EuroSDR Projects - Host Ireland

3.2 Accounting for Different Acquisition Times
Since data acquisition times were not the same for optical and SAR images, the data sets have
marginal discrepancies in some places. Nevertheless, to achieve equal data from both sensor types,
masks were created to cut out objects and terrain not clearly defined (Figure 7). Areas covered by the
masks were not included in the analysis in order to avoid incorrect interpretation of information
content. The same procedure had to be adopted for a part of the airport area of Oberpfaffenhofen.
Figure 7: Areas with different content in SAR and optical images as well as in topographic map
(left); region with different content covered with a mask (right)
In order to achieve quantitative results and due to the fact that data analysis was conducted on raster
images, each layer of the reference maps was converted into a raster data set with pixel-size corresponding
to the imagery. This data conversion was carried out using ERDAS Imagine of Leica
Geosystems.
3.3 Reference Maps
As a basis for data evaluation, object classes were categorized into linear and areal (extended) area
classes. Object classes used for test sites were:
- Built-up - Railways
- Watercourses - Roads
- Agriculture
- Forest
- Alleys
Mask
In addition to these general classes, which were present in all data sets, region-specific object classes
were defined for each data set considering the specific data content (Table 5). For some areas in the
data sets of Copenhagen and Trudering it was not possible to obtain a correct classification. Neither
the image content nor the map information allowed the unambiguous definition of a class. These areas
were classified as undefined spots and not included in the analysis.
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