14 Aerial photographs and satellite imagery
COMMENTARY ON 14.1
Aerial photographs and satellite imagery can be used both in the preparation and revision of maps and plans and for the interpretation of site features or earlier uses of a site. Panchromatic aerial photographs, together with infrared, microwave, multispectral and hyperspectral data from a range of aircraft and satellites might be available. Derived value-added-products, which were once seen as experimental, such as ground displacement maps from interferometry (e.g. InSAR [Inferometric Synthetic Aperture Radar]) can also provide practical information on ground conditions. Archives of imagery can be used to "look back in time" and gain a baseline that might otherwise not be available.
Remote sensing imagery is particularly useful for mapping large, undeveloped sites, notably for projects such as dams, power stations, and highways, although they can also be used for mapping smaller sites with the advent of Very High Resolution (VHR) satellite imagery and data from Remotely Piloted Aerial Vehicles (RPAVs), sometimes also referred to as Unmanned Aerial Vehicles (UAVs). A specialist organization in Earth Observation (EO) can recommend the correct type and scale of data for a particular application.
During the desk study phase, remotely sensed imagery can assist in the identification of geological and geomorphological features on or in relation to a site, and in the interpretation of earlier uses of the site. In the UK, a considerable database of aerial photography has been available since the 1940s, particularly in urban areas, and existing sources should be checked in the first instance (see C.7). Although a non-specialist can often extract a substantial amount of relevant information from aerial photography and satellite imagery, trained interpreters should be used if full information is to be obtained. Interpreters should be carefully briefed on the requirements of the interpretation.
NOTE The first stage is often to consult an online repository of imagery, such as NASA's Whirlwind. A professional license is required for commercial work and the imagery in these repositories is not always accurately geo-located. If required, additional imagery from aircraft or satellite can be acquired and processed by a specialist organization. Derived outputs can include orthophotographs, thermal images, ground displacement maps and elevation models.
14.2 Topographical mapping
Accurate, contoured maps can be produced from aerial photographs by competent survey organizations. Ground control should be provided by placing markers on the ground that can be identified from the air and also measured in plan and level on the ground and/or by GPS (Global Positioning Systems) ground control. Methods are available to make a surface ground model from overlapping aerial photographs that contain markers of known positions (easting, nothing and altitude, i.e. x, y and z).
The scale of photography should be properly related to the project.
NOTE 1 Normally, photography is available at the various scales considered appropriate for level map or plan making. The scales 1:500, 1:1 000 and 1:2 500 are most appropriate for investigations of limited areas, whereas scales of 1:5 000 to 1:20 000 are more appropriate for regional studies. Although much map revision of urban areas is carried out by the Ordnance Survey using aerial survey, specially commissioned surveys are not likely to be justifiable for small urban sites. Commercial companies and other organizations (e.g. the UK Government Environment Agency) undertake aerial surveys and are a source of archive data that can be bought off-the-shelf or new surveys commissioned. This is a situation that is changing with the increasing availability of RPAVs and UAVs.
NOTE 2 Digital Elevation Model (DEM) datasets are available from a range of sources and at different resolutions and formats. The choice of dataset depends on the available data and project requirements and can include Digital Terrain Model (DTM), i.e. bare-earth terrain, or Digital Surface Model (DSM), including vegetation canopy and/or buildings. The definitions of DEM, DTM and DSM vary — often according to data originator and end-user requirements. For example, urban modellers typically prefer to work with DSMs that include building structures without vegetation canopy, whilst foresters might need DSM to show the vegetation canopy. Embankments and bridges can be retained or omitted in some DSM datasets depending on the end user requirements and data supplier.
NOTE 3 Aerial photography requires orthorectification to remove distortions and be sufficiently accurate for use in Geographic Information Systems (GIS) and other software. Orthophotograph raster images can be utilized in GIS to be map-ready and can be combined with other digital datasets, including DEM for 3D visualization.
14.3 Identification of features
COMMENTARY ON 14.3
Remotely sensed images can often be used to identify features of engineering significance such as geological lineaments, for example, strata boundaries, faults, soil and rock types, landforms, drainage patterns and unstable ground, including areas of mining disturbance and swallow holes.
Remotely sensed images are particularly useful in the study of extended sites where ground visibility is limited by obstructions or where access is difficult; however, significant features should be checked on the ground whenever possible.
NOTE Although photographs are best studied stereoscopically (in 3D), much data can be obtained from single (mono) photographs (both vertical and oblique). 3D interpretation using mirror-stereoscope or image processing software allows for greater detail to be derived from the imagery, particularly useful for geomorphological analysis or for site history assessment (e.g. assessing stages of quarry backfill and identifying characteristics of made ground). Additional information can be gained by the study of aerial photographs taken at different times when the direction or nature of light differs, or when soil and vegetation has undergone seasonal changes or different tidal conditions pertain at a site.
14.4 Earlier uses of sites
Historical aerial photography should be inspected, as it can yield valuable information about previous construction and earthworks on a site, and is an essential part of the desk study of former industrial, quarrying and waste tip sites.
NOTE The RAF carried out extensive photography in the early 1940s and many areas of the UK have been subject to repeated photography by commercial sources since that date. Annex C gives details of sources of historical aerial photography, and BS 10175 and BS 8576 describe particular features of importance to be noted on potentially contaminated sites.
14.5 Sophisticated techniques
COMMENTARY ON 14.5
The majority of pre-1980s archive air photography is panchromatic (black and white). Naturacolour (true colour RGB) and false colour can offer particular advantages. Multispectral techniques providing simultaneous images in selective wave bands, including the infrared, are also available. One example of the uses of infrared photography is to identify vegetation "distress" caused by landfill gas, chemical contamination, water deficiency or heat. The use of false colour to enhance such features as vegetation is possible when selective wave bands have been used. Thermal aerial photography can be beneficial for distinction between materials, landfill site assessments and for urban modelling. It is stressed that the more sophisticated, and costly, systems require more highly trained interpreters if their full value is to be realized. Rapid advances are being made in the use of sensor systems, in the conventional airborne field (fixed wing and helicopter), unmanned aerial vehicles (UAVs) and satellite imagery.
Remotely sensed data from a range of sources is available or can be flown and should be used where appropriate (for example, high resolution, LiDAR (Light Detecting and Ranging) and multispectral data).
NOTE 1 There is a need to know and understand when and how remotely sensed data was collected. It is not a substitute for field reconnaissance.
NOTE 2 High resolution satellite imagery can be used for acquiring base imagery (orthoimagery) and for DEM generation.
NOTE 3 LiDAR provides detailed 3D data that can be supplied in different formats for use in topographic analysis. LiDAR can be airborne (fixed wing or helicopter) or terrestrial (tripod or mobile) and is widely utilized for earthworks assessment (roads and railways), coastal studies and city modelling. Existing archive LiDAR data is available from commercial and government archive sources or can be commissioned. LiDAR surveys or terrestrial photogrammetry can be used for building surveys (external and internal) and monitoring of structures (facades, embankments, dams, etc.).
NOTE 4 Multispectral and hyperspectral data can be used to measure and map spectral differences of materials (bedrock, soils, vegetation and man-made objects). Radar imagery (satellite and airborne) is used to create DEM datasets at the detail and scale relating to the method. Airborne radar can produce more detailed, "site specific" data than satellite, which covers larger areas. Advanced data processing techniques for radar data, such as Interferometric Synthetic Aperture Radar (InSAR) allows for measuring surface deformation and can be of benefit for a range of applications, such as monitoring coal mining related subsidence and in some geographies is a useful tool for assessing earthquake and volcanic hazards (see Terrafirma8) — Pan-European Ground Motion Hazard Information Service).