LiDAR data acquisition
swisstopo is planning another LiDAR survey of Switzerland and the Principality of Liechtenstein as the campaign launched in 2017 will come to an end in 2024. Airborne laser scanning is one of the most effective methods for measuring territory in 3D. It provides valuable, high-quality data in terms of both detail and accuracy. As the basis for digital terrain models, these data also contribute to other products and are used by many applications.
Acquisition campaign to be repeated
swisstopo has been using LiDAR technology since the 2000s with the production of the cadastral survey digital terrain model(DTM-AV) – the predecessor to swissALTI3D. Since the laser penetrates into the forest, it is possible to produce a homogeneous and high-quality digital terrain model for the entire territory. In 2017, swisstopo, in cooperation with the cantons, launched a new acquisition campaign to obtain LiDAR data for the whole of Switzerland and Liechtenstein, spread over a six-year period (see below). This campaign will be concluded with the publication of the data by the end of 2024.
Due to the very good experience and high demand, swisstopo has decided to repeat the LiDAR acquisition campaign and thus to update the swissSURFACE3D product. If finances permit, the entire country of Switzerland and the Principality of Liechtenstein are to be surveyed again over a period of 6 years. The provisional planning for the procurement of the new LiDAR data (subject to change) is shown in the following figure. The annual figures correspond to the year of the aerial survey. On average, the data are available about 12 months after acquisition.
LiDAR data form a classified point cloud. It is used in-house to update terrain and landscape models and is also available in our online shop with the product swissSURFACE3D.
The current availability of LiDAR data is given by the layer “Division swissSURFACE3D” in the federal geoportal map.geo.admin.ch.
Characteristics of LiDAR data
- No foliage or snow during acquisition to facilitate ground information and production of a digital terrain model
- Density of points: minimum 5 pts/m2, mean around 15-20 pts/m2
- Classification: unclassified, ground, vegetation, buildings, water, bridges
- Planimetric accuracy: 20 cm (1 sigma)
- Altimetric accuracy: 10 cm (1 sigma)
Cantonal LiDAR data
In addition to its own planned acquisitions, swisstopo sometimes purchases cantonal LiDAR data if they meet its criteria in terms of quality and acquisition period. These data are then also incorporated into the products distributed by swisstopo.
LiDAR, an acronym for Light Detection And Ranging, comprises both a transmitter and a light source receiver that can measure distances. The wavelengths used are in the invisible spectrum and are of no consequence to the human eye. The laser pulses emitted are reflected by the environment and are captured again by the system. The distance is calculated based on the time of flight taken by the pulses to make the round trip.
Mounted in an aircraft, a laser scanner measures the topography by sweeping the ground perpendicularly to the direction of flight. Several hundred thousand laser pulses per second are emitted. This is called laser scanning or airborne “lasergrammetry”.
With a density of several points per square meter, an accuracy of 10 cm and high acquisition speed, airborne LiDAR is one of the best methods to determine the topography of our environment. This data is used, in particular, for the following applications:
- Automatic 3D modelling of buildings and other landscape objects.
- Study of the forest: height, width and inclination of trees, map of cuts, calculation of biomass, planning of cuts, digital canopy models.
- 3D modelling of ground-based elements under vegetation cover such as forest roads or watercourses.
- Monitoring of overhead lines (cables, high-voltage power lines).
- Creation of digital terrain models.
- Creation of digital surface models.
- Visibility analysis.
From raw points to finished products
The volume of measurements acquired by LiDAR requires several processes before it can be used for commercial applications.
Direct georeferencing
Simply knowing the distance between the device and the ground is insufficient for topographical surveys. The measured points must be referenced in a terrestrial coordinate system. In lasergrammetry, only the position of the LiDAR device in flight is used. The position in space (XYZ) is measured by GNSS and the angular position (roll, pitch and yaw) is given by an inertial measurement unit. Georeferencing is then called “direct” because it does not require any reference points on the ground and speeds up production.
Classification
Known as a “point cloud”, the mass of points obtained from LiDAR acquisition describes all the objects that have returned a laser signal and is extremely complex to interpret. To make this information easier to understand and useful, each point is assigned to a predefined class that provides information on the nature of the latter. There are as many classes as there are applications. They depend in fact on users’ requirements. However, most projects contain at least the following classes: unclassified points, ground, vegetation, buildings and surface waters.
Classified point cloud
The classified point cloud is already a finished product. More difficult to handle than some more simplified products (see below), it nevertheless gives more freedom and is suitable for more applications. swisstopo sells a classified point cloud with the product swissSURFACE3D.
The most widely used format for this data is LAS. Developed by the American Association of Photogrammetry and Remote Sensing (ASPRS) in open format, it is very effective for managing large data file sizes and retains information specific to LiDAR points.
Learn more about the LAS format
Thanks to binary data management, the LAS format offers an extremely fast read/write speed. The XYZ coordinates of the points use a translation vector and scaling coefficients to minimize the size of the files.
In addition to the position of the points, the LAS format records all the important information specific to LiDAR. For example, classification is available and predefined with several classes and their associated code.
Digital terrain and surface model
A classified point cloud has a lot of information but is still complex to use. Two simplified products are then often produced with lidar data: the digital terrain model (DTM) and the digital surface model (DSM). In both cases, the information is represented homogeneously in space. The irregular measurements of a point cloud are interpolated on a regularly spaced grid. This operation makes it possible to use a raster format (image) and facilitates processing. Nevertheless, the density of the data is often reduced and the classification lost.
The DTM represents the bare ground, only the “ground” points of the cloud are used. swisstopo distributes such a model with the swissALTI3D product. The DSM uses all the classified points and thus models the surface of the territory with the presence of vegetation and buildings.
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