Seeing the ground from above
Archaeologists have used the view from above for a century or more. Early aerial photographs taken from aircraft showed crop marks and shadows that revealed buried walls and ditches. Today two broad approaches dominate: satellite imagery, which is photography or other sensing from orbit, and LiDAR, an active laser method usually carried on aircraft, helicopters or drones. Both let researchers survey areas too large, too overgrown or too remote to walk systematically.
Neither technique finds sites on its own. They produce maps and images that must be interpreted and then checked on the ground, and their main value is in directing fieldwork and showing patterns across whole landscapes.
How LiDAR works
LiDAR stands for light detection and ranging. An instrument fires a very large number of laser pulses toward the surface and measures how long each takes to return. Combined with precise positioning from GPS and motion sensors, this gives the three-dimensional location of every point struck.
The key advantage in forested areas is that a single pulse can produce several returns. Some bounce off leaves and branches, while others pass through gaps and reach the ground. Software separates the ground returns from the rest and builds a model of the bare surface, effectively digitally removing the vegetation. The resulting maps, often shaded to emphasise low relief, can show house platforms, terraces, causeways, canals and defensive walls that are invisible from the air in ordinary photographs and difficult to see even when standing on them.
Forest cities and field systems
The best-known results come from tropical forests. In Mesoamerica, large LiDAR surveys in northern Guatemala, including a project reported in 2018 that covered roughly 2,000 square kilometres, mapped tens of thousands of structures and a dense network of causeways, fortifications and agricultural terraces. The results suggested that lowland Maya settlement was more extensive and more densely populated than many earlier estimates, although how many of the structures were occupied at any one time remains a matter for further study. The scans sit alongside older fieldwork at places such as the Tikal rainforest ruins, where early mapping had already shown how large a city could be. They also add weight to discussions of Maya trade networks and the roads and causeways that linked communities.
In Southeast Asia, LiDAR surveys around Angkor revealed a more extensive urban pattern than earlier maps, including low-density settlement, roads and water-management features beyond the famous temples. This adds detail to the understanding of the Angkor city plan, and showed planned settlements on the Phnom Kulen plateau as well. In the Amazon basin and in the Upano Valley of Ecuador, laser scanning has recorded earthworks, platforms and road systems that point to substantial pre-Columbian populations, though dating and interpretation come from later excavation.
LiDAR is not limited to forests. In open landscapes, such as parts of Britain and Europe, it shows faint earthworks, field boundaries, Roman roads and burial mounds that have been flattened by ploughing and are hard to see otherwise.
Satellite imagery
Satellite sources vary widely. Declassified American reconnaissance photographs from the 1960s and 1970s, known as CORONA imagery, have been valuable in the Near East, where they show the landscape before large-scale modern irrigation, urban growth and farming destroyed many features. Researchers using them in northern Mesopotamia have traced sunken tracks, often called hollow ways, which radiate from ancient settlements and indicate how people moved herds and goods. This approach has contributed to wider Mesopotamian archaeology by showing that sites were part of larger networks.
Modern commercial satellites provide high-resolution colour images, and multispectral sensors record wavelengths the eye cannot see. Differences in vegetation health, soil moisture and soil colour can reveal buried features. Radar satellites can pick up subtle surface differences and, in very dry conditions, may detect some structures just below the surface of sand. Satellite imagery is also used to monitor looting and damage to known sites, and to follow coastal erosion.
In deserts, images and drone photography have also been important in the study of the Nazca Lines, where researchers have mapped additional figures, including ones found with the help of computer analysis. The Palpa hillside geoglyphs are similar examples of features that are easiest to understand from above.
What these methods cannot do
A LiDAR map shows shape, not meaning. A mound could be a house platform, a tomb or a natural rise, and a rectangle could belong to any period. Without excavation or surface collection, the age of features is unknown, and features from different centuries can appear flattened together in a single image. Archaeologists therefore check selected locations on foot, a process called ground-truthing, and may excavate to test what they see.
There are technical limits too. Very dense vegetation can reduce the number of ground returns, steep slopes can produce errors, and the resolution of the data affects what can be recognised. Interpretation also involves judgement, and published counts of structures are estimates rather than confirmed buildings.
There are ethical and practical concerns. Publishing precise maps of unprotected sites could expose them to looters, so some projects restrict coordinates. The work also involves local communities and authorities, who have a stake in how landscapes are studied and presented.
A tool for asking better questions
The main effect of remote sensing has been to change the scale of questions. Instead of studying a single famous city in isolation, archaeologists can ask how a whole region was settled, farmed and connected. The answers come more slowly, from excavation, dating and analysis. LiDAR and satellite survey provide the map, and the rest of archaeology fills in what happened there.