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Precision product generation using satellite data and GPS based ground control points  S. S Srinivasa Rao, D. S. Prakasa Rao, D. S. Pandit, C. K. Rajender & Y. V. N. Krishna Murthy Regional Remote Sensing Service Centre ISRO, Nagpur rrsscn@nagpur.dot.net.in sitiraju@hotmail.com
Abstract Remote Sensing data is being extensively used for natural resources mapping, management, monitoring and developmental planning. The geometric accuracy of the satellite output plays a major role in generating precise thematic maps. Conventionally, the satellite data is georeferenced using ground control points acquired from topographical maps. This process involves establishment of two transformation models i.e., map-to-digitizer model and image-to-map model, Each of these steps is highly interactive and contributes significantly in the precision of the output. In addition to this, the inherent locational accuracy of the topographical maps and the projection induced errors at the macro level also contribute significantly to the total error budget. With the availability of high resolution satellite data, the corresponding large scale reference maps are not available for georefencing of this data. Apart from this, the latest changes in the infrastructure are not reflected in the topographical maps. This has necessitated the search for alternate ways for precision product generation. The Global Positioning System (GPS) provides the well-equipped user an accurate coordinate at any point on the earth surface, at any time, in any weather conditions. A study has been carried out for collecting Ground Control Points using single frequency GPS receivers. The GPS data was processed in differential mode. The satellite data was rectified using these GCP’s and the second order polynomial transformation model. The total error budget achieved using GPS based GCPs was much lower than the Map-based GCPs. The precision product thus generated can be effectively utilized for thematic and cartographic mapping. Introduction In several parts of the country, improper utilization and management of natural resources especially land and water have set in degradation process through loss of soil productivity, decreasing storages in the reservoirs, rising stream beds, frequent floods, water logging, salinisation and desertification. Associated with these problems, increasing demand on the land resources is gradually leading to land scarce. Large areas of the prime agriculture are being lost due to urbanization and other developmental activities. Forests are being intruded for agricultural activities and exposed relentlessly to grazing pressures. Sectoral developmental efforts of past years with out concern for complementary and other associated resources have caused more harm than benefits. Excessive use of chemical fertilizers, pesticides and insecticides have undoubtedly increased crop production at the cost of fertility of soils. Water pollution through various sources made the problem still worse. In order to optimize and sustain outputs from primary systems to meet the growing demands of the increasing population, developmental plans with integrated approach has been accepted through out the world. This approach helps in optimal management and better utilization of natural resources for improving living conditions of the people. Remote sensing is a multi disciplinary activity which deals with the inventory, monitoring and assessment of natural resources through the analysis of data obtained by observations from the remote platform. The observations are synoptic, provide repetitive coverage of large areas and the data is quantifiable. Satellite remote sensing which meets the requirements of reliability, cost-effectiveness and timeliness is an ideal tool for generating spatial information for natural resources management. Global Positioning System (GPS) Global Positioning System has revolutionized positioning concepts, though it is started primarily as a navigation system. It works on the principle of space resection. It has wide range geodetic, geophysical, navigational marine, military and social applications. The NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System) is a satellite based radio navigation system providing precise three dimensional position, navigation and time information to suitably equipped users every where on a continuous basis. It is primarily a military system with limited access to civilian users. GPS receivers have been developed which observe signals transmitted by satellites and achieve sub-meter accuracy in point positioning and a few centimeters in relative positioning. It has the following advantages over the classical methods.
Remotely sensed images are not maps. Frequently information extracted from remotely sensed images is to be integrated with the map data in Geographical Information Systems (GIS). There are various sources of errors, which degrade the geometric fidelity of the satellite data. The geometric distortion is an error on an image, between the actual image coordinates and the ideal image coordinates, which would be projected theoretically with an ideal sensor and under ideal conditions. Geometric distortions are classified in to internal distortion resulting from the geometry of the sensor, and external distortions resulting from the attitude of the sensor or the shape of the object. The internal distortions are radial distortion of lens, tangential distortion of lens, error of focal length, tilt of projection plane, non flatness of projection plane, alignment error of CCD array, variation of sampling rate, timing error of sampling and variation of mirror velocity. The external distortions are planimetric error of platform, altitude error of platform, motion of orbital position, altitude of platform, variation of attitude, rotation of the earth, earth curvature, terrain relief and atmospheric refraction. The geometric correction is undertaken to avoid geometric distortions from a distorted image, and is achieved by establishing the relationship between the image coordinate system and the geographic coordinate system using calibration data of the sensor, measured data of position and attitude, Ground control Points (GCPs), atmospheric conditions etc. The satellite data recorded at the earth station is corrected to various levels of processing. They are Level 0 - Uncorrected (raw data) Level 1 - Radiometrically corrected and geometrically corrected only for earth rotation (browse product) Level 2 - Both radiometrically and geometrically corrected (standard product)
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