Introduction to Remote Sensing Principles
Remote sensing, in its simplest form, is the acquisition of information about an object or phenomenon without making physical contact with it. In archaeology, this translates to using various technologies to 'see' what lies beneath the surface of the earth without excavation. This non-destructive approach is invaluable for preserving archaeological sites and gaining a broader understanding of past landscapes.
The fundamental principle behind remote sensing is the interaction of electromagnetic radiation (EMR) with the Earth's surface. Different materials reflect, absorb, and emit EMR in unique ways. By measuring and analysing these variations, we can infer information about the composition and structure of the ground, potentially revealing buried archaeological features.
Several factors influence the effectiveness of remote sensing techniques, including:
Sensor Resolution: This refers to the ability of a sensor to distinguish between closely spaced objects. Higher resolution allows for more detailed imagery.
Spectral Resolution: This describes the sensor's ability to differentiate between different wavelengths of EMR. Broader spectral ranges can reveal subtle variations in surface composition.
Atmospheric Conditions: Clouds, haze, and other atmospheric elements can interfere with EMR, reducing the quality of remotely sensed data. Atmospheric correction techniques are often necessary.
Ground Truthing: This involves visiting the site to verify the findings of remote sensing analysis. Ground truthing is essential for validating interpretations and improving the accuracy of future surveys. Learn more about Archeologist and our commitment to accurate archaeological analysis.
Satellite Imagery Analysis for Site Detection
Satellite imagery offers a cost-effective and wide-ranging method for identifying potential archaeological sites. Satellites equipped with various sensors capture images of the Earth's surface in different spectral bands, including visible, infrared, and radar. These images can be analysed to detect subtle variations in vegetation, soil moisture, and surface topography that may indicate the presence of buried features.
Several types of satellite imagery are commonly used in archaeological prospection:
Optical Imagery: This includes imagery from satellites like Landsat and Sentinel. These satellites capture data in the visible and infrared portions of the electromagnetic spectrum. Analysing these images can reveal crop marks (variations in crop growth caused by buried features), soil marks (differences in soil colour and texture), and shadow marks (subtle topographic variations).
Example: A buried wall can restrict root growth, leading to stunted vegetation directly above it. This difference in vegetation health is visible in optical imagery as a 'crop mark'.
Radar Imagery: Radar (Radio Detection and Ranging) satellites emit microwave radiation and measure the amount of energy reflected back to the sensor. Radar imagery is particularly useful in areas with dense vegetation cover, as microwaves can penetrate through foliage to reveal the underlying ground surface. Radar is also sensitive to surface roughness and moisture content, which can be indicative of buried features.
Example: A buried ditch will retain more moisture than the surrounding soil, leading to a stronger radar return.
Multispectral Imagery: These sensors capture data across multiple, specific bands of the electromagnetic spectrum. This allows for a more detailed analysis of surface materials and vegetation health. Advanced image processing techniques, such as band ratios and principal component analysis, can be used to enhance subtle features and identify areas of archaeological interest.
Example: Analysing the spectral reflectance of vegetation can reveal areas of stress caused by buried archaeological features. Our services include expert analysis of multispectral imagery.
LiDAR Technology and Applications in Archaeology
LiDAR (Light Detection and Ranging) is a remote sensing technology that uses laser light to create highly detailed three-dimensional models of the Earth's surface. LiDAR systems emit pulses of laser light and measure the time it takes for the light to return to the sensor. This information is used to calculate the distance to the ground and create a point cloud representing the terrain.
LiDAR is particularly valuable in archaeology for:
Mapping Terrain: LiDAR can penetrate dense vegetation cover to reveal subtle topographic features that are not visible from the ground or in traditional aerial photographs. This is especially useful for identifying earthworks, such as mounds, ditches, and terraces.
Example: LiDAR data can reveal the outlines of ancient settlements hidden beneath dense forest canopies.
Creating Digital Elevation Models (DEMs): DEMs are digital representations of the terrain surface. They can be used to visualise the landscape in three dimensions, identify subtle topographic variations, and create contour maps. DEMs are essential for archaeological site mapping and analysis.
Identifying Archaeological Features: LiDAR data can be used to identify a wide range of archaeological features, including building foundations, roads, and agricultural terraces. The high resolution of LiDAR data allows for the detection of even very small features.
Change Detection: By comparing LiDAR data collected at different times, it is possible to detect changes in the landscape, such as erosion, deforestation, and the construction of new features. This information can be used to monitor the condition of archaeological sites and identify areas that are at risk of damage.
Example: Repeated LiDAR surveys can track the erosion of a coastal archaeological site over time.
LiDAR data is typically processed using specialised software to remove vegetation and other unwanted features. The resulting 'bare earth' model provides a clear view of the underlying terrain, revealing subtle archaeological features that would otherwise be obscured. If you have frequently asked questions about LiDAR, we can help.
Thermal Imaging and its Uses in Identifying Subsurface Features
Thermal imaging, also known as infrared thermography, is a remote sensing technique that measures the thermal radiation emitted by objects. All objects emit thermal radiation, and the amount of radiation emitted depends on the object's temperature and emissivity (its ability to emit thermal energy). Thermal imaging cameras detect this radiation and convert it into an image, where different colours represent different temperatures.
In archaeology, thermal imaging can be used to identify subsurface features by detecting variations in ground temperature. Buried features, such as walls, ditches, and foundations, can affect the thermal properties of the soil above them, leading to temperature differences that can be detected by thermal imaging cameras.
How it Works: Buried features can alter the soil's thermal conductivity, moisture content, and density. For example, a buried stone wall will conduct heat differently than the surrounding soil, leading to a temperature difference. During the day, the wall may heat up more slowly than the surrounding soil, while at night it may cool down more slowly. These temperature differences can be detected by thermal imaging cameras.
Optimal Conditions: The best time to conduct thermal imaging surveys is typically at dawn or dusk, when the temperature differences between buried features and the surrounding soil are most pronounced. Weather conditions can also affect the results of thermal imaging surveys. Clear, calm weather is ideal, as clouds and wind can interfere with thermal radiation.
Applications: Thermal imaging can be used to identify a wide range of subsurface features, including building foundations, roads, and tombs. It is particularly useful for detecting features that are made of stone or brick, as these materials have different thermal properties than soil.
Example: Thermal imaging can be used to map the layout of a buried Roman villa by detecting the temperature differences between the walls and the surrounding soil.
Integrating Remote Sensing Data with GIS
Geographic Information Systems (GIS) are powerful tools for managing, analysing, and visualising spatial data. Integrating remote sensing data with GIS allows archaeologists to combine different types of data, such as satellite imagery, LiDAR data, and thermal imagery, with other spatial information, such as topographic maps, geological data, and archaeological site records. This integration can provide a more comprehensive understanding of the archaeological landscape and facilitate the identification of new sites.
Data Integration: GIS allows for the integration of different types of remote sensing data into a single platform. This allows archaeologists to compare and analyse data from different sources, such as satellite imagery and LiDAR, to identify areas of archaeological interest. For example, satellite imagery can be used to identify potential crop marks, while LiDAR data can be used to map the topography of the area. By combining these two types of data in GIS, archaeologists can gain a more complete understanding of the archaeological landscape.
Spatial Analysis: GIS provides a range of spatial analysis tools that can be used to analyse remote sensing data. These tools can be used to identify patterns and relationships in the data, such as the distribution of archaeological sites in relation to water sources or topographic features. Spatial analysis can also be used to create predictive models of archaeological site location.
Visualisation: GIS allows for the creation of maps and other visualisations of remote sensing data. These visualisations can be used to communicate the results of archaeological research to a wider audience. GIS maps can also be used to manage and protect archaeological sites.
Example: A GIS can be used to overlay LiDAR data with historical maps to identify areas where known archaeological sites may have been destroyed or damaged. Archeologist uses GIS to provide the best possible archaeological insights.
By integrating remote sensing data with GIS, archaeologists can gain a more comprehensive understanding of the archaeological landscape and improve their ability to identify, manage, and protect archaeological sites.