Photogrammetry Labs

Photogrammetry Lab Report 2013

UNIVERSITY OF NAIROBI

COLLEGE OF ARCHITECTURE AND ENGINEERNG

PHOTOGRAMMETRY LAB REPORT Engineering Surveying 2013 NAME : NJOROGE KAGWI MAURICE Reg. No : F16/1334/2010

COURSE WORK F16/ 1334/ 2010 NJOROGE KAGWI MAURICE

 INTRODUCTION: WHAT IS PHOTOGRAMMETRY? This is the process by which ground measurements are derived and determined through photography. In this case, the object measurements and relative positions are determined without touch. Photogrammetry can be divided into  Range photogrammetry (aerial photogrammetry) and  Close range photogrammetry (mostly terrestrial photogrammetry). In close range photogrammetry, cameras are close to the object either hand-held or mounted on a tripod. Aerial photogrammetry is however done from great vertical heights and covers great areas, normally from planes or satellites. Such photographs are termed vertical photographs. Aerial photogrammetry is mainly used to produce large scale topographical maps and digital terrain models (DTM) representing the terrain relief. It makes it possible to prepare contoured plans from the aerial photographs. AERIAL PHOTOGRAMMETRY 1 .PLANNING The area to be surveyed using aerial photography is planned and the flight lines determined. 1

F16/ 1334/ 2010 NJOROGE KAGWI MAURICE

The whole area must be covered by the photographs and therefore there must be overlap. This is called stereo overlap. Forward overlap allowed is 65%+/-5% while side lap allowed is in the range of 25%+/-5%.

2. DETERMINATION OF FLIGHT LINES The area of interest is divided into runs which form the flight lines. The runs are parallel to each other spanning the whole area of study and ensuring overlaps for the photographs are achieved.

3. PROCESS With the aerial camera in the body of the aircraft, photographs are taken along the prearranged flight paths, with the optical axis of the camera pointing vertically down. 4. INTERPRETATION A single photograph contains a lot of detailed information about the area under survey. However, a photograph cannot be a map of the ground or a plan of a building. At best, a photograph shows the view through the camera with distortions due to the optics of the camera, the atmosphere and the position and orientation of the camera. 2

F16/ 1334/ 2010 NJOROGE KAGWI MAURICE

In principal, it is 3D coordinates that define the locations of object points on the ground surface and their relative positions to each other. However, the image (photography) coordinates obtained define the locations of the object in plane. Through photogrammetry, the above process is reversed. It converts, or maps, the flat 2-dimensional images back into the real 3dimensional world. This is derived from the existing ground control points for georeference. Photogrammetry therefore enables the user to derive metric information about the terrain, or any three-dimensional object by making measurements on photographs of that object element image from pairs of photographs. As a minimum, two different photographs are needed to reconstruct the 3-dimensional object. The more photographs taken and used, the extra the information and the process is improved. The 3dimensional coordinates produced from the measurements of multiple photographs are the end result of photogrammetry. Photogrammetry uses the basic principle of Triangulation, whereby intersecting lines in space are used to compute the location of a point in all three dimensions. However, in order to triangulate a set of points one must also know the camera position and aiming angles (together called the orientation) for all the pictures in the set. A process called resection does this. Some camera calibration has to be done in order to define its errors and remove them from the process. The image scale will dictate the magnitude of the linear and radial displacements in the image. The change in the position of an object from one photograph to the next is known as stereoscopic parallax. The amount of parallax depends on the object’s height. Higher points have greater parallax. Given any two overlapping images, a stereo pair, it is possible to view both images simultaneously to replicate a stereo binocular view. 3

F16/ 1334/ 2010 NJOROGE KAGWI MAURICE

5. INSTRUMENTATION Photographs are taken using digitalized cameras or even the analogue type where images are stored in films. Using the stereoscopic pairs of photographs processed; study of the area terrain and configuration can be done using Stereoscopic binoculars which use lenses to integrate the 3d perspective of the photographs. The photographs can also be used to plot the topographical maps of large scale of the areas under study using the various instruments available such as the modern computer work stations or the old stereo plotting machines. Wild stereo plotter and a computer work station available in the lab. GENERALIZED PROCESS

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F16/ 1334/ 2010 NJOROGE KAGWI MAURICE