and digital orthophotos saved on computer storage media.

GLOSSARYAir base. The distance between two image exposure or camera stations.Average flying height. The distance between the camera position at the time of exposure and the average ground elevation. Average flying height can be determined by multiplying the focal length by the image scale. Base-height ratio (b/h). The ratio between the average flying height of the camera and the distance between where the two overlapping images were captured. Block of photographs. Formed by the combined exposures of a flight. For example, a traditional frame camera block might consist of a number of parallel strips with a sidelap of 20- 30%, and an overlap of 60%.

Geometry of an aerial photograph

Exposure station. During image acquisition, each point in the flight path at which the camera exposes the film. Fiducial Marks. Four or eight reference markers fixed on the frame of an aerial metric camera and visible in each exposure. Fiducials marks are used to compute the transformation from pixel coordinates to image coordinates. Photograph Centre. The center of an aerial photo; the intersection point of lines constructed to connect opposite fiducials marks. Focal length. The distance between the optical center of the lens and where the optical axis intersects the image plane. Focal length of each camera is determined in a laboratory environment. Ground Control Point (GCP). An easily identifiable point for which the ground coordinates of the map coordinate system are known.

Nadir. The point on the ground directly beneath a camera . Nadir point. The intersection of the focal axis and the image plane.Parallax. "The apparent displacement of an object due to change in the position of observation. Perspective center. The optical center of a camera lens. A point in the image coordinate system defined by the x and y coordinates of the principal point and the focal length of the sensor. Principal point. The point in the image plane onto which the perspective center is projected.

SCALE DETERMINATIONBefore a photograph can be used as a map supplement or substitute, it is necessary to know its scale. On a map, the scale is printed as a representative fraction that expresses the ratio of map distance to ground distance, For example: RF=MD/ GDOn a photograph, the scale is also expressed as a ratio, but is the ratio of the photo distance (PD) to ground distance (GD). For example:RF=PD/GD

The approximate scale or average scale (RF) of a vertical aerial photograph is determined by either of two methods; the comparison method or the focal length-flight altitude method.The scale of a vertical aerial photograph is determined by comparing the measured distance between two points on the photograph with the measured ground distance between the same two points.The ground distance is determined by actual measurement on the ground or by the use of the scale on a map of the same area. The points selected on the photograph must be identifiable on the ground or map of the same area and should be spaced in such a manner that a line connecting them will pass through or nearly through the center of the photograph

fHABPLbaopositiveOptical axisground2-D view of vertical photograph taken over flat terrain

Scale of vertical aerial photograph over variable terrainABfbaHOAh2h1havghAverage terrainDatumL

GROUND COORDINATES FROM PHOTOGRAPHS

DETERMINATION OF FLYING HEIGHT2 Ground control points with known elevationPoint A (XA, YA, HA) and Point B (XB, YB, HB)Then the distance (D) between the 2 point is

In terms of coordinated D can be expressed as

So H can be determined by solving as

SOURCES OF DISTORTIONS AND DISPLACEMENTOn an accurate planimetric map, all features on the ground surface are portrayed as per their correct horizontal positions. The observer thus has a truly vertical view of every detail shown and the spatial relationship among features is accurate. Unfortunately, this standard cannot be met by aerial photographs because of various sources of distortion or image displacement.Images on vertical aerial photographs are generally displaced from their true plane position.

The main sources of displacement and distortion are

(i) the optical or photographic deficiencies (film and paper shrinkage), (ii) lens aberrations, filter aberrations, (iii) failure of the film-flattening mechanism in the camera focal plane, (iv) shutter malfunction, (v) image motion, (vi) atmospheric refraction of light rays, (vii) curvature of the earth, tilt, and topography or relief.

Relief displacementShift or displacement (d) in the photographic position of an image caused by the relief of the object.The amount of relief displacement is directly correlated with the height or depth of the object and the distance of the object from the nadir. This displacement is inversely correlated with the flying altitude of the aircraft above the datum and the focal length used. AHLAhAAooaarrfDatumd = r - r

Relief displacementHigher altitude photography will produce less relief displacement than lower altitude photography. Relief displacement is radial and either away from (for objects extending above the datum) or toward (for objects extending below the datum) the nadir. Even though relief displacement constitutes a source of errors in measuring horizontal distances on vertical aerial photographs, it is not necessarily a nuisance; because of relief displacement, we can determine the height of objects (or difference in elevation between objects) and to see in three-dimension by viewing stereoscopic pairs of aerial vertical photographs. Relief displacement on aerial vertical photographs also allows us to make topographic maps.

EFFECT OF TOPOGRAPHY- Relief Displacement

DISPLACEMENT DUE TO TILT

A photograph is considered tilted when the angle between the perpendicular projection through the center of the lens and the plumb line is greater than 3o.An aircraft or airborne not perfectly horizontal it causes a rotation of the camera about the x-axis or about the y-axis. Rotation about the x-axis causes lateral tilt or y-tilt, which is due to the aircraft being wing-up-wing-down and displacing the nadir point along Y-axis. Rotation about y-axis causes longitudinal tilt or x-tilt or list, which is due to the nose of the aircraft being up or down causing the nadir point to be displaced along X-axis. Along the axis of tilt there is no displacement relative to an equivalent vertical photograph as this is the line where a tilted photograph and an equivalent vertical photograph would match and intersect on another.

Geometry of a Tilted PhotographN = Nadir PointP = Principal Point (p.p)i = isocentreIsocentre is point where the bisector of the angle of tilt cuts the image plane.The line joining n, i and p is the Principal line.The line perpendicular is the principal line and passing through the isocentre is the axis of tilt.Dist pi = distance between p.p and isocentre = f.tan(/2)Dist pn = distnce between p.p and nadir = f tan Dist ni = distance between isocentre and nadir = f (tan tan /2)

Scale of a tilted photographScale at Principal Point (p) = f / (H Sec )

Scale at Nadir (n) = ( f * Sec )/ H

Scale at Isocentre (i) = f/H

So at isocentre, the scale of tilted photograph is equal to that of vertical photograh

If the point lies on principal plane:Tilt displacement = ia2. Sin / (f-ia. Sin )

If the point doesn't lie on principal plane:Tilt displacement = ib2 . Sin .cos(2) / (f- ib. Sin .cos )LnpibinpanpaDisplacement due to tilt

An equivalent vertical photograph is an imaginary truly vertical photo taken from the same exposure station as the tilted photograph with the same camera.Images on a tilted photograph are displaced inward with respect to their positions on an equivalent vertical photo if they occur above axis of tilt, and they are outward if they occur below axis of tilt.Inward displacements are considered positive and outward are negative.Tilt displacement occur even in photo taken over flat ground.

END

Depth perceptionMethods of judging depth:MonoscopicStereoscopic.

Monoscopic method----Relative size of objectsHidden objectsShadowsDifference in focusing of eye for different objects.

StereoscopyStereoscopy is based on stereoscopic or binocular vision.When the eyes are focused on an object, the optical axes of the two eyes converge on that point intersecting at an angle known as parallactic angle.Nearer the object - greater the parallactic angle and vice- versa.Stereoscopy is the name given to the phenomenon of an apparent 3D "model" created by viewing two photographs of the same object, one photograph with each eye, simultaneously.This stereo-model is a subjective phenomenon and is not physically real, but is formed by the brain using the normal process associated with binocular vision.Depth cues are given by object sizes, perspective, occlusion, movement parallax and change in eye focus. Parallactic angle and inter-pupilary distance between the eyes.

Stereo-photogrammetry is the general term applied to the science of measurement from photographs when an overlapping stereopair of photographs is used.In contrast to single photographs, which can only extract 2D information, stereo-photogrammetry allows 3D information to be extracted.An overlapping stereo pair is a pair of photographs on which the same object or area of terrain is pictured, but from different views or perspectives.Stereo photogrammetry

Epipolar PlaneFor viewing an stereoscopically, the observer should fulfill one condition i.e. the two eyes must represent the camera exposure station and look vertically down such both the exposure station, the image point and the ground point form one plane.This plane is known as the epipolar plane.

Stereoscopes

There are two basic types of stereoscopes; (i) Pocket and(ii) Mirror

Both the pocket and mirror stereoscopes must be physically moved over the photographs to view all parts of the available overlap

Pocket StereoscopeThe pocket stereoscope is a simple lens system with the lenses at roughly the same separation as a normal eye base.The pocket stereoscope is small and portable but has a correspondingly small coverage so that only a portion of a pair of overlapping photographs can be used at one time.

Mirror StereoscopeThe mirror stereoscope effectively widens the eye separation so that the full overlap area of a pair of aerial photographs may be viewed conveniently.

Principle of Mirror Stereoscope It consists of a pair of reflecting prism or mirrors (m & m) and a pair of wing mirrors (M & M).Each are oriented at 45 with the plane of the photograph.A pair of meniscus lens are located above the prism for comfortable viewing.

Advantage of Mirror StereoscopeThe pair of photograph can be viewed completely as the photographs do not overlap over each other.Binoculars provide an enlargement while viewing the pair of photographs stereoscopically.

Base LiningIn order to view the overlapping portion between consecutive and overlapping photographs, the photographs are to be oriented properly, such that the geometry of the photographs similarly to as at the time of photography.This process is known as Base lining.Base lining can be defined as the processing of orienting a pair of overlapping such that the photographs have the same orientation as that the time of photography, so that a stereomodel or 3D model of the overlap portion is created in the mind.

Procedure for Base liningThe first photo is called the LEFT photo and the second photo is known as the RIGHT photo.First of all locate the Principal Point (P.P) of both the photos and let them be designated as p1 and p2 ..Transfer the P.P of left photo (p1) on the right photo and let this be designated as p1 . Similarly transfer the P.P. of the right photo on the left photo and let this be designated as p2 .p1 and p2 are known as the conjugate principal point.p1p2p1p2p1p2p1p2The distance p1 p2 and p1p2 is the air base. Now extend the lines p1 p2 and p1p2 towards the edge of the photos

Procedure for Base liningNow take a white sheet of paper and draw a long line to represent the flight path of the aircraft.Now place the Left photo on the white paper such the extended line p1 p2 is coincident with the line draw on the paper. Now fix the Left photo with cellotape.Now place the Right photo in the same manner and place the mirror stereoscope over the line drawn on he paper and view through the mirror stereoscope and slide the right photo till the overlap portion appears in 3D. Now fix the right photo.The procedure for base lining is complete.

ParallaxThe term parallax is given to the relative movement of objects due to movement of the observerThe most obvious evidence of this phenomenon is the apparent shift of nearby objects relative to distant objects when travelling at speed in a vehicleParallax is often used to detect when an object is correctly focussed in a theodolite or level telescope by moving the eye from side to side - any movement between the object and the cross-hairs indicates that the focus is incorrectParallax is demonstrated in stereo photographs by the shift in position of conjugate images, that is, the same object point imaged on different photographs.

For a constant flying height and tilt-free photographs the parallax will be parallel to the baseParallax will be smaller for more distant objects, or in the case of aerial photography, smaller for points at lower heightsThe perception of depth in stereo photographs is dependent on this parallax, as neighbouring points at different heights will exhibit different parallaxesAs the parallax shifts apply to all points, a continuous 3D impression of the object is given by the stereo photographs

To obtain stereoscopic coverage of an entire area, the minimum overlap for any photography is 50%A mathematical definition of parallax is :p = x - x'wherex = left image coordinatex' = right image coordinateThe coordinates should be measured parallel to the line connecting the principal points on the two overlapping photographs, that is, the line parallel to the baseline.

Ground Coordinates from Parallax MeasurementsL & L camera station so LL is Air base.A point P on ground ( elev hp) is imaged as p on left photo (xp,yp) and p on right photo (xp,yp).Consider s LOW & Low or

In the s Lwp and LWP or

So

Ground Coordinates from Parallax MeasurementsIn LOW & Low and in Lwp & LWP or

So and

Therefore It can be stated that under the given assumptions made, there is no y-parallax.Now and

So or

Thus and

Determination of Air BaseSo by Distance formulaorTherefore

Measurement of ParallaxThe convenient form of measuring parallax on a photograph is with a parallax bar or stereometer.It contains a steel bar having two glass plates at each end.At the centre of each of the glass plate, a reference or measuring mark is provided. These are known as half marks or floating marks.

The Floating MarkThe floating mark is used to carry out quantitative measurements on a pair of photographs.The floating mark is composed of two half marks or dots which are placed over conjugate images on the two overlapping photographs.Within a stereo model the floating mark will appear to lie on the surface of the terrain (see diagram) when the dots are placed exactly on conjugate images.If the dots are moved slightly closer together, the floating mark will appear to rise or float off the surface

The Floating MarkIf the mark floats too far above the surface then stereo fusion will be broken because the eyes can not accommodate the range in convergence, and a double image of the terrain will resultIf the dots are moved slightly apart the floating mark will split into two separate dots as the brain of the observer can not imagine seeing the floating mark beneath the surfaceAgain if the dots are moved too far apart a double image of the terrain will result