Aerial survey is one of the most useful methods that an archaeologist uses for finding archaeological sites. The archaeologist goes up in a plane to see the land below. You can see many things up above that you might not notice walking around on the ground. The archaeologist takes photographs to help show what there might be under the ground.
There are two types of aerial photos called oblique photos and vertical photos.
Vertical means that the photo is taken from directly above the site.
Aerial photographs help show up differences in the land where that land has been disturbed by people, for instance when they have made buildings, ditches, fields or pits.
Some archaeological features, like burial mounds or the remains of castles, stick up out of the ground and are easier to spot. They can create shadows on the ground, which show up on photos, if you take the photo from the right angle.
An oblique photo of Bishopton motte and bailey
© Tees Archaeology
Sometimes archaeological features that are below the ground can also be seen from above and in aerial photos. When archaeological features rot away over time, they leave little traces of themselves behind in the ground. Often this makes the soil a different colour from the rest of the soil around where the feature was. Also, sometimes this can make grass or crops in fields directly above where the feature was grow differently from the rest of the field.
A vertical photo of the Ringlemere Cup findspot - can you see the dark ring?
© Kent County Council
When you look at this from above or in aerial photos, you can see where the grass or crop has grown differently as it can show up as a different colour. The reason for this is that plants will grow best where they can put their roots down nice and deeply as they will get more nutrients and a better water supply. Where plants can't put down strong roots, they will grow less well. So, if a crop is growing over an underground ditch, it will put its roots down a long way and grow thick and strong. This will make the ditch show up as a darker line on an aerial photograph. Or if a crop is growing over the remains of an old building, the plants might not be able to put their roots down that deeply as the building is in the way and the plants won’t grow so well. If a crop is being grown by a farmer on the site, it is best to take the aerial photograph when the crop is fully grown to help show up the crop differences better.
What affects the aerial photos?
- The time of day and the time of year affect an aerial photograph and how good it is, in other words, what it shows up on the photo.
- Oblique photos work best if there are some shadows but not when the shadows are too long, so early morning and early evening are good times to take these.
- For vertical photos, good strong sunlight is best and shadows don't really matter too much so they are usually taken at mid day.
- If a crop is being grown by a farmer on the site, it is best to take the aerial photograph when the crop is fully grown.