Dr. V K Srivastava
Indian School of Mines
Dhanbad
[email protected]
Geographic Information System is a computer based system for storing, checking, manipulating, analyzing and displaying data which are spatially referenced to earth i.e. geographically corrected.. Georeferenced data define objects that have orientation and relationship in two or three dimensional spaces.
Though GIS handles large amount of spatial and aspatial data, it lacks adequate spatial information and is static (Tomilson,1972). However it becomes more effective when real time satellite remote sensing images are incorporated along with sets of other data sets ( Shelton and Estes, 1981).
So satellite remote sensing data are an important source of spatial data of earth surface as an input to GIS but in raw format it contains geometrical distortion which make them unusable as geographically standard data sets These geometric distortions may be in general caused by earth curvature , atmospheric refraction and panoramic distortion. The objective of the geometric correction is to correct for these distortions to produce an image geometric integrity of map. So that map represents surface such that any measurements made on the map agree accurately with the corresponding measurements made on the ground. A map should conform to an internationally accepted type of cartographic projection (PCI Manuals,2000).
Geometric Correction in satellite images
Therefore in order to integrate remote sensing images as geographic data sets to GIS they must be geometrically corrected and transformed into the standard cartographic projection scale. So that it can be overlaid in perfect registration with other cartographic information's. Such accurate registration helps in analyzing multi dates multi spectral images in common geographical frame.
Two basic corrections are, to raw remote sensing images:
- Bulk or Systematic Corrections.
- Precision Corrections
In bulk correction, the images are corrected for errors that are known in advance during the data acquisition stage. These in general include distortion due to the internal sensor characteristics, the attitude and position of the sensor platform and the easterly rotation of the earth.. Correction for these type of distortion are made at the earth receiving station. The bulk corrected data with coordinate marking are in general supplied to users.
Remaining errors are rectified using information based on Image Header file and from the accurate location of ground control points (GCP) for precision corrections, a step known as Georeferencing.
Accurate positioning of ground control points (GCP) using prominent land marks vis a vis features on remote sensing images need to be established.
Accuracy of ground control point for image rectification or geo- referencing depends on few factors including the quantity and distributions of GCP and their reliability which in turn controlled by the quality of GCP sources. Commonly topographic maps are used to select GCP. Buts at some places it is restricted in nature and also some time they are old with varied inconvenient scales and therefore inconvenient and less reliable to be used for geo referencing.. It is here, Global Positioning System (GPS) provides mean to collect accurate ground control points in field for precise correction of spatial remote sensing images.
Global Positioning System and Rectification
The Global Positioning System consists of 24 – Satellites in near circular orbit at about 20000 km altitudes, provides coverage with minimum 4- satellite at any place all over the globe. By receiving signals from 3- orbiting satellites latitude, longitude and altitude of the receiving station can be known where as signal from th one give coordinate Universal Time (Garg,1999; Srivastava,2001).
High precision GPS observations with accuracy up to centimeter and even millimeter can now be achieved effectively for 24 hours a day and anywhere on the earth's surface.
So GPS technology offer many advantages as compared to conventional survey such as coordinates in the field in real time, centimeter level accuracy, no need to post processing the data, one man system and is independent of weather condition.
Rectifications of Landsat TM, SPOT Multi spectral and SPOT Panchromatic digital images have been obtained better by GPS rather than by digitized top sheets as reported by Cook and Pinder,1996 and Kardoulas et al,1996).
Smith and Atkinson, 2001, discussed and established the importance of GCP collection through GPS rather than using digitized Toposheet in change detection study .
According to Gao, 2001, the image rectification in meters is inversely related to image pixel size but positively correlated with numbers of GCP observations. He preferred non differential GPS technique over the differential GPS for collection of ground control points for rectifications of earth resources images. In this process error is minimized by averaging GPS derived coordinates at the same positions.
However according to Smith and Atkinson, 2001; differentially corrected GPS values provide for the optimum corrections of the high spatial resolution data whereas marginally better rectifications have been achieved for coarse image data using uncorrected GPS measurements.
Conclusion
Remote Sensing Image data as input to geographic information system (G I S ) for real time analysis must be georeferenced to represent the real earth surface.. Though so far digitized Toposheets have been used for selecting ground control points (GCP) but due to inherent limitations and non compatibility of scale, render them unsuitable for precise corrections. Therefore with recognized merits such as day and night and all weather data collection capabilities, Global Positioning System is the better technique for collecting ground control points (GCP) for Georeferencing.
Acknowledgment
Author is thankful to Prof B B Bhattacharya, Director, Indian School of Mines, Dhanbad for his kind permission to present this paper in the Asian GPS Conference.
References
- Cook A E Cand Pinder J E 1996," Relative accuracy of rectification using coordinates determined from maps and the Global Positioning System," Photgrammetry Eng and Remot Sensing, Vol 62,No 1,pp73-77.
- Gao,Jay,2001," Non Differential GPS an Alternate source of Planimetric control for Rectifying Satellite Imagery", Photogrammetry Eng and Remote Sensing, Vol 67.. No 1,pp 49-55.
- G C P Works, PCI Geomatics Manuals,2000.
- Garg P K, 1999" Global Positioning System – a Future Scenario, GIS India,,Vol 8,No 4,pp 9-12.
- Kardoulas, N G, A C Bird and A I Lawn, 1996, Geometric correction of SPOT asand Landsat imagery: A comparision of map and GPS derived control points", Photgrammetry Eng and Remote Sensing,Vol 162 , No 10,pp 1173-1177.
- Smith D P and Atkinson S F,2001," Accuracy of Rectification using Topographic maps versus GPS ground control points," Photgrammetry Eng and Remote Sensing, Vol 67,No 5,pp 585-587.
- Shelton R C and Estes J E, 1981," Remote Sensing and Geographic Information System: an unrealized potential". Geoprocessing 1,pp395-420.
- Srivastava V K,2001,"Role of Remote Sensing and Global Positioning System in Land Environment Management in Mining Area," Proc on 4th Conference on Coal Mine Surveying -2001,pp 11-15.
- Tomilson R F,1972," Introduction: In Tomilson R F (ed) Geographical Data Handling UNESCO/IGU, Secondv Symposium in Geographical Information Syastem , Ottawa, Canada,pp 3b -3m.
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