Estimation of forest variables using airborne laser scanning

Airborne laser scanning can provide three-dimensional measurements of the forest canopy with high efficiency and precision. There are presently a large number of airborne laser scanning instruments in operation. The aims of the studies reported in this thesis were, to develop and validate methods for estimation of forest variables using laser data, and to investigate the influence of laser system parameters on the estimates. All studies were carried out in hemi-boreal forest at a test area in southwestern Sweden (lat. 58°30’N, long. 13°40’ E). Forest variables were estimated using regression models. On plot level, the Root Mean Square Error (RMSE) for mean tree height estimations ranged between 6% and 11% of the average value for different datasets and methods. The RMSE for stem volume estimations ranged between 19% and 26% of the average value for different datasets and methods. On stand level (area 0.64 ha), the RMSE was 3% and 11% of the average value for mean tree height and stem volume estimations, respectively. A simulation model was used to investigate the effect of different scanning angles on laser measurement of tree height and canopy closure. The effect of different scanning angles was different within different simulated forest types, e.g., different tree species. High resolution laser data were used for detection of individual trees. In total, 71% of the field measurements were detected representing 91% of the total stem volume. Height and crown diameter of the detected trees could be estimated with a RMSE of 0.63 m and 0.61 m, respectively. The magnitude of the height estimation errors was similar to what is usually achieved using field inventory. Using different laser footprint diameters (0.26 to 3.68 m) gave similar estimation accuracies. The tree species Norway spruce (Picea abies L. Karst.) and Scots pine (Pinus sylvestris L.) were discriminated at individual tree level with an accuracy of 95%. The results in this thesis show that airborne laser scanners are useful as forest inventory tools. Forest variables can be estimated on tree level, plot level and stand level with similar accuracies as traditional field inventories

Foreword

This thesis consists of four papers where the overall purpose is to contribute to the understanding of how local public transport demand is affected by different factors. An underlying theme running trough the thesis is the two-way relationship between public transport demand and the service level caused by the fact that capacity and quality are joint products. In Paper I the relationship between public transport demand and the service level (in terms of vehicle-kilometres) is investigated using panel data from Swedish counties. A Granger causality test is performed in order to test if the level of service cause public transport demand or if demand cause service level, or if they cause each other. It is found that demand and service cause each other, which is to say that there is a two-way relationship between them.In Paper II elasticity estimates for local public transport demand from previous research are summarised and the variation in results is analysed using meta-regression. The variation is explained by model specification, type of data used and origin of data. In Paper III a demand function for local public transport is estimated using panel data from Swedish counties. Instrument variable estimation is used in order to avoid the problem of a two-way relationship between demand and service level (vehicle-kilometres). Demand elasticities with respect to public transport fare, price of petrol, vehicle-kilometres and car ownership are found to be -0.4, 0.34, 0.55, and -1.37. After also taking the effects of income on car ownership into account, it is found that the total effect of income on public transport demand is close to zero. In Paper IV it is found that the strong increase in public transport demand in the town of Linköping between 1946 and 1983, in addition to fare, vehicle-kilometres and car ownership, can be explained by the rapid increase in female labour force participation and the expansion of the city’s outer areas. The city expansion is thought to have increased average trip distance and thereby reduced the number of trips that could be made walking or by bicycle. After 1983, female labour force participation decreased slightly and the expansion of the areas in question has stopped. Without these positive forces to counterbalance the rising levels of car ownership bus trips per capita has fallen by 71%. The effects of a policy change, including peak-load pricing, straighter bus routes, smaller bus size and staggered school hours, is analysed. It is found that the proposed changes would increase public transport travel by 42 % compared to present policy.Denna avhandling består av fyra artiklar där det övergripande syftet är att bidra till ökad förståelse av hur olika faktorer påverkar benägenheten att gör lokala kollektivtrafikresor. Ett genomgående tema i samtliga artiklar är att det sannolikt föreligger ett dubbelriktat samband mellan antalet resor som genomförs och mängden vagnkilometer som kollektivtrafikproducenten tillhandahåller. Detta eftersom kapacitet och kvalitet är intimt förknippade när det gäller kollektivtrafikutbud. Vid en given fordonsstorlek, innebär fler fordon i trafik att fler passagerare kan transporteras men också att väntetider och/eller avståndet till närmaste linje minskar, vilket utgör en kvalitetsförbättring. I den första artikeln undersöks det lokala kollektivtrafikresandet och vagnkilometerutbudet med hjälp av Grangers kausalitetstest. Syftet är att fastställa huruvida orsakssambandet dem emellan går från vagnkilometer till resande, från resande till vagnkilometer, eller om sambandet är dubbelriktat. Materialet som används kommer från svenska län och täcker perioden 1986 – 2001. Slutsatsen är att det föreligger ett dubbelriktat orsakssamband mellan variablerna. I den andra artikeln sammanställs resultat, i termer av elasticitetsskattningar, från tidigare studier av efterfrågan av lokal kollektivtrafik och variationen i dessa analyseras med hjälp av regressionsanalys, så kallad metaregression. Variationen kan delvis förklaras med modellspecifikation, typ av data som använts samt av geografiska skillnader. I den tredje artikeln estimeras en eftefrågefunktion för lokala kollektivtrafikresor med hjälp av data från svenska län. För att undvika problemet med det dubbelriktade sambandet mellan antal resor och vagnkilometer estimeras funktionen med hjälp av instrumentvariabler. Efterfrågeelasticiteter för lokal kollektivtrafik med avseende på pris, bensinpris, vagnkilometer och bilinnehav finnes vara -0.4, 0.34, 0.55, and -1.37. Efter att ha tagit hänsyn till att inkomsten påverkar kollektivtrafikresandet direkt såväl som indirekt via bilinnehavet, konstateras att totaleffekten är nära noll. I avhandlingens fjärde artikel studeras kollektivtrafikresandet i Linköping. Mellan 1946 och 1983 ökade antalet resor per innevånare kraftigt för att därefter minska. Under perioden 1983 till 2006 minskade antalet resor per person med 71 %. Förutom pris, vagnkilometer och bilinnehav kan det konstateras att kvinnlig förvärvsfrekvens och andel av innevånarna som bor utanför stadskärnan utgör viktiga förklaringsvariabler till resandet. Under expansionsperioden, 1946 – 1983, ökade kvinnors deltagande på arbetsmarknaden kraftigt samtidigt som det skedde en snabb uppbyggnad av förorter med hög befolkningstäthet. Sedan 1983 har kvinnlig förvärvsfrekvens upphört att öka och de befolkningstäta områdena i stadens ytterkanter inte längre växer i förhållande till de centrala delarna. Utan dessa positiva faktorer har utvecklingen dominerats av de negativa influenserna från ökat bilinnehav, ökade biljettpriser och minskat trafikutbud. Effekterna av ett åtgärdspaket bestående av differentierad (mellan hög- och lågtrafik) prissättning, uträtade busslinjer, minskad busstorlek och utspridd skolstart utreds. Slutsatsen är att åtgärderna skulle resultera i 42 % högre kollektivtrafikresande

Estimation of 3D vegetation structure from waveform and discrete return airborne laser scanning data

This study presents and compares new methods to describe the 3D canopy structure with Airborne Laser Scanning (ALS) waveform data as well as ALS point data. The ALS waveform data were analyzed in three different ways; by summing the intensity of the waveforms in height intervals (a); by first normalizing the waveforms with an algorithm based on Beer-Lambert law to compensate for the shielding effect of higher vegetation layers on reflection from lower layers and then summing the intensity (b); and by deriving points from the waveforms (c). As a comparison, conventional, discrete return ALS point data from the laser scanning system were also analyzed (d). The study area was located in hemi-boreal, spruce dominated forest in the southwest of Sweden (Lat. 58° N, Long. 13° E). The vegetation volume profile was defined as the volume of all tree crowns and shrubs in 1 dm height intervals in a field plot and the total vegetation volume as the sum of the vegetation volume profile in the field plot. The total vegetation volume was estimated for 68 field plots with 12 m radius from the proportion between the amount of ALS reflections from the vegetation and the total amount of ALS reflections based on Beer-Lambert law. ALS profiles were derived from the distribution of the ALS data above the ground in 1 dm height intervals. The ALS profiles were rescaled using the estimated total vegetation volume to derive the amount of vegetation at different heights above the ground. The root mean square error (RMSE) for cross validated regression estimates of the total vegetation volume was 31.9% for ALS waveform data (a), 27.6% for normalized waveform data (b), 29.1% for point data derived from the ALS waveforms (c), and 36.5% for ALS point data from the laser scanning system (d). The correspondence between the estimated vegetation volume profiles was also best for the normalized waveform data and the point data derived from the ALS waveforms and worst for ALS point data from the laser scanning system as demonstrated by the Reynolds error index. The results suggest that ALS waveform data describe the volumetric aspects of vertical vegetation structure somewhat more accurately than ALS point data from the laser scanning system and that compensation for the shielding effect of higher vegetation layers is useful. The new methods for estimation of vegetation volume profiles from ALS data could be used in the future to derive 3D models of the vegetation structure in large areas

Tree crown segmentation based on a tree crown density model derived from Airborne Laser Scanning

This letter describes a new algorithm for automatic tree crown delineation based on a model of tree crown density, and its validation. The tree crown density model was first used to create a correlation surface, which was then input to a standard watershed segmentation algorithm for delineation of tree crowns. The use of a model in an early step of the algorithm neatly solves the problem of scale selection. In earlier studies, correlation surfaces have been used for tree crown segmentation, involving modelling tree crowns as solid geometric shapes. The new algorithm applies a density model of tree crowns, which improves the model's suitability for segmentation of Airborne Laser Scanning (ALS) data because laser returns are located inside tree crowns. The algorithm was validated using data acquired for 36 circular (40 m radius) field plots in southern Sweden. The algorithm detected high proportions of field-measured trees (40-97% of live trees in the 36 field plots: 85% on average). The average proportion of detected basal area (cross-sectional area of tree stems, 1.3 m above ground) was 93% (range: 84-99%). The algorithm was used with discrete return ALS point data, but the computation principle also allows delineation of tree crowns in ALS waveform data

On track towards improved regional development? – Impacts of the Svealand rail line on labour earnings in the Mälaren region

This paper studies the impacts on productivity following an investment in regional high-speed rail in the Mälaren region in Sweden. It uses wage earnings as a measure of productivity and proposes to capture agglomeration effects by measuring changes in effective labour force density induced by the transport improvement across municipalities in the region. This change in effective labour force density is then used as a continuous measure of treatment, in order to estimate the effect of the Svealand line introduction on labour earnings. While the transport improvement had a large impact on the connectivity and commuting patterns in affected municipalities, we find estimates of wage elasticities with respect to agglomeration that are in line with or smaller than average values from previous literature. The productivity effects, estimated over a ten-year time period, are shown to be rather modest and concentrated to the regions whose connectivity to the greater Stockholm area was directly affected. We also find that the main results do not materialize when a much shorter time period is used for analysis, indicating that a rather long adjustment time needs to occur before any effects can be seen

Stem Quality Estimates Using Terrestrial Laser Scanning Voxelized Data and a Voting-Based Branch Detection Algorithm

A new algorithm for detecting branch attachments on stems based on a voxel approach and line object detection by a voting procedure is introduced. This algorithm can be used to evaluate the quality of stems by giving the branch density of each standing tree. The detected branches were evaluated using field-sampled trees. The algorithm detected 63% of the total amount of branch whorls and 90% of the branch whorls attached in the height interval from 0 to 10 m above ground. The suggested method could be used to create maps of forest stand stem quality data

Counterexamples to a Monotonicity Conjecture for the Threshold Pebbling Number

Graph pebbling considers the problem of transforming configurations of discrete pebbles to certain target configurations on the vertices of a graph, using the so-called pebbling move. This paper provides counterexamples to a monotonicity conjecture stated by Hurlbert et al. concerning the pebbling number compared to the pebbling threshold

Co-registration of single tree maps and data captured by a moving sensor using stem diameter weighted linking

A new method for the co-registration of single tree data in forest stands and forest plots applicable to static as well as dynamic data capture is presented. This method consists of a stem diameter weighted linking algorithm that improves the linking accuracy when operating on diverse diameter stands with stem position errors in the single tree detectors. A co-registration quality metric threshold, QT, is also introduced which makes it possible to discriminate between correct and incorrect stem map co-registrations with high probability (>99%). These two features are combined to a simultaneous location and mapping-based co-registration method that operates with high linking accuracy and that can handle sensors with drifting errors and signal bias. A test with simulated data shows that the method has an 89.35% detection rate. The statistics of different settings in a simulation study are presented, where the effect of stem density and position errors were investigated. A test case with real sensor data from a forest stand shows that the average nearest neighbor distances decreased from 1.90 m to 0.51 m, which indicates the feasibility of this method

Towards low vegetation identification: A new method for tree crown segmentation from LiDAR data based on a symmetrical structure detection algorithm (SSD)

Obtaining low vegetation data is important in order to quantify the structural characteristics of a forest. Dense three-dimensional (3D) laser scanning data can provide information on the vertical profile of a forest. However, most studies have focused on the dominant and subdominant layers of the forest, while few studies have tried to delineate the low vegetation. To address this issue, we propose a framework for individual tree crown (ITC) segmentation from laser data that focuses on both overstory and understory trees. The framework includes 1) a new algorithm (SSD) for 3D ITC segmentation of dominant trees, by detecting the symmetrical structure of the trees, and 2) removing points of dominant trees and mean shift clustering of the low vegetation. The framework was tested on a boreal forest in Sweden and the performance was compared 1) between plots with different stem density levels, vertical complexities, and tree species composition, and 2) using airborne laser scanning (ALS) data, terrestrial laser scanning (TLS) data, and merged ALS and TLS data (ALS + TLS data). The proposed framework achieved detection rates of 0.87 (ALS + TLS), 0.86 (TLS), and 0.76 (ALS) when validated with field inventory data (of trees with a diameter at breast height >= 4 cm). When validating the estimated number of understory trees by visual interpretation, the framework achieved 19%, 21%, and 39% root-mean-square error values with ALS + TLS, TLS, and ALS data, respectively. These results show that the SSD algorithm can successfully separate laser points of overstory and understory trees, ensuring the detection and segmentation of low vegetation in forest. The proposed framework can be used with both ALS and TLS data, and achieve ITC segmentation for forests with various structural attributes. The results also illustrate the potential of using ALS data to delineate low vegetation

Two-phase forest inventory using very-high-resolution laser scanning

In this study, we compared a two-phase laser-scanning-based forest inventory of stands versus a traditional field inventory using sample plots. The two approaches were used to estimate stem volume (VOL), Lorey's mean height (HL), Lorey's stem diameter (DL), and VOL per tree species in a study area in Sweden. The estimates were compared at the stand level with the harvested reference values obtained using a forest harvester. In the first phase, a helicopter acquired airborne laser scanning (ALS) data with >500 points/m2 along 50-m wide strips across the stands. These strips intersected systematic plots in phase two, where terrestrial laser scanning (TLS) was used to model DL for individual trees. In total, phase two included 99 plots across 10 boreal forest stands in Sweden (lat 62.9 degrees N, long 16.9 degrees E). The single trees were segmented in both the ALS and TLS data and linked to each other. The very-high-resolution ALS data enabled us to directly measure tree heights and also classify tree species using a convolutional neural network. Stem volume was predicted from the predicted DBH and the estimated height, using national models, and aggregated at the stand level. The study demonstrates a workflow to derive forest variables and stand-level statistics that has potential to replace many manual field inventories thanks to its time efficiency and improved accuracy. To evaluate the inventories, we estimated bias, RMSE, and precision, expressed as standard error. The laser-scanning-based inventory provided estimates with an accuracy considerably higher than the field inventory. The RMSE was 17 m3/ha (7.24%), 0.9 m (5.63%), and 16 mm (5.99%) for VOL, HL, and DL respectively. The tree species classification was generally successful and improved the three species-specific VOL estimates by 9% to 74%, compared to field estimates. In conclusion, the demonstrated laser-scanning-based inventory shows potential to replace some future forest inventories, thanks to the increased accuracy demonstrated empirically in the Swedish forest study area