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Need For The Scheme
The Corporation of Chennai with an area of 170 sq km, has a population of 6.5 million. Urbanisation is at a rapid rate because of industrialisation and migration of population from rural area. The Corporation is find it very difficult to provide the basic utility services like water supply, sewage, waste disposal and other facilities in the absence of a large scale map. It is also very difficult to regulate the haphazard Urban development and managing the storm water drainage system especially during monsoon period. In order to check and regulate the haphazard development, the Corporation has to prepare detailed development plans for these Velacery area to guide and Control the growth. There is an imperative need to prepare a large scale comprehensive landuse maps to guide and control the development and to effectively monitor the growth of these areas. For effective plan Corporation needs a large scale base maps and hence the work has been entrusted to IRS to prepare the Orthophotos with Contours for part of Corporation area of about 10 sq km.

Methodolgy
NRSA has already taken Aerial photographs for the entire Chennai Metropolitan area (1167 sq km) during 1990 at a scale of 1:6000 for the use of three Organisations namely the CMDA, CMWSSB (Metro Water) and the Corporation of Chennai for the preparation of Master plans.The Corporation is interested in preparing the developmental plans for the Velachery area of about 10 sq km, situated on the south of Chennai city. Ground control survey were carried out using the Global Positioning System (GPS) and EDM, to establish the control points in the Project area and the same has been post pointed on the aerial photographs. The sophisticated equipments like Planicomp PHOCUS P3, Orthocomp Z2 equipments were used to generate DEMs and Orthophotos. These products served as a large scale base maps and helped the Corporation Engineers for preparing detailed development plans.

Ground Control Survey
The TRIMBLE 4000 SSE GPS has been used to traverse the area to determine the planimetric controls and levelling survey were also carried out for establishing the altimetric control. The Electronic Distance Meter SOKKIA is also used for connecting nearby GTS co-ordinates. Both the GPS and Levelling surveys were completed within 3 days. The WILD PUG5 instrument has been used for post pointing in the Aerial photographs. Models were established in the Analytical Stereo plotter Planicomp PHOCUS P3 and the model points including ground control and tie points were measured. Digital Elevation Models (DEM) were measured at an interval of 50m in order to prepare the Contrours at an interval of 20cm and also the input for the generation of Orthophotos. In addition the break lines and skeleton lines of the objects were also measured.

Satelite Data
The PAN and LISS III satellite digital data of IRS-1C satellite were extracted and both the data were merged. Based on the landuse information, extracted from the above merged and classified data and from the derived slope, soil and the morphological maps, the GIS technique has been used to identify the areas which are liable for flooding and areas which requires immediate storm water management (fig.1).

Orthophoto
Orthophoto maps on 1:1500 scale were generated using Orthocomp Z2 instrument for this area. The Orthophoto with superimposition of Contour line provides a very good data base for the developmental plans. The entire work of DEM measurement and generation of Orthophotos tllk only five days. The line maps prepared from mosaicing the Orthophotos is also super imposed on the merged satellite data (fig.2).The above large scale information such as Orthophotos and contour maps have been extensively used for the detailed development plans for the Velachery area and the plans were effectively implemented by the Corporation authorities.

Conclusion
The merged satellite data products of IRS-1C (PAN & LISS III) provides a valuable data base in the extraction of landuse, soil and other resources. The data base is found to be very useful for the GIS analysis, for identifying the problematic areas which requires immediate attention, so as to prepare the large scale Orthophoto maps and Contour maps in these problematic areas for implementing the developmental plans.The Orthophotos and the Contour maps on 1:1500 scale which have been prepared within a very short time and with a reasonable cost of Rs.1.08 lakhs after the ground control survey using GPS instruments, proved to be a effective tool in the storm water management and for implementing the Developmental plans by the Corporation Authorities. It also serves as a data base of various resources and can be updated periodically with the new data products.

Acknoledgment
The authors wish to thank to Deputy Commisioner (Works), Corporation of Chennai and the Officials of the Corporation for sponsoring the project. Thanks are due to Mrs.K.Vani and E.Singaravel for their help rendered in bringing out this paper.

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