Remote Sensing for Archaeological Applications: Management, Documentation and Monitoring

Archaeology is defined as the systematic approach for uncovering the human past and its environment. Archaeology involves not only systematic excavations and surveys, but also analysis of the data collected in the field. In a broader term, archaeology is an interdiscipli‐ nary research. Modern studies in archaeology engage a series of other sciences such as geol‐ ogy, information systems, chemistry, statistics, etc. In recent years, remote sensing has received considerable attention since it can assist archaeological research, along with other sciences, in order to extract valuable information to the researchers based only on non-de‐ structive and non-contact techniques.


Introduction
Archaeology is defined as the systematic approach for uncovering the human past and its environment. Archaeology involves not only systematic excavations and surveys, but also analysis of the data collected in the field. In a broader term, archaeology is an interdisciplinary research. Modern studies in archaeology engage a series of other sciences such as geology, information systems, chemistry, statistics, etc. In recent years, remote sensing has received considerable attention since it can assist archaeological research, along with other sciences, in order to extract valuable information to the researchers based only on non-destructive and non-contact techniques.
Remote sensing is the acquisition of information about an object or phenomenon without making any physical contact with the object (Levin, 1999;Parcak, 2009). According to Sabins (1997), remote sensing involves all the methods that allow the use of electromagnetic radiation in order to identify and detect various phenomena. Based on this definition, many techniques such as satellite remote sensing, aerial photography, geophysical surveys, ground spectroscopy or even terrestrial laser scanners, are considered as remote sensing techniques (Johnson, 2006). Remote sensing has opened up new horizons and possibilities for archaeology. For example, oblique or vertical aerial photography can detect phenomena on the surface associated with subsurface relics, while the use of infrared and thermal electromagnetic radiation can be used in order to detect underground archaeological remains (Bewley et al., 1999;McCauley et al., 1982). Moreover, remote sensing as a non-destructive technique can con-archaeological purposes and concluded that such images can be used as an alternative way in many European archaeological sites, where traditional aerial photography is very limited. Grosse et al., (2005) used CORONA images for mapping geomorphological features in NE Siberia. The combination of ASTER and CORONA images in northern Mesopotamia was also studied by Altaweel (2005).
KVR-100 images from the Russian space program have been available since 1987 and have a high spatial resolution of 2-3 m. Such data are valuable in areas where the landscape has changed dramatically as a result of human activity, such as urban expansion. Even though KVR-100 has been used by several researchers (Fowler and Curtis, 1995;Comfort, 1997), their application is still limited due to their high cost (Parcak, 2009). CORONA and KVR images have been also used to monitor cultural heritage sites in Iran (Kostka, 2002).
Since the 1970s, the launch of new satellite systems coincided with the technological progress of the sensors. In 1972, the Landsat space program was initiated and was followed by the launch of other satellites, including the SPOT satellite in France (Parcak, 2009;Sarris, 2008). The Landsat sensor has been in continuous orbit since 1972 and provides multispectral data for archaeological research. Despite the medium spatial resolution (from 15-80m) Landsat images have a relatively low cost while covering a large area (180 x 180 km) in both the visible -infrared and thermal wavelengths. Landsat images were used to study archaeolandscapes in many archaeological projects and surveys. Vaughn and Crawford (2009) used predictive models in order to identify new areas with potential settlements of Mayans. Barlindhaug et al., (2007) found that Landsat satellite images can be used for monitoring purposes of archaeological sites. Neolithic settlements in Greece were detected using archive Landsat images (Alexakis, 2009;Agapiou et al., 2012a;. Landsat images were also used for monitoring purposes of the surroundings of monuments in Cyprus (Hadjimitsis et al., 2009;. During the 1980's, thermal and radar sensors were also added to satellite sensors (Bewley et al., 1999). In the late 1980's, India launched the IRS 1A, 1B, 1C, 1D and IRS P2 sensors (Tripathi 2005a). Although these data have been used for archaeological purposes in India, such as the identification of the mythic site Dvaraka (Tripathi 2005b) and the observation of Hampi site (Raj et al., 2005), their use is very limited in other regions.
From the 1990's, remote sensing and Geographical Information Systems (GIS) have been used systematically for archaeological research and newer satellites with higher spatial resolution are now available. Indeed, Quickbird, IKONOS, WorldView and GeoEye are capable of providing satellite images with spatial resolution up to 0.5 m.
In addition to the above, hyperspectral images, such as those from EO-HYPERION, have recently made their appearance. Hyperspectral remote sensing analysis is performed over hundreds of narrow bands. The key characteristics of hyperspectral images are its fine spectral and radiometric resolution. Hyperspectral data provides a variety of spectral information, which can be used for the identification of archaeological remains. Alexakis et al., (2009) stated that these new technologies can support the detection of archaeological sites, although it is not always possible to extract a unique archaeological spectral signature due to the heterogeneous presence of vegetation and soil. Lasaponara and Masini (2007a) highlighted the potential benefits of high resolution satellite images in order to detect subsurface monuments through the use of vegetation indices and edge detection techniques. Cavalli et al., (2007) introduced the use of airborne hyperspectral scanner Multispectral Infrared Visible Imaging Spectrometer ( MIVIS) for the detection of subsurface monuments based on spectral anomalies. The study found that the detection of subsurface monuments is possible employing both visible and near infrared part of electromagnetic radiation, and can concurrently detect anomalies using the thermal infrared spectrum. Using QuickBird satellite imagery, Lasaponara and Masini (2007b) Rowlands and Sarris (2007) used airborne hyperspectral scanners (Airborne Thematic Mapper -ATM and Compact Airborne Spectrographic Imager -CASI) and LI-DAR data in order to study the Hellenistic settlement of Itanos in Crete. The data were post-processed using object-oriented analysis. Although the study found several difficulties in relation to the identification of archaeological remains, the continuing use of such methods and applications along with other remote sensing techniques such as geophysical surveys was recommended. In the ancient city Sagalassos, Laet et al., (2007) applied object-oriented techniques and several satellite images (ASTER, SPOT, IKONOS) in order to identify archaeological remains. The results from investigations , in the Piramide Naranjada in Cahuachi (Peru), based on high resolution satellite imagery, geomagnetic surveys and Ground Probing Radar was recently presented by Lasaponara et al., (2011). Currently, several archaeological investigations are carried out using combined remote sensing techniques, such as satellite images, aerial photographs, ground geophysical surveys, and LIDAR measurements. The next section provides an outline of the characteristics of the most important satellite data available today for archaeological research.

Satellite image data
Currently, there is a plethora of satellite images which may be used for supporting archaeological research. However, these images have different resolutions depending on the sensor characteristics. Moreover, many of these satellite systems are nowadays inactive, but their data can be still be used for research. Table 1 summarizes some of the general characteristics of several satellite data regarding spatial, spectral and temporal resolution. As indicated in Table 1, as a result of the space race, satellites have been able to monitor Earth since the 1960's. The Landsat program, which began in 1972 and continues to today, is considered a significant component of remote sensing applications in archaeology.
Prior to the Landsat program, satellite sensors such as CORONA and Zenit 2-8 sensors acquired only panchromatic photographs. These satellites were characterized by non-periodicity; therefore, some areas of archaeological interest may not have been photographed by these sensors. In contrast, the Landsat program has given further capabilities for research since the sensor is able to recover information in the visible, infrared and thermal part of the spectrum. Furthermore, the sun-synchronous orbit of the Landsat satellite enables researchers to study many archaeological sites and monuments in a systematic way. From the beginning of the Landsat program until the end of the century, new multispectral satellite sensors were launched from different countries, including the USA, USSR, France, and Japan, and the spatial resolution of the images was significantly improved. In 1999, the first high-resolution satellite imagery with a spatial resolution of less than 4m was available through the IKONOS space program. The IKONOS satellite was the first satellite operated by a private organization (Space Imaging). In 2000, NASA launched the first hyperspectral receiver, the EO-1 Hyperion, which had the ability to record electromagnetic radiation into 220 different spectral bands.
In the decade that followed, new satellites with higher spatial resolution were available to the scientific community and other countries became actively involved in space technology. Brief descriptions of different satellite sensors characteristics are highlighted in Table 1 and more specific information related to the most popular satellite platforms used in archaeological research are provided in the paragraphs below.

Landsat (MSS / TM / ETM +):
The Landsat program was the result of the combined efforts of NASA and USGS to monitor Earth from space using remote sensing techniques. The first satellite launch was performed in 1972 (Landsat 1) and, since then, another 6 satellites were sent into orbit. According to Parcak (2009), the Landsat satellite program is the most well known satellite used for archaeological purposes due to its relative low cost, global coverage of the satellite data and access to archive data since the 1970's. Landsat satellite images cover an area of about 185 x 185 km. The multispectral bands of the sensor cover both the visible and infrared region of the spectrum while one sensor is able to produce thermal images. The panchromatic band of an ETM+ Landsat image has a spatial resolution of 15 m, while for the rest of the bands the resolution is set to 30 m with the exception of the thermal region (60 m). Landsat data can be obtained via FTP upon request from USGS (http://glovis.usgs.gov/).

CHRIS Proba:
The Proba satellite belongs to a relatively new space program of the European Space Agency (ESA). The Compact High Resolution Imaging Spectrometer (CHRIS) sensor was launched on 2001 and provides hyperspectral images from 63 separate bands at a spatial resolution of 18 m. The objective of the CHRIS Proba is to evaluate new technologies for supporting future satellite sensors (experimental satellite) and to use the data for environmental purposes. The satellite data are acquired in HDF format after approval of ESA committee. A single satellite image covers an area of 13 x 13 km.

IKONOS:
IKONOS is a commercial satellite with high spatial resolution. It was sent into orbit in 1999 and can provide images with spatial resolution up to 1m for panchromatic images and 4m in multispectral bands. The spectral resolution of the sensor extends from the visible to near infrared. Although IKONOS images are widely available to the research community, they are not recorded on a regular basis. The radiometric resolution of the satellite is 11 bit and a single image can cover an area of about 13 x 13 km. IKONOS satellite can provide stereo images in order to support the production of Digital Terrain Models and Surface Terrain Models (DEM, DSM). IKONOS data are available from GeoEye upon request (http://www.satimagingcorp.com/).

QuickBird:
Quickbird is owned by the commercial satellite company DigitalGlobe and was sent into sun-synchronous orbit in 2001. The satellite is currently one of the few satellites with the highest spatial resolution (e.g. OrbView-2, OrbView-3, WorldView-1, WorldView-2 and GeoEye-1). The spatial resolution is up to 0.60 m in the panchromatic wavelength while multispectral bands are acquired at a resolution of 2.4 m. The spectral capacity is equivalent to the IKONOS satellite (visible and near infrared). Moreover, QuickBird images cover a ground area of 16.5 x 16.5 km. QuickBird data is available from DigitalGlobe after request (http://www.digitalglobe.com).
WorldView: WorldView satellite were launched in 2007 (WorldView -1) while a second sensor followed a few years later (WorldView-2). These sensors have a very high spatial resolution (0.5m). The WorldView-2 sensor provides a high resolution panchromatic band and eight Multispectral bands; four standard colors (red, green, blue, and near-infrared) and four new bands (coastal, yellow, red edge, and near-infrared). WorldView data is available from DigitalGlobe upon request (http://www.digitalglobe.com).

GeoEye-1:
GeoEye is the latest high spatial resolution satellite that was sent into space (2008). The spatial resolution of the satellite is 0.41 m and 1.65 m (panchromatic / multispectral bands). The spectral resolution is limited to visible and near infrared wavelength. A Ge-oEye-1 image covers an area of 15 x 15 km.

CORONA:
From 1960 until 1972, the CORONA satellite acquired over 860,000 panchromatic images for US Intelligence. The photographic capsule from the spy satellite was dropped to earth with the help of parachute and then was collected by a special aircraft ( Figure 1). The CORONA images were declassified in 1995, and are now available in digital form upon request.
Remote sensing has been able to assist archaeological research in several ways during the past years, including detection of subsurface remains, monitoring archaeological sites and monuments, archaeolandscapes studies, etc. The next section presents recent developments and applications of several remote sensing techniques for supporting archaeological research. The section includes detection of subsurface remains at the Thessalian plain based on both satellite and ground spectroradiometric measurements. Moreover, remote sensing and GIS analysis as means for monitoring purposes in the area of Cyprus are also examined. Geophysical surveys from various archaeological sites are also presented as well as the results of a study aiming to analyse the impact of atmospheric pollution on archaeological sites. The section ends with discussion of low-altitude airborne systems, as well as 3D laser scanner documentation of cultural heritage site.

Monitoring archaeological sites using satellite remote sensing and GIS analysis
In many areas of the world, cultural heritage sites and visible monuments are monitored mostly with on-site observations, including data collection, periodic observations for archaeological sites and multi-analysis risk assessments. In this way, on-site observations are time consuming and not cost-effective.
Hadjimitsis et al., (2011) highlighted the beneficial integrated use of satellite remote sensing with GIS for exploring the natural and anthropogenic hazard risk of the most significant cultural heritage sites in Cyprus. In order to proceed to overall risk and vulnerability assessment of the archaeological sites in Cyprus due to anthropogenic and natural impact, a risk index was attributed to each different factor such as urban activity, minimum distance of urban activity in the vicinity of an archaeological site, seismic PGA and air pollution impact. They found that, concerning the seismic risk assessment, that significant monuments are located within the spatial limits of the most seismic prone areas in Cyprus. Additionally, regarding sea erosion, the study proved that 50% of the sites examined in the study, are within a distance of only 500 m away from the coastline making them vulnerable to related coastal hazards such as sea water erosion. The creation of buffer zones in GIS environment around CH sites explored the signifi-  (2003) addressed the potential threat that is posed at the historical center of Genoa in the case of failure of the urban drainage system.
This section presents the contribution of remote sensing for monitoring the surroundings of archaeological sites in order the managing authorities or governmental related bodies to be able to conduct a risk assessment analysis of cultural heritage sites in Cyprus. Figure 2 presents some of the most indicative threat parameters. Special attention in this section is given to urban expansion during the past 50 years. Anthropogenic factors, such as urban expansion and air pollution contribute significantly to the destruction of cultural heritage sites. Remote sensing and GIS provide synoptic views of cultural heritage sites which enable policy makers to make appropriate decisions regarding the preservation of cultural heritage sites.

Urban expansion and other hazards as a threat to archaeological sites
In order to study and map urban expansion, a number of significant archaeological sites of Cyprus were examined. These cultural heritage sites are located in the southern coast-al part of the island (from west to east): Tombs of the Kings, Nea Paphos, Palaepaphos (Old Paphos), and Amathus. Urban expansion was monitored with the extensive use of timeseries multispectral and aerial dataset. All images were both geometrically and radiometric corrected in ERDAS Imagine 9.3 software. Moreover, atmospheric correction was also performed based on the Darkest Pixel algorithm (see Hadjimitsis    CORONA satellite images have also indicated the growth of the urban activity around the Amathus archaeological site, including the highway that passes 100 m north of the site (see Figure 5) . Several satellite images were used to examine the threat of urban expansion around the Amathus archaeological site located just east from the outskirts of the city ( Figure 6). The dataset includes Landsat TM/ETM+ images from 1987 until 2009. As shown in Figure 6, urban expansion is clearly observed though interpretation of the images.
It is very important for researchers to understand the dramatic changes that have occurred due to human activity during the last decades. Figure 7 highlights the potential risk of the archaeological sites due to urban expansion of the city of Limassol. Using archive satellite images, the researchers can map this expansion with great detail and accuracy based on classification techniques.   Vegetation indices are also a key parameters that can be used for monitoring dramatic land use changes over time (e.g. urban activities). The Normalized Difference Vegetation Index (NDVI, with range -1 to +1) was applied to the entire dataset (Figure 8). High values of NDVI (indicated with green in Figure 8) are present vegetated areas while low NDVI values (indicated with yellow) are recorded for areas with no vegetation. Since NDVI values may vary throughout time due to the physical phenological changes of the plants, similar periods of Landsat images were examined.
NDVI values were used along with classifications results in order to record NDVI differences in urban classified areas. Figure 9 demonstrates the results of the NDVI difference for the period 1987-2009. Although many areas have indicated no dramatic changes, some other areas represented in yellow and red colour ( Figure 9) indicate dramatic transformation of the initial landscape. Indeed, such changes have been recorded in a very close proximity of the archaeological site of Amathus (see Figure 9 in black square).
Further anthropogenic and natural hazards (e.g. landslides; sea erosion; earthquakes etc) can be monitored in a systematic basis using remote sensing data and GIS spatial analysis. Different studies 2011) have shown the potential of using such methodologies for cultural heritage risk assessment.
Contemporary technological means such as GIS and satellite remote sensing provide efficient and detailed maps of the region of CH sites in the island of Cyprus. This specific study revealed the different kinds of natural and anthropogenic hazards that threaten the preservation of valuable CH sites.

Monitoring air quality in the vicinity of archaeological sites based on satellite and ground measurements
Although cultural heritage sites are documented and preserved, there has been limited monitoring and documentation of how cultural heritage sites are affected by air pollution. Themistocleous et al., (2012a) introduced a new approach for monitoring air pollution near cultural heritage sites. By using a variety of tools, including satellite images, sun-photometers, PM 10 monitors, and laser scanners, the level of air pollution and its effect on cultural heritage sites can be determined. The cultural heritage sites were documented, and using GIS tool, any significant areas of air pollution, including urban areas, industrial areas, and roads were determined. The algorithm proposed by Themistocleous (2011)  Pollution not only deteriorates cultural heritage sites but may also cause irreversible damage that prevents the proper salvation of the monument (Skoulikides, 2000). Therefore, improving air quality is critical for the preservation and maintenance of cultural heritage sites.
The study area was the Limassol Castle, located in the center of Limassol, Cyprus. The study utilized a variety of remote sensing tools to measure air pollution. Landsat TM/ETM+ and MODIS satellite images, as well as the GER 1500 spectro-radiometer, were used to directly or indirectly retrieve AOT, as were ground measurements using the Microtops II handheld sunphotometer and the Cimel sun-photometer located at the Cyprus University of Technology, which is part of the AERONET program. Air particles' measurements were correlated to the AOT levels to verify the level of pollution. Last, visual observation of the Limassol Castle identified the damage caused by air pollution and laser scanning to document and monitor the damage was conducted. Results from satellite remote sensing identified that the centre of Limassol contains high levels of air pollution, with values of AOT higher than other surrounding areas. Determination of AOT measurements using MODIS and Landsat satellite images found that the centre of Limassol, where the Limassol Castle is located, experiences the highest level of AOT values ( Figure 10). A PM 10 /PM 2.5 in situ measurement campaign in the area of the Li-massol Castle found that for the majority of the time periods, the PM 10 readings exceeded the limit value (50 μg/m3), indicating a high level of air pollution in the area. A similar approach was followed for the Paphos town using daily MODIS AOT data. The results have shown that 54% of the measurements for air quality was above the threshold of AOT 300 (AOT 0.300) (see Figure 11). This analysis suggest that cultural heritage sites near the Paphos town (e.g. Nea Paphos, Tombs of the Kings etc) are exposed to air pollutants half the time.

Detection of archaeological sites based on remote sensing techniques
Several Neolithic settlements ("magoules") are located in the Thessalian plain in central Greece. These sites are typically found as low hills raised up to 5-10 m. Alexakis et al., (2009;2011) has recently shown that the detection of several unknown sites is possible based on remote sensing and GIS analysis. The study aimed to combine several types of remote sensing data (e.g. Landsat TM/ETM+, ASTER, Hyperion, IKONOS) and DEM in order to improve the detection of these subsurface remains ( Figure 12). The satellite data were statistically analyzed, together with other environmental parameters, to examine any kind of correlation between environmental, archaeological and satellite data. Moreover, different methods were compared for the detection of Neolithic settlements. The results of the study suggested that the complementary use of different imagery can provide more satisfactory results.
Further to the Alexakis study, Agapiou et al., (2012a) argued that the detection of the settlements is possible based on ground spectroradiometric measurements. Several spectroradiometric measurements have indicated that each magoula has its own spectral characteristics related to its own morphological characteristics. The study has found that the highest peak of the magoula tends to give high NDVI and SR values (similar to the flat -healthy regions) while the slope of the magoula has lowest NDVI and SR values (and for the other indices as well). The extraction of each magoula requires further analysis and enhancement techniques in cases where the spatial resolution of the satellite image used is low. Local histogram enhancements can identify magoules as a small difference of NDVI values at the same parcel ( Figure 13). Similar results were found following the application of the Tasselled Cap algorithm ( Figure  14 to a series of Landsat TM/ETM+ multispectral images. The Tasselled Cap transformation is used to enhance spectral information for Landsat images, and it was specially developed for vegetation studies. The first three bands of the Tasseled Cap algorithm result are characterized as follow: band 1: brightness (measure of soil); band 2: greenness (measure of vegetation); band 3: wetness (interrelationship of soil and canopy moisture).  Phenological studies of crops for the detection of buried archaeological remains were also evaluated  It was found that the phenological cycle of crops for 'archaeological' and 'non archaeological areas' can be used as a "remote" approach in order to locate buried architecture remains. In Figure 15, the phenological cycle of an archaeological site (Almyros II) and the phenological cycle of a healthy site (Site 3) are examined. A small NDVI difference is evident (Case A, Figure 15) which is associated with buried archaeological remains. This is due to the fact that soil over the archaeological remains seems to have a different moisture content compared to their surroundings. Therefore, although there exist similar climate characteristics and crop cultivation techniques, there is a difference in amplitude of the NDVI cycle of the archaeological and nonarchaeological areas.

Documentation of cultural heritage sites using remote sensing techniques, GIS and laser scanning
Contemporary techniques and methods such as computer graphics, virtual reality, multimedia technology, and information technology can be integrated in Web GIS technologies, in order to act as a uniform digital tool for documentation, protection and preservation of cultural heritage (Agapiou et al., 2010c;Hadjimitsis et al., 2006). In order to document and map known archaeological sites and monuments, several techniques may be used, including laser scanning, 3D modelling and GIS. In this section, applications from several monuments in Cyprus are presented.

Integrated use of GIS and remote sensing: a pilot application at the archaeological sites of Paphos
Local cadastral maps were used to support the documentation of cultural heritage sites in the Paphos district, SW Cyprus. In general, each monument may be located in a different sheet /plan; therefore, spatial analysis from such data is a very difficult task.
In order to overcome such limitations, a GIS geodatabase was developed using the ArcGIS 10 software. A GIS system is a computer system (software) that collects, stores, manages, analyzes and visualizes spatial information and upgrades to other information systems. Therefore, GIS can be used as a tool for modelling and analysis of complex research and as a system that supports decision making. Important advantages of GIS include: (a) The data can be stored in a small digital space, (b) Both the storage and the recovery can be achieved with lower costs than traditional ways, (c) Analysis can be carried out much faster, (d) GIS allow synthetic analysis of data without any particular problems and (e) GIS offers the digital environment for an integrated process, where the collection, analysis and decision process are in a continuous flow. The most important advantage of the GIS environment is that it can connect both spatial information (e.g. place, coordinates) along with a-spatial (non-spatial) information (e.g. type of the monument, chronology etc). In this way, further spatial analysis can be performed ( Figure 16).

Terrestrial laser scanning for documentation, reconstruction and cultural heritage structural integrity
Due to their high data acquisition rate, relatively high accuracy and high spatial data density, terrestrial laser scanners are increasingly being used for cultural heritage recording, architectural documentation studies, research of cultural heritage with photogrammetric methods and engineering applications that demand high spatial resolution. Terrestrial laser scanning process can be considered as a part of remote sensing methods. In this section, the results from three different cases studies are presented: Saint Theodore, Tomb I at the Tombs of the Kings and the Church of Kyrikos and Ioulitis For the documentation of the church of Saint Theodore in Idalion village, central Cyprus, the 3D laser scanner Leica C10 was used ( Figure 19). Pre-processing of the point clouds was performed at the Cyclone software. The latest includes the noise removal of the initial point clouds and the registration using scan targets (Agapiou et al., 2010b). A single scan station was also used for the interior of the Tomb I, located at the Tombs of the Kings, archaeological site. The data were then processed at the Cyclone software. The initial point cloud of the Tomb I was further analysed and a 3D mesh was finally created (Figure 20). Using the 3D mesh several sections can be drawn in order to study in detail the architecture of Tomb I.
The third example is the Saint Kirikos and Ioulitis church. Specific laser scans were acquired from the exterior and the interior of the church. The use of laser scanner can provide accurate geometric documentation of such buildings through time and monitor them. One such example is the crack presented in the background of fresco of Christ in the church of Saint Kirikos and Ioulitis ( Figure 21). Repeated accurate measurements of the order of magnitude of a few mm can identify if the crack is gradually increasing in size.
The combination of 3D model and WebGIS applications was also presented by Agapiou et al., (2010c). The "Digital Atlas of Byzantine and Post Byzantines churches" application consists of a WebGIS tool, using the ArcGIS Server software. The WebGIS includes a detail 3D reconstruction of the surrounding area of the monuments using grayscale high resolution orthophotos, a digital elevation model (DEM) of a high accuracy of (± 2m) and 3D digital "light" models of the monuments, produced in Google SketchUp software, after applying topometric methods for measurements. Moreover, the application includes non-spatial information about the monuments, such as relevant bibliography, photos of the interior and exterior of the monuments and also audiovisual data. Finally, this digital tool provides to the end-users a brief, time-stamped, historical background information about the Byzantine and post-Byzantine monuments of central Cyprus (www.byzantinecyprus.com).   Moreover, laser scanners can be used for monitoring purposes as shown by Themistocleous et al., (2012a). In order to monitor the effects of air pollution, the Limassol Castle is being documented every year with the 3D laser scanner. Areas of the castle which show deterioration on the 3D laser scanner will have samples taken to determine the chemical analysis of the surface to establish if the deterioration was caused by air pollution or natural causes. Photographs of the castle were also taken and applied to the 3D laser scanned point cloud.
A direct visual comparison between the intensity of the laser scanner and close range photographs of the cracks in the Limassol Castle indicate that observation of intensity values can indicate the presence -or not-of possible cracks in the monument. (Figures 23 and 24). Similar conclusions can be drawn when laser scanner intensity is compared with ultrasonic measurements.

Geophysical prospection techniques: From mapping to CRM
In terms of ground based remote sensing, there is a wide range of surveying techniques that are focus targeted towards the shallow or medium mapping of the subsurface antiquities or even of the deeper geological layers that may have covered the cultural strata. The various methods, including magnetometry, soil resistance or electromagnetic methods (EM), ground penetrating radar (GPR), and seismic, are based on the measurement of different physical quantities and the complementary application of them (the manifold approach) produces datasets that can match each other and maximize the information content of the geophysical interpretation (Sarris, 2012). Depending on the method and the configuration of the techniques, it is also possible to have different penetration depths and operation in diverse environmental settings (rural or urban) to address a various topics related to the mapping of archaeological sites and archaeoenvironment, the preservation of monuments, e.t.c. Geophysical approaches can be applied in planned excavations, rescue archaeology, archaeolandscape studies, building conservation and cultural resources management (Sarris & Jones 2000).
In general, magnetic techniques using the measurement of the total geo-magnetic field intensity or of the gradient of it or one of its components can be helpful in identifying architectural relics or residues of habitation and workshop activities. Magnetometry techniques have been successfully used to map the relics of settlements and reveal the town planning system. Mud brick foundations of Late Neolithic houses together with pits and other details were recorded around the tell of Sceghalom-Kovácshalom in E. Hungary. The organic material gathered in the pits was responsible for the enhancement of the magnetic susceptibility, resulting in the good registration of the pits from the measurements of the vertical magnetic gradient. Even stronger was the magnetic signature of the foundations of the fired daub foundations and walls of the farmsteads that were recorded as thermal targets, but which at the same time were not able to register to the GPR measurements due to the high conductiv-ity of the soils (Monahan & Sarris 2011, Sarris, 2012) ( Figure 25). The same type of thermal signature is shown in the investigation of workshops and kilns belonging to different chronological periods. In other cases, such as in Sikyon, Peloponnese (S. Greece), the difference of the construction materials of the structural remains of the Hellenistic/Roman city in terms of the magnetic minerals they contained was responsible for providing an accurate plan of the ancient city. Due to the soil conditions and the preservation of the site, the magnetometry survey specified the street layout and the city quarters, tracing numerous monuments inside and outside the agora limits, including temples, porticoes, a basilica, street lines, houses and industrial installations Gourley et al., 2008).
Similar is the operation of the EM and soil resistance methods, which, together with the GPR, are considered ideal to resolve features related to structural remains, champers, voids and tombs. These methods are considered to be active measuring techniques. The particular methodology has been used successfully in resolving the foundations of buildings, road networks, and funeral residues. Of particular interest is their ability to operate in different frequencies (EM and GPR) or configurations (soil resistance) allowing a larger or smaller penetration depth. In this way, it is possible to provide valuable information regarding the subsurface stratigraphy. For example, the decrease of the GPR antenna frequency can provide a larger penetration to the soil strata. In addition, the multiple reflections of the GPR electromagnetic signals originating from adjacent (usually parallel) transect can create images of the subsurface layers (of various widths) by increasing depth (depth slices) (Figure 25). In a similar way, vertical electric soundings measure resistivity variations with depth by increasing gradually current electrode separation while the center of the electrode configuration, remains stationary. Based on the same principle, the electrical resistivity tomography provides information for both the lateral and vertical variations in the resistivity of the soil and, based on 2D or 3D inversion algorithms; it can produce a 3D reconstruction model of the subsurface , Sarris 2008).
The use of the EM, electrical resistivity tomography (ERT) and seismic techniques is more appropriate for the deeper mapping and their employment is usually applied in archaeolandscape studies. This was the case of Priniatikos Pyrgos, where the integrated application of ERT and seismic tomography techniques processed by 3D inversion algorithms were capable to contribute to the archaeoenvironmental reconstruction of the Priniatikos Pyrgos at Istron, E. Crete, providing indications regarding the ancient harbor of the nearby settlement (Shahrukh et al 2012). The particular methods were the only solution to provide information about the deposits that exist in the coastal area of Priniatikos Pyrgos: carstic formations of medium to high permeability and alluvium deposits of variable permeability, probably originating by past landslide episodes and periodic flooding of the Istron River, have covered the ancient harbour at depths varying from 20-40m below the current surface. Similarly, electromagnetic and soil resistance measurements revealed the movement of the older Istron River branches, which appeared to be directed to the sea from both sides of the settlement, leaving probably a small path to the mainland from the SW direction. The above results were also supported by the sedimentologi-cal analyses and OSL dating of cores taken from the region and the use of geophysical techniques in the study of the dynamics of the landscape evolution (Sarris et al 2012) (Figure 26).
GPR and soil resistance techniques (including ERT) also can be used in an urbanized context in contrast to the rest of the geophysical approaches (Sarris 2008;Linford 2006). Due to a high level of ambient noise from the background anthropogenic activities and the high disturbance of the upper soil layers, the particular techniques can be adapted to resolve a number of issues in question Papadopoulos et al., 2009). Thus, the above methodology can be used during the course of private construction activities but also for even larger civil construction works that can deal with highways, squares, pedestrian roads, etc. In a number of instances they can even be applied within historical structures and monuments to conclude on the integrity status of the monuments. The geophysical techniques can also contribute to a more generalized risk assessment model, since it can provide information for the tectonic regime and the classification of geological strata either in terms of their resistivity (ERT), velocity of propagation of acoustical waves (seismic techniques) or even the seismic amplification factor (micro-noise horizontal to vertical spectral ratio -HVSR) (Sarris et al., 2010).  Although current trends have emphasized the fast reconnaissance of the archaeological sites through multi-sensor, multi-electrode or multi-antenna systems, the manifold approach, which is the amalgamation of multiple geophysical techniques, as well as the fusion of the geophysical data with other types of remote sensing techniques, such as satellite imagery, LIDAR or laser scanning and orthophotos aiming towards a better and more holistic visualization of the area and a better reconstruction of the underground monuments will continue to be of crucial importance in the geophysical prospection of archaeological context (Sarris 2012).

Low altitude systems for supporting archaeological investigations
Further to satellite and ground investigations, research has indicated the need for a low altitude airborne imaging systems in order to support archaeological research. This is due to the fact that such systems of low cost, with a stable platform for imaging sensors and have the ability to lift a payload equivalent to sensor equipment (Patterson & Brescia, 2008;Voerhoeven, 2009;Kemper, 2012;Nebiker et al., 2008;Bento, 2008;Georgopoulos, 1982;Hailey, 2005). In this study, several technologies were merged to create an innovative low altitude airborne system supporting remote sensing and photogrammetric applications, which includes the ability to conduct spectroscopy and aerial photography using a helium filled balloon. The complete low altitude airborne system is shown in Figure 27. A helium-filled balloon with a 3 m. diameter was used which was able to be raised to a height up to 200 m with a payload of up to 6kg. The Spectra Vista GER 1500 spectroradiometer was attached to the aerial platform and operated remotely. The balloon was raised to varying heights and spectroradiometric measurements were taken of the same target at different elevations. Concurrent to the spectroradiometric measurements, aerial photographs were taken using two digital cameras, one with infrared filter. The integration of the various techniques was used in order to detect subsurface archaeological remains by examining ground anomalies identified through spectral signatures. Previous campaigns in Cyprus found that field spectroscopy can support the detection of archaeological crop marks based on the retrieved spectral signatures over agricultural areas which are characterized as archeological areas (see Agapiou and Hadjimitsis 2011). Possible identification of subsurface archaeological remains is based on spectral signatures anomalies. Such anomalies are observed in crops when the vegetation is under stress due to subsurface relics. Therefore, spectral signatures anomalies are expected in the red and VNIR part of the spectrum.
The low altitude airborne imaging system was tested at the Agricultural Research Institute in Paphos, Cyprus, where a simulated archaeological test field was constructed. Spectroradiometric measurements and photographs in the visible and infrared range were taken over the target area. Preliminary results found that there were no significant differences in the spectral signatures in the visible range, while there was a significant difference among the spectral signatures in the NIR range as the balloon was moving up-wards ( Figure 28). The study found that the spectral signature of the target can changed as a function of altitude, with higher reflectance indicated as the elevation increased.

Conclusions
Remote sensing can contribute in several ways to archaeological research. This chapter presents some results from different cases studies in Cyprus, Greece and Hungary using several techniques of remote sensing, including satellite images, archive aerial images, geophysical surveys, 3D terrestrial laser scanners, ground spectroscopy, atmospheric pollution, WebGIS and GIS analysis for monitoring purposes.
The results have shown the potential use of satellite remote sensing and ground spectroscopy for the identification of buried archaeological remains through crop marks. Moreover, monitoring archaeological sites and risk assessment can be performed for several threats including urban expansion and air pollution. As demonstrated in this chapter, a dramatic land use change has taken place in several archaeological sites during the last decades. Such investigations are very important for studying archaeolandscapes since can provide valuable for information for areas that are nowadays vanished. Furthermore, the potential use of ground geophysical surveys for the detection of subsurface remains was also demonstrated through several applications in Greece and Hungary, was also demonstrated. Documentation, mapping. 3D modelling and WebGIS applications for archaeological sites and monuments are also demonstrated in this chapter.