Editing Shadow marks (section)

== Light and shadow ==
The formation of shadow marks relies on the basic principles of physics and optics, especially how light interacts with differences in the earth's [[surface topography]].<ref name=":0" /> When sunlight strikes an uneven surface at a low angle, it casts long shadows that contrast high and low areas more.<ref name=":0" /> This optical effect is integral to observing slight [[Archaeology|archaeological]] features since it permits a visualization of the ground’s irregularities that otherwise would not be apparent under direct, overhead illumination conditions.<ref name=":1" />
[[File:Aerial_photograph_of_Maiden_Castle_from_the_west,_1937.jpg|thumb|This aerial picture shows [[Maiden Castle, Dorset|Maiden Castle]], a prehistoric site located in Dorset, which was taken from the west in 1937. The low angle of the light creates long shadows across the earthworks, and the long shadows highlight the fort's intricate ramparts and ditches. This photo demonstrates shadow marks in aerial archaeology, where slight variations are made visible through the effects of shadow and light on the landscape.]]
The angle of solar illumination is the primary agent for creating shadow marks. In the morning and the evening (late afternoon), the sun is low in the sky, and shadows are longer, meaning even small ground undulations are apparent.<ref name=":0" /> Because of this, archaeologists will conduct aerial surveys during this time to best use shadow marks. However, the effectiveness of shadow marks depends upon the [[latitude]], season, and atmosphere, each of which can affect the scattering of light and shadow clarity.<ref name=":0" />

[[Albedo]], or [[surface reflectance]], is also a key factor in photographs or in other records of shadow marks. Different materials have different albedos with regard to both absorption and reflection of incoming light and thus vary in their ability to create contrast, which may either increase or obscure the shadows.<ref name=":6" /> For example, when a buried stone structure or wall has changed the physical or chemical properties of the surrounding soil—whether that is by compacting it or influencing [[moisture]] retention—the area above is likely to reflect sunlight differently than surrounding, unaltered soils.<ref name=":4">{{Cite journal |last=Verhoeven |first=Geert |date=2017-09-14 |title=Are We There Yet? A Review and Assessment of Archaeological Passive Airborne Optical Imaging Approaches in the Light of Landscape Archaeology |journal=Geosciences |language=en |volume=7 |issue=3 |pages=3–5 |doi=10.3390/geosciences7030086 |doi-access=free |bibcode=2017Geosc...7...86V |issn=2076-3263}}</ref> The variations in surface reflectance, or albedo, sometimes will be observable in aerial photographs as differences in tonal contrast or shadow intensity.<ref name=":4" /> A ploughed field is an example of an exposure that can reveal faint outlines of a buried foundation in the soil due to slight differences in shadow or color even if the foundation remains hidden underground.<ref name=":4" /> Furthermore, the [[moisture content]] may reflect light differently, thus making shadow marks more or less detectable depending on prior weather conditions.<ref name=":1" />