Assessing the Extent and Severity of Erosion on the Upland Organic Soils of Scotland using Earth Observation
A GIFTSS Implementation Test
In 2009 the Scottish Government launched its Soil Framework for Scotland which highlighted the need for a monitoring scheme to identify trends in soil condition. A central theme for this framework concerns the link between soil erosion and climate change and the large soil carbon reserves that exist in the soils of Scotland.
Environment Systems were commissioned by the Scottish Government, working with the British National Space Centre (BNSC) under the GIFTSS programme (Government Information from the Space Sector), to evaluate satellite earth observation as a cost-effective method for assessing the extent and severity of erosion in the upland organic soils of Scotland.
Environment Systems specialise in the development and use of geographic information in the environment, agriculture, land and property sectors. The company has a strong track record with government and corporate organisations across the UK and the rest of Europe. This has been developed through delivering independent, professional and cost effective services and solutions with the use of geographical information systems (GIS), remote sensing, database and internet technologies.
The GIFTSS study was set up to deliver an implementation test of the mapping of peat erosion using earth observation. The study was based on the Monadhliath Mountains which are located within the westernmost range of the Grampian Mountains in the highlands of Scotland.
Peatlands are one of the most important ecosystems in terms of carbon retention. The spatial extent and mosaic of exposed peat, vegetated bog surfaces and pools is a highly important component in regards to the potential modelling of the carbon budget of peatlands and related climate change phenomena. As long as the erosion rates of organic matter within the peat bog are lower than the rates of accumulation, it functions as a carbon sink.
Peat erosion features range in scale from local events to the extensive degradation of plant cover and associated exposure of bare peat. This exposure leads to the surface layers of the peat mass becoming less structurally cohesive through the action of frost and desiccation. Rain may then penetrate down these desiccation cracks leading to the development of gully systems (Figure 1). Other erosion features prevalent in peatland environments include rills and sheet features, peat slips and peat bursts.
For the study area SPOT5, IRS P6 and ASTER satellite imagery were prepared; including full geometric and atmospheric correction. Critical to the success of the project was the availability and full integration (into the automated processing chain) of current digital aerial photography. This image data was then complemented by GIS datasets that, whilst often historical and at differing scales and nomenclatures, provided a set of core topographic and thematic information.
Definiens eCognition object orientated rule-based classification software was used to classify the imagery into a set of ‘core level’ data, which in turn was used to produce ‘application level’ data; in this case peat erosion maps. eCognition was chosen due to its ability to take into account both the spatial and spectral information in high-resolution remote sensing imagery, its relative ease in realising the processing of a large remote sensing dataset, its ability to include ancillary information in the segmentation process, and its fast execution Outputs include standard vector and raster formats which can be easily used as inputs within future analyses.
The segmentation procedure involved the creation of three levels, which were used to classify object features at different scales. Within the upper level (Integration Level), all external datasets (i.e. OS MasterMap) were synchronised within a chessboard segmentation procedure and excluded from further classification. The multi-resolution segmentation algorithm was then used to segment the remaining areas of the image to aid in the removal of all other areas not associated with peat erosion features (i.e. Woodland, Non Peat vegetation) to separate the areas of blanket bog.
This segmentation was subsequently carried down to create an Earth Observation (EO) level A finer multi-resolution segmentation within the SPOT data applied to areas of blanket bog to aid in the classification of smaller peat features within the EO data.
Finally, within the lowest level (Air Photo Level), a very fine multi-resolution segmentation using the aerial photography was performed to delineate peat erosion features present within the aerial photography (Figure 2). This segmentation was applied within the areas previously classified to be potential peat erosion regions within the EO layer. The integration, of EO and Air Photo levels forms the basic data building blocks for the production of a range of maps at the level of detail required. Figure 3 shows how the classifications could be incorporated into an assessment of erosion risk.
Specific small scale peat erosion features could not be identified through only the use of moderate EO data such as SPOT and ASTER. It was however possible to establish the larger areas of bare peat which are present.
The use of aerial photography enabled the smaller scale peat erosion features to be delineated with features such as peat gullies classified to a high level of detail. The use of aerial photography alone though did not enable the discrimination between areas of bare peat, and dark, vegetated areas dominated by species such as Calluna vulgaris. Therefore it was necessary to utilise the greater spectral information of satellite based EO data to initially target areas, within which the finer resolution aerial imagery could be applied.
Figure 2 – Air photo level classification
Figure 3 – Erosion risk level classification
Overall map accuracy was calculated at over 84%. The classified map and both the in-situ field data and aerial imagery clearly coincided with one another. Within the accuracy assessment for the land cover maps several layers of uncertainty exist. The primary layer of uncertainty was found in the field and consists of ecotones between one land cover type and another e.g., where a degraded bog becomes a peat erosion feature is not necessarily a ‘hard’ line on the ground. In many cases, the slump at the end of a peat hag has resulted in clumps of bog vegetation within the bare peat area. Where these are frequent, the area is eroding bog; where they are infrequent the area would be considered ‘bare peat’, but the land in-between has a degree of uncertainty.
The success of the project is based on not simply considering this as an ecological project, or as a remote sensing project. Rather, the coupling ecological knowledge with remote sensing expertise and EO information content provided a solution to a complex mapping challenge. EO, and in particular the combination of airborne and spaceborne imagery, offers the opportunity for consistent, objective mapping over a range of mapping scales.
It is proposed that the Scottish Government should adopt the use of EO for mapping and monitoring peat erosion across Scotland, initially through a short project to confirm the transferability of the approach to a second area of Scotland. This could take the form of a ‘rapid implementation’ to assess and confirm the biogeographical effect on erosion processes. A project of this nature would also provide an additional short term output for the Scottish Government, including further examination of additional ‘application levels’ that could usefully be generated from the ‘’core levels’ in support of policy delivery, whilst full funding is being considered.
The full report and downloadable PDF can be found here
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