nbul 2004 
2ummlers 
thms and 
Scientific 
‚S., 2002. 
.pdf 
TOPOGRAPHIC CORRECTION FOR DIFFERENTIAL ILLUMINATION EFFECTS ON 
IKONOS SATELLITE IMAGERY 
K.H. Law*, J. Nichol ® 
Department of Land Surveying and Geo-Informatics, 
The Hong Kong Polytechnic University, 
Hunghom, Kowloon, Hong Kong 
* samuel.law@polyu.edu.hk, °Isjanet@polyu.edu.hk 
KEY WORDS: Remote Sensing, IKONOS, Algorithm, Correction, Test 
ABSTRACT: 
The problem of differential terrain illumination on satellite imagery has been investigated for at least 20 years and has not been 
solved satisfactorily. Most past research has been conducted on Landsat imagery where the look angle is nadir. There is no research 
on topographic correction of IKONOS imagery, which has a higher spatial resolution. 
The high spatial resolution of IKONOS imagery requires a more accurate digital elevation model (DEM) than is obtained using 
standard interpolators. This paper assesses some existing topographic correction methods and applies them to off-nadir multispectral 
IKONOS-2 imagery in the mountainous terrain of Lam Tsuen country park in Hong Kong. The IKONOS-2 image was ortho- 
rectified using PCI Ortho Engine achieving an accuracy of approximately 4m over the whole image. Slope and aspect parameters 
derived from the terrain model were applied in different topographic correction algorithms. This research makes a contribution to 
existing knowledge on the subject. It demonstrates existing algorithms for topographic correction of satellite image can be 
successfully adapted to IKONOS images. ; 
1. INTRODUCTION 2. BASIS OF THE STUDY 
1.1 Background 2.1 Image Used & Study Area 
The spatial resolution of satellite images has increased in The satellite image used in this study is a 4m multi-spectral 
recent years. However, many images are affected by the system IKONOS imagery taken on 7 Sept 2002, with solar azimuth: 
itself and by the environment e.g. geometric distortion and 128.7341 degrees and solar elevation: 65.16463 degrees. 
shadow due to topography. In order to interpret the satellite 
images effectively, these effects need to be minimized. Lam Tsuen Country Park is chosen to be the study area. The 
mountain is oriented northeast to southwest and is located at 
This study examines the application of existing topographic the northern part of New Territories, Hong Kong. It has a 
algorithms on IKONOS satellite image. Most past research relative relief 566m over an area of 5km x 5km . Its land cover 
about topographic correction has been conducted on Landsat types are mainly forest, herbaceous, grass, bare soil and 
imagery, e.g. (Teillet et al., 1982; Justice and Holben, 1979; buildings. 
Civco, 1989) As IKONOS satellite imagery has 4m spatial 
resolution, which is comparatively high to traditional satellite 2.2 Creation of Digital Elevation Model (DEM) 
sensors such as SPOT and LANDSAT, the accuracy 
requirements of both geometric correction and digital elevation 
model (DEM) for topographic correction need to be higher than 
before. 
Digital contour maps of 1:5000 scale produced by the Land 
Information Centre (LIC) of Lands Department in Hong Kong 
were used as a source of height information. The height 
1.2 Objective separation of the contours in the digital map is 10 meters. A 
DEM with a resolution of 2m x 2m was created. 
The objective of this study is to compare three existing 
methods of topographic correction and see how successfully From the DEM dataset, information about the slope, aspect and 
they can be applied to IKONOS imagery. illumination according to the sun angle and elevation were 
generated for input to the topographic corrections algorithms. 
2.3 Geometric Correction 
The IKONOS image used in this study was geometrically 
corrected to the HK80 grid coordinate system, which is the 
same as the digital maps. 
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