Poli, Daniela 
  
CLOUD-TOP HEIGHT ESTIMATION FROM SATELLITE STEREOPAIRS FOR WEATHER 
FORECASTING AND CLIMATE CHANGE ANALYSIS 
Daniela POLI, Gabriela SEIZ, Manos BALTSAVIAS 
Federal Institute of Technology, Zurich 
Institute of Geodesy and Photogrammetry 
(daniela, gseiz, manos) O geod.baug.ethz.ch 
KEYWORDS: Climate Change, Image Matching, Cloud Top Heights, MOMS, ATSR. 
ABSTRACT 
Within the EU project Cloudmap, cloud-top heights should be estimated using satellite stereo images at very high 
temporal resolution. Since such images are not provided by operational sensors, MOMS-2P and ATSR2 images of lower 
temporal resolution were used instead. The MOMS stereopair was oriented with subpixel accuracy using GCPs from 
1:50,000 topographic maps and Kratky's sensor model. Preprocessing for noise reduction, cloud stripes removal and 
contrast enhancement was applied. For the estimation of cloud-top heights, the images were resampled to 288 m and 
geometrically constrained least squares matching, using image pyramids and an interest operator, as well as varying 
parameters was used. The results were checked by visual inspection and comparison to semi-automatically measured 
points in the original resolution images. Automatic blunder detection using two tests were also applied. Matching led to 
large blunders in land areas between clouds or close to cloud boundaries. Excluding these blunders (error » 1100 m), 
matching showed an RMS of ca. 0.2 pixel, exhibiting a very high accuracy potential. A matching geometric 
transformation using rotations and radiometric equalization during the iterations showed slightly better results compared 
to the other matching versions. ATSR2 images were matched with a similar approach, however without geometric 
constraints, as the input images were rectified. Due to differences between the images which vary spatially, varying 
matching parameters are optimal for each image region. First steps in combining matching results from such varying 
matching versions have been performed. Both datasets showed similar matching problems due to surface discontinuities, 
mixing of surfaces than are neighbouring in image space but differ in height, and often large illumination differences 
(even with along-track stereo and small time acquisition differences). 
1 INTRODUCTION 
CLOUDMAP is a EU project for the detection and mapping of cirrus clouds and airplane contrails using satellite sensors 
for weather forecasting and climate change analysis. The scientific objectives of the project are to provide new cloud 
products (height, type, optical thickness, fraction etc.), compare different techniques for the extraction of some of these 
products (brightness temperature, stereoscopy and Oxygen A-band) and to validate them using airborne sensor 
underflights and ground-based remote sensing instruments. The requirements for cloud-top heights are very high 
temporal resolution, low geometric resolution, high processing speed and an accuracy of 100 to 500 m, depending on the 
application. 
This paper will describe the results obtained in cloud-top height estimation from satellite sensors using stereo images at 
various resolutions (MOMS-2P and ATSR2) and spectral content (ATSR2). The exact knowledge of cloud-top heights is 
still a problem in satellite meteorology. The cloud heights derived from the radiation temperatures of infrared channel 
data include uncertainties mainly because many clouds are not blackbody sources, do not always fill the full pixel size 
and there is insufficient knowledge of the atmospheric temperature profiles. Photogrammetric methods based on 
stereoscopy are independent of these factors and are, therefore, useful for an improvement of cloud-height 
determinations. (Lorenz, 1985). Different authors (e.g. Hasler, 1991) have shown that cloud-height measurements made 
from stereoscopic satellite images can be used for a broad spectrum of meteorological processes. 
2 CLOUD-TOP HEIGHT ESTIMATION FROM MOMS-2P 
The MOMS stereopair (Fig. 1) was taken over South Germany on March 14th, 1997, from the MIR - Priroda mission, 
from a height of approximately 400 km. MOMS-02 is a three-line sensor (Ebner et al., 1996; Kornus, 1998), with along- 
  
1162 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000.