Coherence image of Bonn area (Germany)
series of ERS-1 SAR (Slant Range Complex) data sets of an area near Bonn, acquired
during early spring 1994 has been analysed using interferometric methods. As
the images were sampled at 3 days intervals, good phase-coherence between image-pairs
could be expected. Images between 1st and 28th March were considered and totally
9 coherence images from different image-pairs have been generated.
Agricultural fields appear in general quite bright in the coherence images,
however some appear dark, indicating low coherence. In this cases, one can expect
that coherence was lost due to farmer activities during the time between the
two data acquisitions.
In order to verify this all coherence-images were provided to agricultural experts
of the Bonn-University. They inquire about the farmers activities during March
1994, especially with respect to the dark-appearing fields in the coherence
images. With respect to the relevant dates all fields were listed for type of
crop and farmers activities.
In all cases a reason for the loss of coherence could be identified.
To demonstrate the farmers activities over a longer period a multitemporal colour
image was produced (see figure) choosing 3 of the 9 coherence images, namely.
-07/03/1994-10/03/1994 shown in red image channel.
-10/03/1994-13/03/1994 shown in green image channel.
-13/03/1994-16/03/1994 shown in blue image channel.
All fields with equal grey-values in all three channels, i. e. with no change
in coherence over the whole period from 07/03/1994 to 16/03/1993 appear not
coloured, in black and white. The coloured fields result from different grey
values in at least one of the three image planes (red or green or blue). The
red fields have low coherence in the green and blue image plane, and hence no
contribution from those to the colour-combination.
In fact the farmer ploughed the field twice, once between 13/03/1993 and 16/03/1993
and a second time between 16/03/1993 and 19/03/1993.
In yellow appear the fields with no coherence in the blue image plane, i. e.
farming activities in this case must have taken place after the 13/03/1994.
In fact the field verification proved that farmer activities on these field
were carried out between the 13th and the 16th of March. In the same manner
all other colours could be explained.
Not only farmers activities, but also meteorological effects (rain, wind-fall),
irrigation and of course all changes linked to vegetation growth can be reasons
In our case, a loss of coherence especially related with growth can be neglected
as vegetation canopy in March, if available at all is still very low and further,
only a rather short time interval between two ERS-1 acquisition was considered.
(Wegmüller, Werner, Nuesch, RSL, Univ.Zurich)
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,