Airborne laser scanning

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Airborne laser scanner on the helicopter

Laser scanning (also known as LiDAR = Light Detection And Ranging) is a remote sensing method . It uses the properties of the scattered light to characterize distant objects. With Airborne Laser Scanning (ALS) , the scanning unit is attached to or on a flying object (usually on / on an airplane or helicopter). The earth's surface is scanned using a laser beam. The distance between the recorded point on the earth's surface and the sensor is determined. The surface models created from the height information obtained are now used in many specialist areas.

history

The beginnings of ALS can be found in the USA and Canada. They go back to the 1970s. At that time it was already known that airborne LiDAR systems can measure the distance between the aircraft and the ground surface with an accuracy of less than one meter. However, airborne laser elevation measurements have not been used for topographical mapping for two reasons. One of the problems was that the vertical position of the flight system and the horizontal position of the light cone on the ground surface could not be recorded with the required accuracy. This problem was solved by GPS in the late 1980s. By using a differential global positioning system (DGPS) , the horizontal and vertical position of the scanner could be determined with centimeter accuracy. Laser scanning from the air has also become feasible thanks to the technical advancement of the laser. Pulse lasers were now able to emit light in the near infrared wavelength range, which the receiver could clearly register again after scattering and reflection on the surface of the ground. The high geometrical accuracy of the method and the potential that it represents for the creation of digital elevation models was proven by tests at the University of Stuttgart between 1988 and 1993. Thanks to important knowledge about the system parameters, the devices and the method have developed rapidly since then. Nowadays, ALS has become indispensable in many areas and is used in numerous disciplines.

Components

An aircraft-based laser scanning system consists of at least the following components:

  • Laser distance meter: this contains the laser, the transmitter for the laser beam, signal receiver for the reflected beam, amplifier and timer;
  • a system for georeferencing: GPS receiver and inertial navigation system (INS)
  • Storage medium for the laser, GPS, INS data and possible image data;

The systems can optionally be combined with other sensors such as digital cameras and video cameras in order to record image data in addition to the height information. These components are attached to the aircraft with a bracket. The scope of delivery of a laser scanning system also often includes the software for flight planning and for evaluating the raw data (from the laser scanner and GPS). Parameters such as measuring rate, scan angle and frequency can be set on the respective scanning system. Together with variable flight altitudes and flight speeds, the required data density can be tailored to different areas of application.

functionality

A laser scanner is an active system that emits light pulses that are reflected from object points. The object point must be visible from at least one direction. Diffuse reflection on the surface is a prerequisite. This technology works independently of the sun lighting. The use of laser scanning systems enables large amounts of 3D information to be obtained about the earth's surface at very fast acquisition rates. Depending on the recording of the reflection, a distinction is made between two types of sensors: `Discrete Echo´ sensors, and` Full-waveform systems´. The former detect only a small number of echoes, while the latter are able to register the entire time-dependent variation in the received signal strength. This means that additional parameters, such as the signal amplitude or the echo width, can be derived from `full waveform 'data. The investigation area is flown in individual, overlapping flight strips. These usually have a length of a few kilometers and a width of several hundred meters, depending on the altitude above ground and the maximum scan angle.

With pulse lasers, the distance is measured using the transit time measurement: 

Distanz = Laufzeit/2 * Lichtgeschwindigkeit.

The data points are oriented using differential GPS (DGPS) and INS. The GPS system provides the absolute position of the sensor, the INS the position of the platform (roll, pitch and yaw angles).

Important parameters

  • Point density: The point density depends on the flight altitude and the properties of the scanning system, e.g. B. Speed ​​of the platform, field of view, scanning frequency.
  • Laser footprint: Illuminated area on the earth's surface, resulting from the beam divergence. In addition to the beam divergence, the footprint size is directly dependent on the flight altitude.
  • Signal amplitude: Parameter for the strength of the recorded echo, depending on the target area and the degree of reflection of the surface.

Data processing

The result of a scan flight is a three-dimensional point cloud that is first referenced. This step is necessary to transform the acquired data from one coordinate system (which can be an internal, instrument-defined system) to another. The data is then processed further in order to generate different object models. Two important examples are, on the one hand, the digital surface model (DOM), which provides information about the nature of the earth's surface, including all objects located on it, such as vegetation or buildings, and, on the other hand, the digital terrain model (DGM), which represents the bare terrain surface. Such terrain models represent an important basis for topographical analyzes. In order to distinguish terrain points from non-terrain points, filter methods are necessary. These can be roughly divided into three main groups: a) Based on mathematical morphology, b) Based on progressive compression of a triangular network, and c) Based on linear prediction and hierarchical robust interpolation.

Advantages and limitations

Despite some limitations, ALS technology has proven to be an effective method for creating digital terrain models. The main advantages are the high density of measuring points and measuring accuracy, rapid data acquisition and the penetration of vegetation. The advantages over other remote sensing methods can be seen due to the high density of measurement points, for example in areas of application in which the surfaces have a low roughness, such as ice and snow surfaces, sand, swamps and wetlands; also in the investigation of vegetation, since the vegetation height can be calculated by recording the first and last reflected signal. The ALS enables the mapping and, above all, the automatic detection of small objects such as power lines. Since laser scanners are active systems, they do not depend on sunlight compared to passive methods such as photogrammetry, but can theoretically be used 24 hours a day to collect data. However, the laser beam can also hit obstacles such as clouds or fog and vegetation, which it only penetrates to a limited extent. While the emitted laser beams can reach the surface of the ground through deciduous forests, especially in winter, dense coniferous forests or multi-storey rainforests show the limits of the ALS. Compared to airborne photogrammetry, in which the carrier systems can range from small balloons to geostationary satellites, airborne laser scanning systems are installed on drones, helicopters and aircraft. The minimum and maximum flight altitude is limited due to the safety for people on the ground, especially the possible health-damaging effects on the eyes, and due to the energy of the laser and the power of the sensor. This can be within 20 to 6000 m, but mostly between 200 and 1000 m.

Sources of error

The vertical and horizontal accuracy of the altitude data obtained by ALS is 0.05 to 0.2 m and 0.2 to 1 m, respectively. Causes of errors can be found in the calibration of GPS, INS and scanner data. Position errors on steeply sloping surfaces can also lead to incorrect information on the height. Another frequent source of error is the multiple reflection of the laser beam, for example if the laser is deflected again from an object after being reflected on the floor surface before it reaches the sensor. The accuracy of the measured values ​​can also be influenced by errors in data processing, such as when transforming into a different coordinate system.

application areas

The technical progress of the past few years has made "small-footprint full-waveform laser scanning" (FWF), laser scanners from which additional parameters such as signal amplitude and echo width can be derived, more accessible and applicable. As a result, this airborne laser scanning (ALS) technology has since been used in many different areas of application. Regardless of the area of ​​application, the target objects are classified according to their characteristics (signal amplitude, height, etc.) so that they can be defined and analyzed over large areas within a short time.

Vegetation-geographical analyzes

ALS data acquisition with an airplane over the Brazilian rainforest.

Since 2004 FWF ALS systems are in the forest sciences applied to vegetation to quantify and thus to model the dynamic change. The specific information of individual trees is important for sustainable forest management, which is obtained across the board using this method and with the help of which one can examine the relationship between vegetation and climate , but also gain scientific knowledge about individual tree species. Due to the comparatively easy and fast data acquisition and processing, the application of FWF ALS also enables biomass and its changes to be evaluated and evaluated over a large area. Areas can be classified according to their dominant plant species, which in turn provides information about succession , microhabitats , and the state and functioning of ecosystems . This is why this method has become an important part of the monitoring of nature conservation projects.

Analysis of building surfaces

Buildings can be extracted automatically using ALS data. First of all, buildings are classified and then, if necessary, the geometry of buildings is reconstructed. The bigger a building, the more reliable and high-resolution its geometry can be derived from the data. The information obtained is also used in urban research . Since the method enables quick and uncomplicated use, it is used for quick mapping and damage assessment after natural disasters .

Analysis of solar potential

Photovoltaic system on the roof of a Viennese school.

In recent years there has been an increasing need for inexpensive generation of sustainable energy for private households. With the help of airborne laser scanning, the solar potential of roofs can be determined. The roof surfaces are extracted from vegetation and other building surfaces. The extracted roof surfaces can then be analyzed with regard to their orientation and inclination, so that suitable areas for photovoltaic systems can be determined so that they can be used as effectively as possible.

Analysis in water management

In the field of water management and hydrology , ALS is primarily used for the creation of specific basic data that enable a significant improvement in hydrological products. This ranges from watercourse terrain models, hazard zone planning , designation of flood areas, to various water biological issues. Compared to runoff modeling that is not based on ALS-based data, ALS data can be used to achieve greater accuracy. However, it is always important to ensure that it is up-to-date and that the terrain has changed (e.g. flood protection measures, pouring, etc.). When measuring entire rivers, ALS has largely replaced traditional measuring methods such as terrestrial or photogrammetric techniques. The advantages lie in the high degree of automation of large-area recordings, a uniform point density of several points per square meter and height information with an accuracy of approximately 10 cm. Thanks to these improvements, detailed terrain structures can be recorded largely automatically and precisely. As part of an INTERREG project, a high-resolution re-measurement of Lake Constance took place in 2015 , which was carried out for the first time worldwide with this accuracy for a larger inland body of water . In addition to water management, the products obtained are also used in archeology , shipping and recreational use for further analyzes.

Analysis of geomorphological questions

The development of FWF in particular resulted in a significant increase in publications in the environmental sciences that are based on high-resolution and versatile ALS data. ALS data are used in different scales and degrees of processing from a few points to complete digital terrain models . They occur in different degrees of integration, with the possibilities ranging from simple visualizations and visual interpretations to integration in various process modeling (e.g. mass movements , hydrology, etc.) and automatic classification and mapping processes. Initially, ALS was mainly used for mapping and thus classification and delimitation of various landscape forms and processes. The more precise and detailed techniques and the resulting products make it possible to observe spatial and temporal changes, mainly via the height differences in various sub-areas of geomorphology .

  • Glaciers and ice masses : A relatively simple application is the mapping of ice surfaces and thus visual interpretations of the expansion or reduction of the ice surface. By comparing multitemporal digital terrain models, changes in the volume of ice and snow masses can be determined on the one hand, but also on the other hand Validation and support of other or conventional mass balance calculations can be used.
  • Coasts and sea level : In view of climate change , observations on coastal changes and sea level rise have moved into the focus of research. High-resolution ALS data allow the coincidence and mutual influence of geomorphological processes on the coast to be analyzed more precisely. In addition to using level measurements and regional geoids , the height of the sea surface can also be determined using ALS data. The information obtained can help to understand the influence of the tides and processes on the coast on a regional level and to relate them to the rise in sea ​​level .
  • Erosion processes : By using multitemporal ALS data, the sediment dynamics in an area, which are expressed in the form of accumulation and erosion , can be recorded. For example, the erosion rates of gullies , as they often occur in the Mediterranean region, can be calculated and estimated for the future. But sinkholes and various subsurface subsidence can also be detected better with ALS data than with photogrammetry , as more terrain points can be identified through the entire point cloud and the vegetation can thus be calculated out by filtering.
  • Modeling - risk research : ALS data are used in various models as they contain, among other things, essential information on the development of an area over time. Especially in the reconstruction, prognosis and risk assessment of rock falls and landslides , ALS data and their analyzes form an important basis. Geotechnical measurements on site and vulnerability analyzes of the objects at risk can expand investigations.
  • River morphology : In the area of ​​river morphology, ALS data can represent information on the vegetation of river landscapes, the water level and the boundary between land and water surfaces , changes in the bank area and the roughness . The different bands for the specific wavelength enable a more precise differentiation between vegetation, water and dry areas. Terrestrial laser scanning is increasingly being used for the detailed investigation of individual river sections , as it can achieve even greater accuracy.

Further areas of application

The explanations already given show that ALS data is used in a variety of ways. In addition to the areas of application shown, ALS data are also used to produce various map and navigation services, but also in the areas of archeology, spatial planning , settlement, geology , forestry and agriculture and in many other disciplines, ALS data are used both as original point clouds, derived products (such as digital terrain models) or used as input variables in model calculations to estimate various processes.

Web links

Individual evidence

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