At Risk Cultural Heritage

Overlooking Portland Harbour, England, is Sandsfoot castle and its at risk of collapse thanks to coastal erosion. What can drone photogrammetry do to help mitigate the risk?

Dating from the reign of Henry VIII and completed in 1541 its creation was triggered by Henry’s decision to split with the Catholic church. Thanks to family connections to Charles V the Pope took things personally and formed an alliance intended on invasion. Henry responded by building a series of defensive castles along the south coast, of which Sandsfoot was one.

Since 1583 erosion has been a concern and now there is a debate about the castle’s future. The castle is now at risk of further loss and is a perfect example of how photogrammetry can preserve the remains, and offer some far deeper analysis that can offer comparative measurement.

Just before Christmas 2022 AccuPixel visited the site. A couple of short drone flights were enough to capture the extent of the castle and shoreline with 364 aligned images processed in 3DF Zephyr.

Lighthing conditions on the day would be described as harsh. The low winter sun cast some very strong shadows but nevertheless Zephyr worked well extracting details. The model can be viewed on Construkted.com and is embedded here:

After creating the mesh and deriving ortho mosaic and DEM the files were imported into our favourite GIS tool – Global Mapper – for further analysis.

Layers

Global Mapper segregates information using layers and this can include formats such as the elevation DEM and raster ortho mosaic, vector data points and lines, or online tiled map services such as World street View:

Contours

Using the DEM as source contour lines can quickly be generated as a separate vector layer:

Sections

Taking a slice through the site can help visualise the terrain. In our example four section lines have been drawn and each can then be used to create a path profile:

Volume

The castle originally had a gun gallery facing south east but was lost to the sea sometime during the 18th or 19th century. This section of the castle is still present on the beach and is very much in kit form…a lot of masonry is still there.

One chunk of this section does remain and its volume can be accurately estimated using Global Mapper’s analysis tools.

First step is to draw an area feature around the block of masonry which is then used as a boundary for analysis:

Analysis of the block on the beach indicates there is a total of 23.281m³ of fallen masonry inside the area feature. Volume analysis can work on lines too but area analysis works best when handling clearly defined features such as this.

Manually Rectified Layers

Not all content carries geo referencing but nevertheless has significant value.

Global Mapper allows manual rectification of raster source layers and we can use this to load an outline map into the project and place it in approximate context:

Using these layers are great for communication and understanding of how much has been lost to the sea. Sandsfoot’s gun gallery has gone as the cliff has eroded by approx 10m.

Summary

Data gathered from subsequent drone flights can be compared to this baseline and future changes measured and analysed in Global Mapper. Whilst not always constant the rate of erosion may be estimated and hopefully a plan put in place to secure the future of the castle ahead of further collapse.

For comparative work such as this photogrammetry is a very cost effective and accurate tool that can quickly record and preserve the as-is condition and our work with mountain biking trails clearly demonstrates its application for objective measurement.

If you require more information or detail please use the contact us page to get in touch.