The Rectification of High Resolution Remote Sensing Satellite Imagery
Chao-hsiung Wu
Assistant Professor, Digital Earth Research Center
Chinese Culture University
55, Hwa-Kang Road, Yang-Ming-Shan, Taipei 111
Tel:(886)-2-28623538 Fax: (886)-2-28623538
E-mail : chwu@derc.pccu.edu.tw
TAIWAN
Key Words
Remote Sensing Image, Geometric Correction, Ortho-Rectification
Abstract
Since the successful launch of the IKONOS in 1999, its high ground resolution has draw attentions from earth resource management communities. This study involves geometric correction of the IKONOS Geo-product image. Two correction models are applied to the image. 1: without ground control, the relative geometrically corrected image shows a visually acceptable picture. 2: with accurate control points and DEMs' yields ortho-rectified image. The results show that this image can be used for map revision at large and medium scale cost effectively. Further investigation of using satellite onboard data to precisely determine the imaging geometry is recommended.
1. Introduction
Since the advance of earth observation satellites in the late 70's, remotely sensed imagery has been widely used in many fields of earth science. Due to its limited ground resolution, applications in large-scale mapping, planning, zoning and evaluation are not yet readily for business purposes. The successful launch of the U.S. IKONOS in 1999, high ground resolution image is since then commercially available. It provides digital image of 4 meters ground resolution in color and 1 meter in black and white. The revision cycle of 1-3 days makes it possible to periodically monitor the changes of the earth surface in an ever-closer manner. This brings the detailed earth observation scope into a feasible and operational stage that medium/small scale image such as Landsat and SPOT image are not compatible.
The preprocessing of remotely sensed image consists of geometric and radiometric characteristics analysis. By realizing these features, it is possible to correct image distortion and improve the image quality and readability. Radiometric analysis refers to mainly the atmosphere effect and its corresponding terrain feature's reflection, while geometric analysis refers to the image geometry with respect to sensor system.
This paper investigates the geometric characteristics of the IKONOS satellite image. Two scenes were processed and the results were evaluated. Three experiments of various numbers of control points and distribution pattern were conducted to evaluate the planimetric accuracy. The results can be used when this image is to apply at large scale observation and measurement whose geometric requirement is in the order of one meter.
2. Mathematical Model
2.1 Space Transformation
During the satellite imaging process, the projection, the tilt angle, the scanner, the atmosphere condition, the earth curvature and the undulation etc., will cause the satellite image distorted. It is necessary to correction these distortions before one can really use it as a precise measurement in the large scale operations. In this paper, as previously stated, the orbital parameters were unknown. The mathematical model used to compensate the distortion correction is the so-called rubber shifting method. It neglects all the sources of distortions but deal with the present ones with the help of control points. This also makes the correction procedure easier in the circumstance of insufficient parameters. In this study, due to the limitation of the software on hand, one assumption was made in formulating the relationship between image coordinate system and the ground control system. It is assumed that the geometry of the IKONOS imaging system is similar to that of the SPOT, for both scanners are optical system. The import source of the software is image coordinate system. This system is then transform to the photographic coordinate system and again transform to the ground control system.
The transformation model between satellite image coordinate system and the photographic coordinate system is the widely applied Affine transformation or the 6 parameters transformation. The mathematical equation is:
where
x,y - satellite image coordinates
u,v - photographic coordinate system
a
1~c
2 - transformation parameters
The model used to link the photographic system to the ground control system is the famous condition: the space resection condition, the co-linearity equation as following:
in which
x,y - photographic coordinate, f - focal length
X
0,Y
0,Z
0 - exposure station in ground system
X,Y,Z - control point in ground system a
11-a
33 - rotation matrix
