Digital Photogrammetric Data Extraction – A Way of Advanced Mapping

--Mohammad Masudur Rahman, Deputy Director

Photogrammetry is a science that deals with the technique of obtaining reliable information about physical objects and environment through process of recording, measuring and interpreting photographic images. In short, it is the measurements of distances on the photographs.06. Masuour Rahman

Survey of Bangladesh (SOB) is the National Mapping Organization (NMO). It has been using analogue Photogrammetric mapping system from 1960. SOB has switched on to digital mapping system with a limited number of hardware and software in 1997. Considering the growing demands of the country, SOB has under taken a project titled “Improvement of Digital Mapping System of Survey of Bangladesh (IDMS)”. Under this project SOB has taken Digital Aerial Photograph (50 cm GSD & 25 cm GSD) using Digital Camera integrated with GPS/IMU system. Aerial photographs will be used for 1:25,000 scale mapping for all over Bangladesh and 1:5000 scale mapping for major divisional city area.It will be done by the latest digital photogrammetric mapping technology. Using Digital Photogrammetry SOB is carrying out Aerial Triangulation (AT), Stereo Plotting, DTM and Orthophoto.

Latest AT software is used for Aerial Triangulation. AT is used for assigning coordinates to a sufficient number of points for an area with stereo coverage of a large number of photographs bundled in a block. Goal  of AT  is  to use only a few  number of ground control points and generate huge number of control points by triangulation.

Survey of Bangladesh uses CAD software in photogrammetric workstations for Stereo Plotting. Topographic features are extracted accurately from undulated and urban areas. This system is more accurate and effective for creating topographic maps. Vast area can be controlled at a time and Stereo Plotting work can be done faster than the previous analogue system.It saves both time and money. After completion of Stereo Plotting, the map is sent for field verification.

SOB is creating DTM points at 20 m interval. The Earth’s surface takes many different shapes like mountains, foothills, basins, ditches, cuttings and embankments; elevation of terrain changes from place to place. Nature has created this terrain relief and we try to map it.

An orthophoto is a family member of photo mosaic. A photograph covers a limited part of the Earth’s surface. Taking number of aerial photographs in a ‘block’, we can join several images to make a single image covering a large area. This process is called “mosaicing” and the product is called a “mosaic”.

If mosaicing is done with unrectified images or improperly rectified images, there can be considerable mismatches along the cutline where two images are stitched together. The first advantage is that an orthophoto is georeferenced, i.e. users can read the coordinates of a point from an orthophoto. The second advantage is that true horizontal scale and bearing are maintained over the whole orthophoto, i.e. users can obtain true ground distance and bearing on it like a map. An orthophoto can be in digital format as well as in hardcopy.  After creating DTM points SOB generates orthophoto. This orthophoto can be used as a map if it has given topographic treatment.

Uses of Photogrammetric products in Bangladesh:

i) Large-scale stereo plotted topographic maps for all types of planning and construction.

ii) Highway design and layout planning.

iii) Location surveys

iv)  Measurement of structural deformations of civil works.

v) Analysis of slopes, ground coverage, watershed areas in order to determine surface water runoff.

vi) Analysis aerial and terrestrial photographs and other imagery for the measurement and interpretation of ruins dating to antiquity.