Green Volume 2020
To calculate the green volume, the data sets of vegetation heights were intersected with the ISU5 areas (block, block segment areas and road areas) (intersections formed, cf. Figure 2) and the vegetation data was transferred to the ISU5 areas.
Green volume of blocks, block segment areas and road areas
Each ISU5 area was used to trim each corresponding vegetation area. The area size of the trimmed vegetation area was then multiplied by its mean height (= green volume of a trimmed vegetation area) (cf. Figure 2).
The sum of these for each ISU5 area yields the green volume of all individual block, block segment and road areas (m³). Dividing the green volume by the area sizes (m²) then results in the respective green volume number for each area (m³/m²).
The green volume number also serves as the mean vegetation height (in m) relative to the entire block (segment) area or the non-built-up parts of the block (segment) or road area.
Green volume of non-built-up parts of block (segment) areas
To determine the green volume of non-built-up block (segment) areas, the buildings above ground were first assigned to their respective areas. These buildings were then used to trim the individual areas.
The stock of vegetation assigned to the areas extend beyond the non-built-up parts (vegetation in road areas, canopies above building level and green roofs).
Trimming the vegetation share based on the non-built-up areas yields the desired vegetation of the non-built-up block (segment) areas and road areas (Figure 3).
The green volume was calculated further according to the process described above.
Dataset from 2010: comparison of the old and new surveys
When updating the dataset to the version of 2020, the entire procedure of recording vegetation heights was switched from object-based sectioning (aerial photography flights in 2009/ 2010) to grid-based sectioning. Key advantages include the size and completeness of the dataset and therefore its suitability for further processing. Using this method, the 2010 dataset could also be reclassified and recalculated. The two periods could therefore be compared without methodological discrepancies while also minimising potential errors (cf. project report, only in German, SenStadtWohn 2021).
In the following, the two different datasets from 2010, from the old and new survey will be compared (cf. Table 1). First of all, it is clear that the old survey underestimated the green volume in all use groups. The largest discrepancy here occurs for the built-up area types. The new survey recorded an additional 148.3 km³ of green volume for these groups. This is due to the fact that the green structures in built-up areas are rather fragmented and were eliminated in the course of the segmentation process of the old mapping in which areas were combined. By switching to a grid-based segmentation for the new survey, these smaller green structure fragments could be recorded in more detail.
The old mapping of the forest areas also omits a green volume of 88.4 km³. For other green and open spaces and road areas, the green volume difference is smaller, amounting to about 21 km³.
Overall, the total green volume for 2010 that is recorded in the new mapping is therefore 5,157 km³, exceeding that of the old survey by 279.8 km³.