Survey of Bangladesh (SOB), with the approval of GOB, is now implementing a project titled “Improvement of Digital Mapping System of Survey of Bangladesh (IDMS)” to produce 1:25,000 scale digital topographic maps and data covering whole Bangladesh territory using digital aerial photos and satellite images, and 1:5,000 scale digital topographic maps and data targeting 5 major cities using digital aerial photos by digital photogrammetric mapping systems.
For the production of 1;5,000 scale digital topographic maps and data at 5 major cities, 25 cm GSD digital aerial images have been taken. Also, for the production of 1;25,000 scale digital topographic maps and data covering whole Bangladesh territory except border, 50 cm GSD digital aerial images have been taken.
The digital photogrammetry, especially, aerial triangulation and digital plotting are the most important key work in the digital mapping.
At the first stage of digital photogrammetric mapping, aerial triangulation will be executed. After completion of aerial triangulation, results of aerial triangulation and digital aerial images will be sent to digital plotting section. Digital plotting section will obtained the data such as river line, road line, vegetation boundary and so on by 3D observation of stereo models of digital aerial photos.
After implementation of filed check (identification), both data such as digital plotting data and field checking data will be sent to GIS section. And then, data re-classification based on field identification data and also, data structuration such as polygon making will be executed by GIS section. This data is called as GIS data base or GIS basic data.
After completion of GIS section`s work, the data will be sent to cartography section. Cartography technique such as symbolization, transposition, generalization and so on will be applied to prepare 1;25,000 scale digital topographic maps. And marginal information such as sheet number, annotation and so on will be added to the digital topographic data, then final 1:25,000 scale digital topographic maps will be created by cartography section.
DIGITAL TERRAIN MODEL(DTM)/DIGITAL ELEVATION MODEL(DEM)
Digital terrain model (DTM), which is a digital representation of the terrain surface, given by
a. Gridded data points, where a uniform grid is established in the horizontal (XY) plane, and the elevation at each grid intersection is given, or
b. Random data points, at locations where significant elevation changes occur.
IN SURVEY OF BANGLADESH
DTM points interval = 20 meter
Accuracy of DTM points = +/- 50cm
COMMON USES OF DIGITAL TERRAIN MODELS INCLUDE