Unit - 4
Photogrammetry Surveying
Photogrammetry is a mapping and surveying procedure or technique which has many applications in Transportation Department.
Applications of Photogrammetry are as follows: -
The term “photogrammetry” is made of by 2 words which is “photo” and “meter” means measurements from photographs.
The photogrammetric process consists of following phases: -
The basic or fundamental principle used by photogrammetry is the triangulation. Clicking photograph from at least 2 different area, so-known “lines of sight” can be developed from each camera to points on the object. This LOS (lines of sight) are mathematically crossed or intersected to made the 3-D coordinates of the points of interest.
Parallax
The movement or displacement of an object caused by a change in the point of observation is known Parallax. By taking photographs of the same object but from the different point is Stereoscopic parallax. aircraft’s motion causes the Change in position of image from one photo to the other or next.
Aspects of stereoscopic parallax are as follows: -
Types of photogrammetry
The camera is mounted in an aircraft, usually pointed perpendicular towards the ground with the camera axis vertical or nearly so.
Many photographs are taken with the clincher – built concept. Later the processing of these photographs done using stereo-plotter.
Photos are also being used in for Digital Elevation Model (DEM) creation.
Photographs are taken from a stable, usually known as, position on or near the ground and with the camera axis horizontal or nearly so.
The position and orientation of the camera are frequently measured directly at the time of exposure. Instrument used for exposing such photograph is known photo theodolite.
In this type of photogrammetry, satellites are used.
Global coverage of satellite image is taken in lesser time with a high-resolution data.
Images are studied and identification is done for judging their significance with standard and careful analysis.
Characteristics: -
An aerial photograph is a central projection, in which the bundles of perspective rays meet at a point of origin known perspective centre. A map is an orthogonal projection, where every point on the ground is projected perpendicular on to the map on a reduced scale. One of the greatest advantages of central projectionist its ability to produce a three-dimensional model of the terrain under the stereoscopes.
1.Fiducial marks:
Fiducial marks are marks built into aerial cameras which can be seen on the sides or in the corners of the photo (or both), and which are used to determine the position of the principal point.
2.Fiducial axes:
Lines joining the fronting fiducial marks on a photograph are known fiducial axes.
3.Principal point:
It is a point where the perpendicular projected through the center of the lens meets the photo image.
4. Conjugate principal point:
It is the principal point of an aerial photograph represented on adjacent aerial photograph.
Principal point (PP) and conjugate principal point (CPP)
5. Nadir point:
It is a point vertically below camera Centerior aircraft.
6.Isocentre:
The point on an air photo which lies midway between the Principal Point and t Nadir point. On a true vertical aerial photograph all these three would be at the same point. There is no such thing as a true vertical aerial photo. Each and every air photo have some degree of tip or tilt.
7.Air base (B):
Line joining two air stations is known as Air base.
8.Photobase (b):
Air base is known photo base on aerial photograph and it is the interval between the principal point of one photograph and conjugate principal point of adjacent photograph.
9.Forward overlap/End lap:
The quantity of overlap in the middle of successive photos in a flight line to allow for stereo viewing (usually 60 –70%).
10.Sidelap/Lateral overlap:
The amount of overlap in the middle of air photos in adjacent flight lines (usually 20 –30%).
11.Tilt: I
it is the angle in between the camera axis and vertical. It is also known as rotation of the camera away from the vertical, about the x-or y-axis.
12.Focal length (f):
Distance from the optical centre of the lens to the focal plane.
13.Flying height/altitude (H):
It is a perpendicular distance of aircraft above the datum (usually mean sea level) during flight. It is measured using an instrument known Altimeter mounted in an aircraft.
14.Scale of aerial photograph:
Scale of aerial photograph is known as the ratio of photo distance to the corresponding ground distance.
Viewing perspective of a map-
On a map objects are both planimetrically and geometrically accurate.
Hence objects are located on the map in exactly same position relative to each other as they are on the surface of the Earth, except in change in scale. Because that maps use an orthographic projection and constant scale to represent features.
(Orthographic projection is using parallel lines of site)
Whereas Aerial photographs are formed by perspective projection. Therefore, the relative position and geometry of the objects depicted depends upon the location from which the photo was taken.
Hence due to this we get certain forms of displacement and distortion in Air Photos
From (1) and (2)
If the point is taken to datum, (havg – 0)
Shift of Image of a point on photograph, if the point is taken to datum is called relief displacement.
Tilted aerial photograph
f -focal length
n -nadir point
s –swing
p -principal point
t -angle of tilt
i –isocentre
tan (t/2) = (pi)/f; (pi) = f tan (t/2)
tan (t) = (pn)/f; (pn) = f tan (t)
Photographs are taken from a stable, usually known as, position on or near the ground and with the camera axis horizontal or nearly so.
The position and orientation of the camera are frequently measured directly at the time of exposure. Instrument used for exposing such photograph is known photo theodolite.
Usually, a stereoscope is used for image interpretation. There are several types of stereoscopes, for example, portable lens stereoscope, stereo zoom transfer scope and many more.
The process of stereoscopy for aerial photography is as follows. At first the center of both aerial photographs, known the principal point, should be noted. Secondly the principal point of the right image should be plotted in its location on the left image. At the same time the principal point of the left image should be also plotted on the right image. The principal points and shifted points should be aligned along a straight line, known as the base line, with an appropriate separati5.on.
The advantage of stereoscopy is the power to extract three-dimensional information, for example, classing between tall trees and low trees, terrestrial features such as height of terraces, slope gradient, detail geomorphology in flood plains, dip of geological layers and so on.
The principle of height measurement by stereoscopic vision is based by the use of parallax, which is the distance between image points, of the same object on the ground, on the left and right image. The height difference can be computed if the parallax difference is measured between two points of different height, using a parallax bar
Ground control can be categorized as targeted and photo-identifiable (picked) control points, and can also be known as horizontal control, vertical only control, or as 3-D control. Horizontal and vertical controls need different configurations to make them serve their intended purposes. The use of only ground control is now restricted to small projects, such as bridge sites, borrow areas and where only one or two models are needed. Photo identifiable control points are needed. The surveyor must know what type of control is known for when he or she attempts to pick or photo-identify the point. Accessibility for surveying should also be considered while selecting the position for control points
Ground Control
Application of aerial photography:
Mapping: Itis a valuable tool for soil mapping. It is found to fast, precise, indispensable in inaccessible areas and it is cost effective in the long run.
Interpretation: Photointerpretation has revolutionised the method of data collection in many disciplines. It decreases the field work and thereby the cost. The information is reliable and acceptance for most studies for example as in the fields of geology, water resources, geomorphology, hydrogeology, forestry and ecology, and urban and regional planning.
The term applied to the process of finding out x, y, and z ground coordinate of each separate points by measuring from the photograph.
The principle behind this application is extending ground control through strip for use in subsequent photogrammetric operation.
Method of Aerial triangulation: -
Procedure of Aerial triangulation: -
Benefits of aerial triangulation: -
Photogrammetry is the science of building measurements from photographs. The input to photogrammetry is photographs, and the output is typically known as a map, a drawing, a measurement, or a 3D model of some real-world object or scene. Aimé Laussedat is known as the "Father of Photogrammetry".
4.12 Mapping using Paper PrintsPhotogrammetry can be applying to collect a variety of data, presented in the following formats:
Planimetric maps – Planimetric maps are maps which represents only the horizontal features of the mapped area. Planimetric maps shows features such as roads, sidewalks, buildings, river banks, shore lines, manholes and many more. No elevation information appears on planimetric maps.
Topographic maps –These are the maps on which both horizontal and vertical features of the mapped are shown. In addition to the mentioned planimetric features, a topographic map shows elevation information as contours and/or as spot elevations.
DEM's – Digital Elevation Model (DEM) or Digital Terrain Model (DTM) are densely heavy networks of spot altitude represented by X, Y, Z coordinates. The DEM points are collected in a regular grid with break points which shows the characteristics of the topography. DEM's are mostly used to draw contours and is essential ingredient for the production of orthophotos.
4.13 Mapping using stereoplotting instrumentStereoscopic Photography:
A stereoplotter uses a stereo photograph to find elevations. It has been the primary method to plot contour lines on topographic maps since the 1930s. The specific devices have advanced technologically, but they are all based on the apparent change in location of a feature in the two stereo photographs.
A combination of photographs taken from two camera stations but cover a common area makes a stereo pair which when viewed under a stereoscope in a certain way with parallel eye axes give a 3-D view of the common area.
The pocket stereoscope, usually has planoconvex lenses with a focal length of 100 mm. The parallel rays, entering the eyes meets at infinity. Normal viewing distance is 250 mm, the magnification due to closer view is 250/100 = 2.5. The pocket stereoscope is cheap, portable, has a larger field of view, good image quality due to the simple optical system.
The limitations are as follows:
(i)It have limited magnification. Pocket stereoscope with more than 3 times magnification can’t be equipped with simple planoconvex lenses due to the large increase in lens aberration.
(ii) The space between the eye and the photograph is too small for adequate illumination and working space for tracing etc.
(iii) The space between the corresponding points on the photographs must be equal to or smaller than the eye base, which is difficult with simple photograph unless they are bent or folded.
4.14 Mosaic:In a situation where there are no adequate large-scale maps available, aerial photographs can take map substitutes in the formation of photomaps.
Limitations:
References:
1 Madhu, N, Sathikumar, R and Satheesh Gobi, Advanced Surveying: Total Station, GIS and Remote
Sensing, Pearson India, 2006.
2 Manoj, K. Arora and Badjatia, Geomatics Engineering, Nem Chand & Bros, 2011
3 Bhavikatti, S.S., Surveying and Levelling, Vol. I and II, I.K. International, 2010 4 Chandra, A.M., Higher
Surveying, Third Edition, New Age International (P) Limited, 2002.
5 Anji Reddy, M., Remote sensing and Geographical information system,
6 Arora, K.R., Surveying, Vol-I, II and III, Standard Book House, 2015.
