Data RESULTS AND DISCUSSION

Figure 5: Classification of reference waveforms ground. After the classification we extract only those points lo- cated between vegetation waveforms from all forest floor points. This way we eliminate waveforms originating from forest tracks, clearings or fields. To exclude the influence of the incidence an- gle, only waveforms recorded close to the nadir of the scan lines are taken into consideration. The reference value is derived from the remaining waveforms, analyzing the integrals of the signals mean or maximum value. The advantage of the method is the full automation as well as the fact that it can be applied on a complete data set including ur- ban areas with roads, buildings and fields. The prerequisite for the success of the method is a sufficiently large number of laser pulses completely penetrating through the vegetation without any interactions. Due to the assumption that all surfaces, which inter- act with the emitted laser pulse, have a similar reflectance sec. 2. the reflectivity of the forest floor plays an important role. Us- ing the forest floor waveforms for the estimation of the reference value entail that the reflectivity of the forest floor is set as com- mon reflectivity for all materials in the footprint.

3.2 Semiautomatic Method

The forest floor is of minor relevance for the characterization of the vertical vegetation structure. Therefore, it seems reasonable to take the reflectivity of vegetation as a basis for the assumed common reflectivity of all targets. Although there are waveforms with single reflections in the canopy, those waveforms do usually not meet the requirements for reference waveforms extended tar- get, horizontal orientation to the scanner. Therefore, the refer- ence value cannot be estimated reliably from those single veg- etation echoes. Alternatively, we investigate the integrals of all vegetation waveforms more than one peak. We assume that the maximal possible backscattered signal corresponds to the wave- form with the highest occurring integral. This waveform results from the interaction with vegetation and forest floor and is thus influenced by attenuation effects. Therefore, the reference value is still smaller than the actual value but closer to it than the refer- ence value derived with the automatic method in section 3.1. Beside this advantages, the drawback of the approach is that solely vegetation waveforms may be included in the analysis of the in- tegrals. In comparison to the first approach, the procedure has a lower automation level, as the required segmentation of a region of interest in the data set may need a certain level of user inter- action. Another problem is caused by materials with very high reflectivity remaining in the segmented data set. Those materi- als may distort the reference value estimation and have a negative effect on the attenuation correction. Figure 6: Asphalt road in aerial photo left and laser scanner point cloud right

3.3 Interactive Method

This method combines the reference value estimation from echoes of extended targets with an accurate adjustment to the actual re- flectivity of vegetation. The approach, requiring user interaction as well, works as follows: A point cloud is derived from the digi- tized waveform data with conventional methods. Afterwards, ref- erence areas of known reflectivity ρ ref are segmented manually, e.g. asphalt areas and grasslands. If possible, the reference areas should be located close to the nadir of the scan lines. Figure 6 shows a suitable asphalt road in the aerial photo and in the point cloud with color coded amplitudes. The relative reflectivity ρ ref of a specific ground cover depends, among other things, on the wavelength of the used laser scanner system. Characteristic val- ues for different types of ground cover are given in manufacturer specifications Pfennigbauer and Ullrich, 2011. Finally, we ana- lyze the integrals of the reference area waveforms and adapt the mean values mean ref to the relative reflectivity of vegetation ρ v Eq. 1. For better understanding we can consider the following example: The relative reflectivity ρ ref of the chosen reference area is half of ρ v . Thus, the mean value mean ref has to be dou- bled to perform the adjustment to the reflectivity of vegetation. rv = mean ref · ρ v ρ ref 1 The advantage here is that the reference value is close to the actual reflectivity of vegetation. Unfortunately, the method re- quires a high level of user interaction and is difficult to auto- mate.Furthermore, appropriate materials with known reflectivity have to be present in the data set.

4. RESULTS AND DISCUSSION

In this section, the developed methods sec. 3 are applied on different data sets. Our aim is to invent methods which are ap- plicable to all kinds of forests. For the reference value estimation factors such as vegetation structure, forest species composition and foliage conditions play an important role. To ensure a struc- tured analysis of all influences, not too many parameters should be varied at a time. First of all, we have focused on different foliage conditions. The data used for our investigations are intro- duced in subsection 4.1. Afterwards the results of the reference value estimation are presented and discussed.

4.1 Data

For our experiments data of two different flight campaigns have been used. The main characteristics are presented in table 1. The This contribution has been peer-reviewed. The double-blind peer-review was conducted on the basis of the full paper. Editors: S. Oude Elberink, A. Velizhev, R. Lindenbergh, S. Kaasalainen, and F. Pirotti doi:10.5194isprsannals-II-3-W5-65-2015 68 data set A data set B Location Oberlausitz Wildacker Tharandt Forest Recording Time March 2010 May 2014 Foliage leaf-off leaf-on Scanner LMS Q680 LMS Q680i Altitude over Ground 800 m 500 m Pulse Repitition Rate 100 kHZ 400 kHz Table 1: Description of data sets 500 1000 1500 2000 500 1000 1500 2000 2500 I mean integral freuquency Figure 7: Histogram of the integrals of all waveforms with three interactions in the leaf-on data set Riegl scanners used in both flight campaigns operate with the same laser having a wavelength of 1550 nm. The data sets dif- fer with regard to foliage, flying altitude and pulse repetition rate. Due to the dependence of the amplitudesintegrals on the altitude over ground and the pulse repetition rate Ahokas et al., 2006, the absolute values are not comparable. Although the altitude over ground is a little lower in data set B, the amplitudes are gen- erally lower because of the considerably higher pulse repetition rate. Data set A was recorded in March 2010 in the area of Ober- lausitz Germany under leaf-off conditions, whereas data set B, recorded in May 2014, contains fully foliated deciduous trees. To simplify the reading flow, subsequently, the data sets are termed ”leaf-off” or ”leaf-on” referring their most interesting property. For our experiments we have selected two equally sized test re- gions covered with mixed forest spruce, beech, oak, birch, which have a similar vegetation structure and forest species composi- tion. Due to the different foliage conditions, the recorded wave- forms nevertheless differ considerably. To evaluate the data, we calculate the integral and determine the number of interactions via a simple peak detection algorithm for all waveforms. The further analysis is done in classes seperated by the number of in- teractions. Figure 7, for example, shows the histogram of the in- tegrals of all waveforms with three interactions in the leaf-on data set. Additionally, the mean integral I mean = 793 is marked. Figure 8 shows a summary of the evaluation of all waveforms in the test regions. Each bar represents a class of waveforms with a specific number of interactions. The bar length corresponds to the number of waveforms included in the class, the position specifies the mean value of all integrals of a class. Single peak waveforms are additionally divided into forest floor and canopy. The ma- jority of waveforms consist of one to three interactions. In the leaf-off data set, the proportion of the waveforms with more than five interactions is higher because the laser pulses can penetrate deeper into bare trees. Compared to each other, the scattering of the mean integrals of the leaf-off data set is higher than the value for the leaf-on data. This is caused by the heterogeneous char- acteristic and the greater variation in reflectivity bare trees and needles vs. needles and leaves of the leafless and indeciduous trees. Looking at the mean integrals of the individual bars, an- other interesting fact becomes obvious: The greater the number of interactions a waveform has before the reflection on the forest floor, the higher is the mean integral of the waveform. This is related to the different reflectivity of vegetation and forest floor. Moreover, we closely examined the single pulse waveforms by separately analyzing waveforms from ground and vegetation re- flections. The comparison of the mean integral in figure 8 shows that the values for ground reflections are higher than those result- ing from a single vegetation interaction. This indicates either a signal loss in the canopy or reflections at targets with a low re- flectivity like branches and the trunk.

4.2 Estimation of Reference Value