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Peat Depth Model Methodology:
The peat depth model has been generated using ArcGIS 10.3.1 geographical information system (GIS) software, a widely available, industry standard software package. ArcGIS provide several different methods of interpolating a surface with varying values (in this case peat depths) across an area from real, measured data points.
The method used to create the peat depth model for this project has involved creating a Triangular Irregular Network (TIN) which connects real measured data points via a series of edges to form a network of triangles (ESRI, 2016). The TIN is subsequently converted from a TIN into a ‘Raster’ (i.e. a grid of cells of equal dimensions bases on a specified resolution, such as 1m by 1m) to allow further analysis (RWE, 2013a)).
This method has been chosen following detailed analysis at the Carnedd Wen Wind Farm site in Mid-Wales, which showed that, of the various methods available in ArcGIS, the TIN to raster method was preferable due to:
• Its mathematical simplicity
• The reduced likelihood of it reducing the size of, or ‘smoothing out’ completely, smaller areas of deeper peat
• It being true to the measured dataset from which it is created, in that the value of the peat model surface at a measured data point will always be equivalent to the value at that data point (RWE, 2013a).
Additionally, work at Carnedd Wen found there to be little discernible impact on the model between specified raster resolutions of 5m, 10m and 20m. At Carnedd Wen, a raster resolution of 5m was used so that the peat depth can be more accurately represented where data points are closely spaced, such as at known proposed infrastructure locations. For the peat model created for the Proposed Scheme, a resolution of 1m has been used. It is acknowledged that such a resolute raster may ‘over-represent’ the resolution of the survey, i.e. give an impression that more data has been collected than is actually the case. However, by using such a resolution, sudden step changes in peat depth (where none is present in reality) are avoided, calculations of volumes are more straightforward as each raster grid cell represents a 1m2 area (rather than a 25m3 area) and inaccuracy in volume calculation caused when the footprint of infrastructure elements overlaps partially with a grid cell. At the Carnedd Wen site, ‘barriers’ were introduced to the model to reduce the peat depth to zero where streams were known to be incised to substrate, and where lakes and streams existed. Such barriers have not been employed for the Proposed Scheme here, as this technique was resulting in areas being interpolated as zero peat depth, where this might not be the case. Instead, any such features including watercourses incised to substrate, existing road or tracks where no peat is present, have been ‘reduced to zero’ from the peat model. Embankments and cutting slopes have deliberately not been ‘reduced to zero’ peat depth to account for the possibility that dressing of these slopes has been undertaken with peat or peaty soil.
Peat Depth Model Iterations and Testing:
Numerous peat model iterations have been produced as new data has been acquired at various stages of survey. Whilst no specific testing has been undertaken to verify the accuracy of the peat model using ‘redundant’ points, where new data has been collected in areas where peat depth has been interpolated the change has usually been marginal and the new data has broadly confirmed rather than contradicted interpolated peat depths. Nonetheless, as new data becomes available, the model should be updated to ensure the highest accuracy possible.
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