International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
COMPARISON OF DISCRETE RETURN AND WAVEFORM TERRESTRIAL LASER 
SCANNING FOR DENSE VEGETATION FILTERING 
A. Guarnieri, F. Pirotti, A. Vettore 
CIRGEO-- Interdepartment Research Center for Geomatics, University of Padova, 
viale dell'Università 16, 35020 Legnaro (PD), Italy - cirgeo@unipd.it 
Commission VII/ WG7 
KEY WORDS: Terrestrial Laser Scanner, DTM, full-waveform, analysis, vegetation filtering, multi-target capability. 
ABSTRACT 
In this paper we present the results of the comparison between two terrestrial laser scanners (TLS), a discrete return system (Riegl 
LMS-Z620) and an echo-digitizing system (Riegl VZ-400), employed for the survey of a dense forested area, in the italian Alps. The 
site is actually undergoing a strong debate among the inhabitants and local government authorities about the exploitation of the area 
as a huge quarry to produce building material. The dispute originates from the uncertainty about the instability of the underlying 
mountain slope, which was interested in 1966 by a landslide. The whole area was surveyed with the two laser scanners on February 
2011 during the vegetation dormant period. A slight different processing workflow was applied to the collected datasets: the VZ-400 
scans were pre-filtered by exploiting the “calibrated relative reflectance" readings and the multi-target capability provided by this 
laser scanning system. Next, two different spatial filters were applied to both the resulting georeferenced 3D models, in order to 
eliminate as much vegetation as possible: iterative filter and a custom morphological filter, developed by the authors. Achieved 
results show that for both datasets, the iterative and the morphological filters perform quite well for eliminating the vegetation, 
though some manual editing is still required since vegetation does not feature a prevalent growing direction. Furthermore, the 
comparison between the number of points left in the final DTMs shows that the VZ-400 provided a one order of magnitude denser 
point cloud wrt. the LMS-Z620. This demonstrates that a TLS with multi-target capability can potentially provide a more detailed 
DTM even in presence of dense vegetation. 
1. INTRODUCTION points over an area of user-defined size; points are then added 
using a criteria on the new triangle slope. The last group is 
Distinguishing vegetation characteristics in a terrestrial laser based on the method proposed by (Kraus and Pfeifer, 2001) 
scanner dataset is an interesting issue for environmental where a surface model is defined using linear prediction and 
assessment. Most of the methods developed so far for hierarchic robust interpolation. 
discriminating between ground and off-ground points have In recent years the Riegl company has developed a new line 
been applied to datasets derived from airborne laser scanner of terrestrial laser scanners (VZ-series), based on pulsed 
(ALS), while minor interest has been shown towards the Time-Of-Flight (TOF) technology, providing some 
application to terrestrial laser scanners (TLS). The extraction interesting features which may help to solve the problem of 
of an accurate digital terrain model (DTM), digital surface generating reliable DTMs in forested areas. Indeed, these 
model (DSM), as well as correct information on vegetation instruments provide real time echo digitization and online 
density and height, is a challenging task which calls for waveform processing, thus combining the advantages of 
consideration on some significant aspects. For example, in analogue detection systems (immediate results wihtout the 
densely vegetated areas the obstruction of the laser beam is need for post-processing) with those of airbome echo 
particularly relevant, as well as the rapid drop of point digitizing systems (multi-target capability). 
density as the distance from the laser sensor increases. For In this paper we present the results of the application of two 
these reasons, different issues, with respect to ALS filters, related to the first and second class of methods 
applications, must be taken into account in the development mentioned above, for the removal of the vegetation present in 
of methods for the processing of TLS data acquired in these the datasets acquired with two terrestrial laser scanners in a 
natural environments. An overview of ground filtering dense forested area, in the italian Alps. The employed laser 
methods which have been applied to aerial laser scanner point scanners are both based on pulsed TOF technology but they 
clouds is presented in (Maas, 2010) and can be divided in adopt different measurement recording approaches: analogue 
three main groups: one based on mathematical morphology, discrete return (Riegl LMS-Z620) and echo-digitizing 
one on progressive densification of a triangle mesh, and caability (Riegl VZ-400). 
another on linear prediction and hierarchic robust 
interpolation. The first group derives from the work of 
(Haralick and Shapiro, 1992) which propose erosion e and 2. STUDY SITE AND DATA ACQUISITION 
dilation d filters in succession either for opening d — e or 
closing e — d operations. The second group of filters is well In order to evaluate the potential of a full-waveform (FWF) 
represented by Axelsson’s algorithm (Axelsson, 2000) based system to provide higher quality DTMs with respect to 
on the progressive densification of a triangular mesh. The conventional single-echo TLS systems, we selected a small 
initial triangle network is created using a set of local minima dense forested area in the italian Alps. 
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