DIGITAL PHOTOGRAMMETRY INTRODUCTION

2.1 Definition Photogrammetry is the "art, science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant imagery and other phenomena" (ASP 1980).

Geometric Distortion Raw aerial photography and satellite imagery have large geometric distortion that is caused by various systematic and non-systematic factors. Photogrammetric processes eliminate these errors most efficiently, and provide the most reliable solution for collecting geographic information from raw imagery.

Photogrammetry is unique in terms of considering the image forming geometry, utilizing information between overlapping images, and explicitly dealing with the third dimension i.e. elevation.

2.2 Transition In Photogrammetry Photogrammetry was invented in 1851 by Laussedat, and has continued to develop over the last years. Over time, the development of photogrammetry has passed through the following phases: 1. Stereo photogrammetry and analog stereo plotter 2. Analytical photogrammetry 3. Computer-assisted photogrammetry 4. Digital photogrammetry

Analog photogrammetry Measurement and drafting were done manually. Classical analog stereo plotters have disappeared from the market and are not being manufactured anymore.

Analytical Photogrammetry the most important development in this period was the invention of the analytical stereo plotter by Helava (1957). The analytical stereo plotter is essentially an instrument with a built-in digital computer as its main component, which handles the physical and mathematical relationship between object (ground) space and image space. The analytical plotters were introduced into the market during 1976 International Society of Photogrammetry and Remote Sensing (ISPRS) Congress.

Computer-assisted Photogrammetry Computer-assisted photogrammetry has brought computer assisted design (CAD), workstation based systems. These systems had graphic displays that provided on-line graphics for reviewing and editing digitized data.

Softcopy or digital photogrammetry By digital photogrammetry, we mean input data are digital images or scanned photographs. Digital photogrammetry has its root in the late sixties when Hobrough (1968) began experimenting with correlation, even though the solutions were analog in nature.

For almost 20 years, correlation techniques remained the only noticeable activity in digital photogrammetry efforts in photogrammetry have increased tremendously recently due to the availability of digital cameras, satellite imagery, scanners, increased computing power, and image processing tools A digital photogrammetric system should automate some processes that are usually performed by operators.

. 2.4 Advantages of Digital Photogrammetry i. It can handle inputs from other non-traditional sources such as: Digital camera output, Remote sensing stereo imagery, LIDAR imageries and, other such imageries from active sensors. Video camera output

ii. Since digital photogrammetry accepts digital input and generates digital output, it is closely integrated with RS as well as GIS Unlike Analog and Analytical instruments. iii. the DPWS offers other photogrammetric products such as Orthophotos, Digital Elevation Model (DEM) etc. iv Since computers carry out the photogrammetric operations in Digital photogrammetry, many operations have been automated.

2.5 Photogrammetric Software: Digital photogrammetry software configuration varies provides the following capabilities: • Enhanced images for brightness and contrast. • Rotate, flip, and transpose imagery. • Display overview, full resolution, and detail imagery. • Measure fiducials, pass points, and control points; manually, semi-automatically, or automatically. • Interior, relative, absolute, exterior orientation and bundle adjustment.

• Create epipolar stereo models (if necessary) and image pyramids. • Display a digital stereo model for compilation, DEM generation, and three-dimensional feature extraction. • Automatic aerial triangulation, DEM collection and linear feature extraction

• Manual collection of breaklines and other map features. • Graphic updates, while reviewing, roaming, and editing. • Several editing options for a quick model set-up

2.6 Input In Digital (Softcopy) Photogrammetry: The following inputs can be used for a digital photogrammetric task. Scanned aerial photographs. Stereo imageries from various remote sensing platforms. Multi sensor stereo imageries. Output from Digital Aerial, video and terrestrial cameras .

For the input of first kind a Scanner is absolutely necessary For the input of first kind a Scanner is absolutely necessary. A Photogrammetric Scanner is of high precision and resolution capable of providing high spatial resolution from 5-10 microns size of picture elements (PIXELS) and excellent positional accuracy.