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Guide Sidmouth Beach Management Plan: Frequently Asked Questions

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26. Why does some of the analysis show the cliffs “growing towards France”?

Inaccuracy within historic mapping techniques and the poor resolution of historic aerial photography when compared to modern images can lead to uncertainty when determining the position of landmarks over time. This error is normal when using historic data sets, with inaccuracy possible both within the survey undertaken at that time and when referencing older data to more recent mapping. These data sets were used to provide a long-term analysis of erosion for the beach management plan.

The large variation in resulting erosion rates could appear on to show the cliffs advancing as opposed to retreating (or not moving) as would be expected.

The transect method for measuring erosion has been used by CH2M partly to avoid areas of the largest error with the historic datasets. This method involves focusing on the same fixed cross sections of cliff over time, and comparing the measured positions of the cliff top and toe. This approach allows for the position of individual features to be mapped over time, and for the transects to be positioned along lengths with relatively low error, for example, where aerial photographs are clearer (fewer shadows or vegetation obscuring terrain), allowing the position of the cliffs to be more easily identified.