Terrestrial Laser Scanning
The technique of terrestrial laser scanning (TLS) entered geodesy at the end of the 1990s. While initially only usable for the short range, today it is also at least for the most sophisticated systems to cover a distance range of several hundred meters up to one kilometre. The method can be automated to a great extent and constitutes an outstanding supplement to terrestrial photogrammetry and photographic documentation. Today there is a multitude of approaches that deal with the automated processing and modelling of the considerable amount of data.
How does it work?
Terrestrial laser scanning (TLS) is an automated measurement technology. Starting from the viewpoint of the laser scanner, angles and distances are measured with a laser beam in all surrounding spatial directions. In this way an object becomes discretized in its geometric form through a great number of measuring points (point cloud). The degree which is used to reduce the object to its individual three-dimensional coordinates, can be generally chosen freely. Thus the scanner provides a detailed one-to-one copy of the object’s surface (e.g. a building) within a few minutes, which can be further processed in a variety of ways. Sections, views, surface reliefs, target-performance comparisons, CAD models or collision analyses belong to the application opportunities.
Terrestrial Laser Scanning in Epigraphic Research
In the IBR-project the geometric process of TLS is complemented with radiometric (close range) photogrammetry. In order to speed up the extremely complex evaluation of the unstructured point clouds, larger amounts of TLS data are only displayed as a photogrammetric 2D projection (panoramic scene) – for example as a complete full circle scan of 360 degrees. Only in a few cases (e.g. for inscription-bearing-objects) are three-dimensional models of higher abstraction developed solely on the basis of point clouds.
Technology of TLS
Characteristics of TLS
- Measurement possible without scaffolding
- True-to-scale measuring data on a scale of 1:1
- Objectivity – no interpretation by the land surveyor
- Speed – up to one million points per second
- Completeness – registration/recording of all objects that are visible from the scanner’s standpoint
- Precision in the millimetre range
- High resolution through point to point distance of just a few millimetres
- Independence of lighting conditions
- Large measuring range und high spatial range
- Point cloud can be combined with data of other origins
Disadvantages of TLS
- Stable standpoint of the laser necessary during measurement
- No immediate colour information (as in the case of a digital camera) available
- Unstructured point cloud can require time-consuming manual post-processing
- Object edges are depicted less ‘realistically’ in the point cloud than structures on an areawide basis
- Relatively high purchase price for hard- and software