Digital Elevation Models (DEM) for the Analysis of the paved surface of Linear Infrastructures

The proposed study aims at analyzing effective surveying techniques and methodologies to acquire more detailed metric data and to support traditional surveying techniques on linear infrastructures.

In particular, from measurements acquired with LiDAR technique, it is possible to obtain an accurate 3D model of the infrastructure surface, which can be used to have complete information on its distress conditions.

The particular plano-altimetric development of the road belt makes the classic methods used for DEM extraction unsuitable, on which most modelling software is based, which reconstruct the trend of a given variable according to a regular grid of nodes starting from discrete measured and irregularly distributed values. This is done by means of interpolation techniques with which we arrive at a statistical or deterministic surface, usually in matrix format with a resolution chosen by the user; each element of the matrix corresponds to an elevation value. However, it is evident that a grid structure oriented according to the North-South cartographic grid is not effective to represent the curvilinear development of a road infrastructure.

Therefore, we want to introduce a first methodology to generate a particular curvilinear abscissa DEM, called DEMc, suitable for road pavements, which optimizes not only the computational effort but also the organization and extraction of profiles (longitudinal and transversal) and the plano-altimetric analysis. The construction of this model, represented by a ‘raster’ matrix, is semi-automatic. The elevation value of each single node of the two-dimensional grid is estimated through specially modified spatial and local interpolation processes. The process has been implemented in a Matlab environment.

A more advanced example of the digital paving model was based on the study of the deviation of the paved surface from a reference plane. The process involves the creation of a two-pitched flat surface constructed so as to lay on the real surface (theoretically, a road cross-section is represented by a double pitch to allow water flow). The building of the planes is carried out on road sections as wide as the entire carriageway and between 3 and 5 m long. To ensure that the pitch lays on the surface, an iterative algorithm has been implemented; at each iteration the algorithm excludes the points below the plane obtained by previous interpolation. In this way, in the next cycle, the new plane will be built by interpolation on the basis only of the data that were above the plane at the previous iteration; this method makes the plane orient itself according to the number of points remaining at each iterative cycle. The adjacent pitches, in the direction of travel, are built in such a way as to be mutually joined. This process has been implemented in the Matlab environment as well.