538 
OPERATORS USED IN THE COMPUTATION OF 
COHERENCE MAPS FOR THEMATIC INFORMATION 
EXTRACTION. 
Gerrit Huurneman 
LT.C., the Netherlands 
Keywords: ERS-tandem mode, coherence map, filters, land use 
Abstract 
The application of repeat- pass SAR interferometry for the creation of DTM's and in combination with optical 
data for the extraction of thematic information is well known. The realization of these products, the DTM and 
the thematic information, is based on the coherency of the input data sets. Coherence maps are created from the 
SAR data sets that are in complex format. Each element of the map is the level of coherency in the local region 
of that element. The resolution of the coherence map is related to the size and contents of the operator kernel used 
in the computation of the coherency. In this paper, the influence of some operators on the resolution of the 
coherence map, is tested, described and visualized with a data-set form the island of Ameland situated in the 
northern part of the Netherlands. 
1. Introduction 
Optical and microwave data can provide 
complementary information about objects that 
cover the Earth surface. The image elements 
(pixels) of optical sensor can be seen as vectors of 
which the components represent the reflection in 
the different bands. Image elements of microwave 
data consist of two components, the magnitude and 
the phase, which are stored as complex numbers in 
two "layers". If optical and microwave data sets are 
correctly combined, the resulting product will 
convey more information and could prove to be 
more useful then either image alone. 
Phase information of microwave image data is 
mainly used in the field of interferometry. 
Interferometric processing of SAR data from space 
combines images from two passes of a sensor 
system or combines the data from two sensor 
systems in tandem mode. This process derives 
precise measurements of the differences in path 
length to the two sensor positions. The main output 
of interferometry of SAR data is topographic 
information related to terrain heights or the 
monitoring of positional changes of the Earth 
surface. A strong relation exists between the quality 
of these products and the correlation of the 
complex data sets. In those areas where high, 
correlation exists, an accurate Digital Terrain 
Model can be realized. On the other hand, height 
information cannot be extracted in areas with low 
correlation. So the quality of the products is 
characterized by the "interferometric correlation", 
which is a measure of the variance of the 
interferometric phase estimate. The amount of 
correlation is a function of the system noise, the 
volume scattering, baseline configuration and 
temporal change. Consequently, the interferometric 
correlation itself contains significant thematic 
information that can be useful for several other 
applications. In SAR images, the magnitude and 
the phase of each element of an image are the 
coherent summations of the backscattering and 
phase of the individual scatterers inside a 
resolution cell. 
In a changing structure and/or chemical 
composition of the ground-cover, the amount of 
temporal decorrelation will vary and this variation 
can contain information about the type of ground- 
cover or the situation in which it exists. The 
influence on temporal changes, in case of 
spaceborne sensor-systems, can be minimized by a 
high temporal resolution of the system or by a 
combination of two "identical" systems in a tandem 
mode. The ERS-1 and ERS-2 systems satisfy this 
last requirement. 
Correlation between the data from a cell is 
expressed in terms of the summated phase and 
intensity of the resulting backscatter. A cell is 
considered to contain a set of individual 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998