The comparison of aerial images taken at different times is a quick and cost-effective way to detect landscape changes. In the WBS module Remote Sensing, changes in approximately 7000 sites representing biotopes of national importance are investigated by comparisons between past and recent aerial images. An early-warning system enables federal and cantonal offices for the environment to identify positive and negative changes and, if necessary, to initiate measures.
In the first period of the project WBS (2012-2017), the conditions of sites of national importance were assessed using black-and-white stereoscopic aerial photographs taken at the time of the inventory in the 1980s and 1990s. These were then compared with site conditions as assessed from recent imagery (colour infrared stereoscopic photographs) of the period 2010-2015. Since 2018 the former state is compared with the state of up-to-date infrared stereo aerial pictures.
The interpretation of changes is carried out manually by comparing the two images using a 3D screen. The data illustrate and quantify both negative processes (e.g. shrub encroachment due to land-use abandonment), as well as positive developments following regeneration measures or adapted management regimes.
Two slightly different protocols are used to evaluate the state and the evolution on aerial images, both reproducibles and allowing the spatial differentiation of changes within sites. For dry grasslands, raised bogs and fens, amphibian spawning sites and alpine floodplains, a 50m x 50m grid was laid over each site. Within each grid cell, simple indicators such as the percentage cover of open soil/water/woody plants, as well as the type of woody plants (e.g. single trees, linear tree structures, groups of trees) and the occurence of infrastructure elements are visually estimated.
For riparian and lakeside floodplains as well as for river deltas of national importance, the interpretation is based on formations rather than a grid, with the assessment of categories such as gravel banks without vegetation and different forest types.