Suitability of laser data for DTM generation: a case study in the context of road planning and design

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Abstract

Laser range data acquired from a helicopter are evaluated in terms of the information that can be derived from them and the accuracy. The objective is to study the suitability of laser data to generate a DSM for road planning and design in The Netherlands. The conclusion is that high-density laser measurements allow the reconstruction of the terrain relief with the required accuracy. Nonetheless, they do not allow the extraction of all the information required, particularly semantic information. Thus, the combination of laser data with existing information is a prerequisite. This process of combining laser data with existing geographic information is not trivial. The rate of success depends much on the quality of the individual datasets and the method used to combine them. This problem appears in a much broader context, that of spatial data fusion, and should be the object of future research.

Introduction

Rijkswaterstaat is involved in the planning, design, construction and maintenance of the highways in The Netherlands. Presently, Rijkswaterstaat maintains approximately 3000 km of highways, having at a minimum 4 lanes.

Because of the importance of the road network for further economic development, new highways are being proposed and their impact, especially on the environment, needs to be evaluated. If the planned road has to be changed, alternative routes have to be planned rapidly. Thus, the method to acquire the new information has to be fast. At present, road planning and design are carried out with the help of a DSM generated by photogrammetric means. Because photogrammetry is a time consuming technique, information is acquired over a much wider area than that needed to plan the initial road.

Laser scanning could fulfill the desire for faster methods. In this article, we investigate the suitability of laser data to generate a DSM for road planning and design in The Netherlands.

Over the past years, Rijkswaterstaat has been experimenting on using laser scanning data to map the coastal regions of The Netherlands (Huising et al., 1996; Huising and Vaessen, 1997; Vaessen et al., 1998), and to produce a DTM of the complete country in a grid format of 1 point per 16 m2 (Ministry of Transport, Public Works and Water Management, 1997). Obviously, these types of applications are less demanding than that of producing information for road planning and design. Although all applications require filtering of both blunders and undesired objects (Fritsch and Kilian, 1994; Pfeifer et al., 1998), the extraction of information from laser measurements for the purpose of road planning and design requires more sophisticated procedures to recover and identify the objects on the terrain (Weidner, 1996; Haala and Brenner, 1997).

In the following sections, we will first present briefly the current practice of DSM production for road planning and design at Rijkswaterstaat. In addition, we will determine which type of information, both semantic and geometric, can be extracted from high-density laser measurements. The planimetric and altimetric accuracies of the laser data are also assessed by using reference measurements. The article finishes with some conclusions and recommendations.

Section snippets

Current practice of DSM production for road planning and design at Rijkswaterstaat

At the Survey Department of Rijkswaterstaat, the terrain relief is extracted by measuring pertinent morphologic features. These features consist of both single points, and strings of points on flat surfaces and on places where the terrain slope or curvature changes abruptly (category 1 in Table 1). Attached to the coordinates of these features is also semantic information that classifies the feature, e.g., as a single point or a breakline. This semantic information is however not further used,

Study area and data

The laser data were acquired by the contracted survey company Eurosense (P.O. Box 4923, 4803 EX Breda, The Netherlands). This company was responsible for the flight procedure, the calibration of the laser system, and post-processing of the laser measurements. The latter included conversion of the coordinates of the points to the Dutch national reference system, and filtering of blunders and of measurements made on trees and houses. Filtering was carried out by means of automatic procedures

Information content of the laser data

In this section, it is assessed which of the information in Table 1 can be extracted from laser measurements. For this purpose, the laser-DTM and the laser-DSM mentioned above are used. The information that cannot be extracted from the laser data has to be collected from existing sources of information, such as databases. In this context, some considerations are also given to the combination of laser data with Dutch 2D and 3D databases.

Accuracy assessment of the laser data

As it can be observed from Table 2, the terrain relief can in principle be reconstructed by using the same features that are part of the photogrammetric DTM. Obviously, the procedures to extract all these features either automatically or manually are very demanding. Instead, the problem of reconstructing the terrain relief may be approached in another way. Considering that the required height accuracy of the DTM for road planning and design is 25 cm (7.5 cm for hard surfaces like roads), and

Conclusions

Based on the results of this study, a DSM derived from laser measurements with an average density of 4 points per m2 has sufficient quality to represent the terrain relief for the purpose of road planning and design. For the specific tested situation, conditioned by the type of laser system used and the specific terrain, the height accuracy is 29 cm (RMSE), which approaches the 25 cm required.

When compared to the current DSM product used for road planning and design, almost all features can be

Acknowledgements

The authors are very grateful to their colleague René van Heerd at the Survey Department for preparing some of the figures and for carrying out some of the computations in Section 5.

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