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Berlin Environmental Atlas

01.02 Impervious Soil Coverage (Sealing of Soil Surface) (Edition 2012)

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Rule-Based Classification

Under rule-based classification, the results of spectral classification are combined with ISU data (section types) to yield degrees of impervious coverage derived at the pixel level. For this purpose, we first proceeded by using the set of rules developed for the 2007 Edition, and carried out a preliminary mapping process for 2011 was. Figure 4 shows a schematic overview.
In order to improve the comparability between two mono-temporally derived rule-based classifications, a second step was carried out involving a multi-temporal change analysis of satellite image data between 2005 and 2011.

Figure 4
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Fig. 4: Diagram of Rule-based Classification

The classes and the NDVI categories were then assigned to degrees of impervious coverage. A reliable delimitation of completely vegetation-free and completely vegetation-covered areas was achieved in the NDVI categories 1 and 12 (lowest and highest NDVI values, respectively). The corresponding threshold values were derived automatically by means of reference sections.

  • NDVI Category 12 "Vegetation - Certain:"
    Under the rules, such sections were classified as 0 % impervious. This applied to all section-type categories.
  • NDVI Category 1 "Vegetation-Free - Certain:"
    Vegetation-free spaces were only considered to be 100 % impervious once they had been determined to not be neither "Sand" nor "Track Gravel."

The range of values between these NDVI limits is broken down via interval scaling into ten additional NDVI categories of "Vegetation – Uncertain". In order to obtain a reliable assignment of degrees of vegetation and impervious coverage, they had to be interpreted differently, by section-type category or section type. Thus, three assignment variants were established (Table 2). with the mean percentage value (5 %, 15 %, ..., 95 %) the conversion factor for each NDVI and impervious coverage category.
Recommendations from the concept study, the evaluation results of Haag 2006 and findings from aerial image interpretations, and terrain inspections were incorporated, and results of the procedural validation process (cf. Validation, Edition 2007) were taken into account for the iterative process optimization.

  NDVI (categories and degree of vegetation)  
Kat 1 2 3 4 5 6 7 8 9 10 11 12
% 0 5 15 25 35 45 55 65 75 85 95 100
Degree of
impervious
coverage
% 100 0 0 0 0 0 0 0 0 0 0 0 A Assignment
variant
% 100 95 85 75 65 55 45 35 25 15 5 0 B
% 100 100 100 100 100 100 0 0 0 0 0 0 C
Conversion factors for the calculation of pixel values:
Degree of impervious coverage 100 % = 1.00; 95 % = 0.95 etc.


Tab. 2: Assignment Variants: Degree of Vegetation – Degree of Impervious Coverage

The assignment variants were oriented toward certain section types, which are characterized by the spatial interconnection and proximity of certain surface materials and types of buildings.

  • Assignment Variant A: Vegetation and pervious vegetation-free sections.
    The intermediate stages of the degrees of vegetation coverage (5% - 95%) were interpreted as mixed signatures of vegetation and other pervious surface types. The corresponding sections were therefore classified as pervious.
    Examples: Fallow areas, Forest, Farmland.
  • Assignment Variant B: Vegetation and impervious vegetation-free sections.
    The characteristic surface materials suggest a low share of vegetation-free pervious sections. Intermediate stages of the degrees of vegetation were therefore interpreted as mixed signatures of vegetation and impervious surfaces. The gradual increase in degree of vegetation per category thus corresponded to an adequate drop in degree of impervious coverage.
    Examples: Allotment gardens, traffic areas, block-edge buildings.
  • Assignment Variant C: Vegetation and impervious vegetation-free sections – block type "Airports".
    A variety of impervious surfaces characterized this block type. Some materials, such as concrete, showed strong spectral coincidences with sand and open soil. Such blocks indicate runways, parking areas etc. Within the airport area; green spaces were largely delimited as separate blocks. To achieve certain separation, it has proved useful to classify sections with low degrees of vegetation as completely impervious (NDVI categories 2 through 6).

The new rule-based classification for 2011 and that of 2005 were thus available as intermediate results. These sets of map data were interlinked, and also linked to the current ISU block map, in order to obtain reliable information on changes of degree of impervious coverage at block level.

Methodologically, the following aspects had to be taken into account in this process:

  • Ascertainment of changed sectors and elimination of pseudo-changes by means of multi-temporal change mapping
  • Comparability of the blocks in terms of geometry and sector type category.

For the reliable ascertainment of suspected sectors, the satellite image data of 2005 and 2011 for the non-built-up sectors were first of all evaluated, and secondly, the ALK data on possible changes in the built-up impervious sectors were examined.

Figure 5 provides an overview of the derivation of the results of the 2011 rule-based classification:

Figure 5
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Fig. 5: Diagram of the 2011 Rule-based Classification

The ascertainment of changed sectors was carried out taking the various phenological effects pertaining at the times of photography of the scenes, May and September respectively, into account. The spectral reflections of vegetation-covered services could vary depending on the development and/or vitality of the vegetation. Unchanged surfaces are accordingly described differently in the satellite images, and could therefore lead to mis-mapping in the automated evaluation, due to so-called pseudo-changes.
Using the procedure of principle component transformation (cf. Principal Component Analysis/ PCA, (Wikipedia 2012), the respective NDVI channels for 2005 in 2011 were analyzed, and suspected sectors, i.e. those which, based on their NDVI values, indicated possible changes in impervious coverage, were statistically derived.
The conclusive rule-based classification in 2011 was derived from a set of rules from the rule-based classification of 2005, and from the 2011 intermediate results. For unchanged blocks, the 2005 classification was retained. The rule-based classification in 2011 was adopted in the following cases:

  • changed blocks (changes of the ISU sector type, or major changes of block geometry)
  • suspected areas within unchanged blocks (changes in spectral properties, taking into account the phenology, ascertained by means of principle component transformation)
  • previously built-up areas which, according to the current ALK, no longer contain any structures (demolition).

The conclusive result of the rule-based classification system in 2011 for the non-built-up blocks corresponded to the final result of the satellite-image classification process. The category "non-built-up impervious sections" has been described in the classification with the 12 impervious coverage-degree categories, a Shade class and a Track-Gravel class.

Figure 6 shows the result of the 12 impervious coverage-degree categories, a Shade class and a Track-Gravel class, and the built-up impervious sections from the ALK, on a grid basis. Based on this intermediate results (grid data), the mean degrees of impervious coverage per block section were then calculated (cf. Calculation of the Degrees of Impervious Coverage).

Figure 6
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Fig. 6: Uncorrected Degree of Impervious Coverage (Grid Data) – Intermediate Results of Rule-based Classification

The intermediate result published in the FIS-Broker as the Map Impervious Soil Coverage (Edition 2012) raster data2011 Impervious Coverage Map (uncorrected degrees of impervious coverage, grid data) shows the distribution of impervious coverage within the blocks. The effect of shade in the various blocks can also be seen (Map 01.02, by contrast, shows the mean degree of impervious coverage per block area).

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