Lidar

Lidar (Light Imaging, Detection and Ranging) is a surveying method which measures distance to a target by illuminating the target with a laser light. With airborne Lidar, the laser scanner creates a 3-D digital elevation model of the landscape with a highly accurate level of detail. A major advantage of the technique over conventional aerial photography is its ability to filter out reflections from vegetation to reveal underlying surfaces, such as rivers, roads and archaeological sites, normally concealed by trees.

In the UK, Lidar images of the landscape are now freely available from the Environment Agency, at resolutions of 2m, 1m, or 50cm  depending on location. These can be downloaded and further processed using specialist software such as QGIS to add shadows to enhance detail.

For the Longtown area, only 2m resolution images are available, which lose sharpness quickly as objects on the ground are enlarged by the software.  Even so, some interesting features emerge.

The top image shows the ridge between the Olchon brook (left) and Monnow river, with the village of Longtown running up its spine. Longtown Castle is the square structure near the top. The bottom image shows the Ponthendre motte and bailey towards top right. with the confluence of the two rivers just to its right.

Looking at the pictures in closer detail shows the position of the medieval burgage plots which lay to the east side of the road south of Longtown castle. The extensive area of ridge and furrow ploughing to the west of the castle is also apparent. Little of this is visible to the casual observer.  If anything, the area to the west of Ponthendre (lower picture) shows even greater coverage  of ridge and furrow and it may be that in the medieval period, arable farming decreased at higher altitudes, despite the favourable climate of the time.