Traditional survey and terrestrial laser scanning basically differ in the surveying method and require different methods for analysis and interpretation.
Survey with the total station
For the survey with a total station, the surveyor fixes the points to be captured (e.g. corners of building). For this purpose, a target (e.g. prism), for instance, is placed to the respective corner of a house, that has be surveyed, and is tachymetrically determined (frequently indirectly/eccentrically). The, the resulting coordinates of these single points are processed to a model (e.g. a site plan) by a CAD technician. The CAD technician may just base on the points which have been captured on site. Additional information or forgotten measures cannot be taken from the captured data and usually can just be defined with a further survey on site.
Survey with the terrestrial laser scanner
In the case of laser scanning, however, many survey points are arbitrarily placed on the object to be surveyed by the laser scanner. The user has no bearing on the points’ position; in fact, the single positions of the points are defined by the horizontal and vertical drive of the laser scanner. Furthermore, the positions of the laser scanner as well as the chosen resolution of the scanner play an important part. The large number of survey points (modern laser scanners provide up to 1 million points per second) and the unique detailing are advantageous. Thus, the CAD technician receives almost all information in millions of point coordinates and, additionally, a photorealistic documentation. During the analysis and interpretation, the operator derives the desired project-related information from the laser-scanning data.
Different approaches in the analysis and interpretation of data (total station/laser scanner)
According to the various surveying methods, approaches, in which point clouds are analysed and interpreted corresponding to tacheometric surveying, have to be called into question. The following aspects must be considered:
Manual clicking of unfit survey points (single points of the laser-scanner point cloud)
- Non-classified survey points (laser-scanning survey points) cannot be merged to a plan by manual clicking since the survey points have been placed arbitrarily on the object to be surveyed by the laser scanner.
- The large number of single points (up to 1 million points are captured per second) makes it impossible for the operator to interpret them manually. Finally, no one can merge millions of single points.
- The accuracy of a point cloud results from the interpretation of total of points. The interpretation of single points does not correspond to a laser scanner’s operating principle.
Using the example of the corners of a house, the analysis and interpretation of laser-scanning data can be explained as follows:
- a: Point cloud of two house walls (point cloud in CAD software – Image 1)
- b: Selection laser-scanner survey points of the first building wall
- c: Generation of the building edge in the top view (centre line of all points on the wall)
- d: Selection of all survey points of the second building wall
- e: Generation of the second building edge in the top view as a centre line of all scan points
- f. Cutting of both lines to a building corner
The quality of the analysis and interpretation increases significantly in proportion to the number of survey points. A CAD technician should base on as many laser-scanner survey points and always analyse and interpret them using the appropriate laser-scanner software. Working with strait cuts carries a big risk since most of the point cloud (the source of information) is simply excluded.
Conclusion
The survey with the total station is especially suited for objects, of which only few survey points are needed, and for projects, in whose progression, in all probability, no further points must be captured on site at a later date.
If, however, complex objects with many details have to be surveyed within a short period of time, laser scanning should be applied as surveying method. Due to the complete data capturing and the large number of survey points, no measures can be forgotten. Just the desired data needs to be analysed and interpreted; further analyses can be realised without additional expenses whenever you want. The analysis and interpretation of data is automatically accomplished with the appropriate software and reduces both time and expenses. The realistic replica of surveyed objects can immediately be used in a CAD system and allows absolute security in planning.