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GPS for geometric correction of remotely sensed imagery: possibilities after termination of SA
S. K. Katiyar
Ph.D. student
Civil Engg. Department
I. I. T. Kanpur- 208 016
Email skatiyar@iitk.ac.in
Tel. No. 0512-597937
Onkar Dikshit
Associate Professor
Civil Engg. Department
I. I. T. Kanpur- 208 016
Email onkar@iitk.ac.in
Tel. No. 0512-597937
Krishna Kumar
Professor
Aerospace Engg. Department
I. I. T. Kanpur- 208 016
Email kkumar@iitk.ac.in
Tel. No. 0512-597060
Abstract
Precise and reliable information extraction from remotely sensed data requires higher accuracy geometric correction. In the pre-processing of high-resolution satellite imagery like SPOT, IRS-1C/D and IKONOS geometric accuracy requirements are more demanding. Geometric correction process utilizes some GCPs for the removal of image distortions, and the correction accuracy depends on the accuracy of GCPs used. In the developing countries like India, the large-scale maps are not available and getting a high accuracy GCP is a big problem. GPS is an alternative tool for the determination of precise GCP coordinates. The differential mode observations are more accurate and reliable as compared to stand-alone mode. In higher accuracy requirements (sub-meter/centimeter level), the dual frequency (L1 and L2) receivers are used in differential mode, which are very costly. The hand-held GPS receivers are very cost effective and after termination of selective availability (SA) of GPS signals, accuracy of these receivers has been improved considerably.
This research paper investigates the accuracy of hand held L1 frequency GPS receiver (GS5), for its suitability in deriving GCPs for the geometric correction of IRS-1C/D, PAN sensor imagery. In order to verify the accuracy of GPS receiver, eight different stations were established in a localized area of about 1.5 ´ 1.5 km. The true distances between these stations were measured with the help of total station, and GPS measured distances were computed from the mean values of repeated observations taken for a period of about one month. At one of the above stations, observations were also taken with the help of dual frequency receiver (SR530). The difference between GPS computed and total station measured distances is about 1 to 4 meter. The coordinates observed by hand-held receiver are deviating from the corresponding values computed from differential GPS by about 5 to 7 meter. The GCP-based geometric correction of one PAN imagery, using hand-held GPS receiver derived GCPs, has yielded sub-pixel RMS error. In absence of SA, the hand held GPS could be a cost effective solution for the determination of GCP coordinates.
Introduction
Satellite images are vital tool in various applications like land use and land cover mapping, Geographic Information System (GIS) etc. Remotely sensed images are often considered as maps of the radiometric properties of the earth's surface. However, these are not usually map-like. A variety of factors involved in the process of image acquisition, introduce geometric distortions, which are removed by pre-processing of the digital imagery. Geometric correction is the process of rectification of geometric errors introduced in the imagery during process of its acquisition.
A major concern in remote sensing information extraction and data handling is to ensure proper geometric properties of the acquired image. The objective of geometric correction of remotely sensed data is to make the image features geographically correct. Methods chosen for geometric correction rely on the purpose of applications, based on geometric precision required. Precision geometrically corrected products are basic necessity for mapping smaller features in the urban area and use of high resolution satellite data like SPOT, IRS-1C/D and IKONOS. In the geometric correction process, precise ground control points (GCPs) are required and accuracy of geometric correction depends on the precision of GCP coordinates.
As technology progresses, the raster picture element (pixel) resolution of digitally acquired remotely sensed imagery continue to become finer in spatial resolution. This dictates the need of ground control points (GCPs) with increased spatial accuracy to geometrically correct remotely sensed data. Conventionally GCPs are derived from maps, but in the developing countries most of the existing maps and medium scale maps are out of date due to large population growth, rapid changes in urban areas and economic development. In order to achieve the sub-pixel level accuracy in the geometric correction of high spatial resolution imagery required scale maps might not be available. In such situations, Global Positioning System (GPS) can prove an alternative tool, which can provide even sub-meter accuracy GCPs.
The use of GPS derived GCPs in geometric correction of satellite imagery, is very common. In the literature, various researchers have reported the use of GPS derived coordinates in the geometric correction of remotely sensed satellite imagery. GCPs were derived for the geometric correction of Landsat TM imagery of Ituri rain forest of northeastern Zaire, by using Magnavox 4400 GPS receiver (Wilkie, 1990). The average precision hand held GPS receiver (Magellan NAV 1000 PRO) was used in differential mode mode, for the geometric correction of SPOT imagery, with accuracy sufficient to meet planimetric accuracies for 1:50,000 scale production from satellite imagery (Clavet et al., 1993). The Landsat TM and SPOT, MSS and PAN images were geometrically corrected with the help of GCPs derived from Trimble Pathfinder Professional GPS receiver (Cook and Pinder, 1996). GCPs were determined by Magellan NAV 5000 PRO GPS receiver in stand-alone mode, for the rectification of Landsat MSS and TM and SPOT images (Kardoulas et al., 1996). Differential global positioning system (DGPS) was used to derive GCPs for the geometric rectification of fine resolution (4m × 4m pixel) aircraft scanner data (Witter et al., 2001).
In the results of above investigations, accuracy of GCP coordinates determined from stand alone single hand held receiver was not very good due to selective availability (SA) of GPS signals. In view of the termination of SA, the accuracy of stand alone hand held GPS receiver require investigations. In order to have cost effective GPS solution, accuracy requirements need to be thoroughly assessed and accordingly decision should be taken on the GPS configuration.
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