3. Photos and LIDAR
Photographs are very useful to archaeologists to show how places have changed over time. We tend to look at three different categories of image:
Aerial and satellite photos, and
A specialised type of imagery called LIDAR.
When we did the Alton Big Dig a few years ago, some of the town's brewery owners there had been among the first photographers in England and we had some photos dating back to the mid-1800s. In the case of Petersfield, the most useful source of old photos of buildings has been the photographic archive of the Petersfield Museum. These are examples of their historic photos.
Houses in Sheep Street. © Petersfield Museum & Art Gallery 2023
Aerial view towards The Spain. © Petersfield Museum & Art Gallery 2023
Aerial and satellite imagery
Satellite images can sometimes be useful but usually they are too recent. Of more value are aerial photographs. There are online archives of aerial photos, examples of which can be accessed through these websites:
Historic England has some aerial photos that date back to just after the First World War but most of these are of too poor quality to be useful for archaeology. We tend to visit their archive in Swindon (by appointment) to see the original copies of the images. The most useful photos tend to be the high-quality images taken by the RAF at the end of the Second World War, when they undertook a national survey.
LIDAR is like radar except that it scans using a laser beam instead of a radio beam. An aircraft flies across the countryside at a constant height and flies in a zigzag pattern to cover the whole of an area. While doing so it repeatedly fires a laser vertically downwards and times how long it takes for the reflected beam to be bounced back. Because the laser beam is travelling at the speed of light it is possible to calculate accurately the height of the ground below. The big advantage of using a laser is that the beam penetrates through any foliage to the ground surface, so it is possible to see the shape of the terrain even if covered in trees or scrub.
This has been a blessing to archaeologists in recent years, because it is able to reveal lumps and bumps in the ground not visible to the eye at ground level, even if covered by forestry.
The easiest way for the public to see LIDAR imagery in t5he UK is through a website called lidarfinder.com. Originally the Environment Agency arranged the scans, mainly of those areas where it was considered there might be a risk of flooding, but the coverage is now being steadily extended and updated across the whole of the country. We use the LIDAR data in specialised mapping software called QGIS, so we download the data from the DEFRA website.
This is our working copy of the LIDAR plot for the Causeway Farm area of Petersfield, overlaid onto mapping in the QGIS software referred to.