Biased and unbiased estimates based on laser scans of surfaces with unknown deformations

The estimates based on laser scans of surfaces with unknown deformations are biased and not reproducible when changing the scanning geometry. While the existence of a bias is only disadvantageous at some applications, non-reproducible estimates are never desired. Hence, this varying bias and its origin need to be investigated – since this situation has not been examined sufficiently in the literature. Analyzing this situation, the dependence of the estimation on the network configuration is highlighted: the network configuration – studied similarly to geodetic networks – rules about the impact of the deformation. As pointed out, this impact can be altered by manipulating the network configuration. Therefore, several strategies are proposed. These include manipulations of the least-squares adjustment as well as robust estimation. It is revealed that the reproducibility of the estimates can indeed be significantly increased by some of the proposed least-squares manipulations. However, the bias can only be significantly reduced by robust estimation

TIMBER VOLUME ESTIMATION BY USING TERRESTRIAL LASER SCANNING: METHOD IN HYPERDIVERSE SECONDARY FORESTS

High accuracy in timber volume estimation in tropical forests is required to support sustainable management. Terrestrial laser scanners (TLS) can provide high-quality estimates from tree structural variables. We compared stem variable estimations obtained by TLS and traditional methods at tree level and adjusted volume equations using data of a secondary seasonal semideciduous forest (Atlantic Forest). We also discuss the feasibility of TLS in hyperdiverse and secondary forest fragments. Traditional measurements (Method I) and TLS-based measurements (Method II) were performed on 29 trees belonging to 10 species. Volume equations based on the Schumacher and Hall (SH) and Spurr models were generated. DBH (diameter at breast height) was equal for both methods. Total height (TH) was overestimated by Method II, and commercial height (CH) showed a low correlation between the two methods. The adjusted volumetric equations were different for both methods, and those based on the SH volume model showed the best fit. Our results lead us to infer that in hyperdiverse secondary forests, tree structural variables should be obtained via TLS. However, attention should be given to the occlusion of target trees by the regenerating understory and to height estimates, which can be biased by the crown characteristics of the dominant species. &nbsp;</p

Mobile Laser Scanning Elevation Data Accuracy in Forest and Open Sky Areas

Bakalaureusetöö Geodeesia, maakorralduse ja kinnisvara planeerimise õppekavalAntud bakalaureusetöö eesmärgiks on saada teada, kas mõõdistatava ala asukoht horisondi avatuse suhtes mõjutab mobiilse laserskaneerimise andmete kõrguslikku täpsust ning kui mõjutab, siis kui palju. Andmed saadi 2016. aasta talvel läbi viidud projektist, kus mõõdistati Tartu-Põlva maanteel Vastse-Kuuste – Põlva lõiku mobiilse laserskanneriga. Uurimistöö eesmärgi täitmiseks valiti kaks polügooni kogu mõõdistatud maanteest. Üks polügoon asub lagedal alal, kus mõlemal pool teed on põld ning ühtegi kõrget taimestikku ega objekti ei ole. Teist lõiku piirab mõlemalt poolt kõrge mets. Mobiilse laserskanneriga saadud andmete kõrgusliku täpsuse hindamiseks mõõdeti mõlemal maanteelõigul kontrollpunktid elektrontahhümeetriga. Uurimistöö tulemustest selgus, et mõlemal maanteelõigul oli kõrguslik täpsus parem kui üks sentimeeter. Lagedal alal oli keskmine ruutviga 4,69 mm ning metsaga piiratud maanteelõigul oli keskmine ruutviga 6,85 mm. Mõlema ala keskmiste ruutvigade vahe on ligikaudselt 2,17 mm. Uurimistöö tulemustest võib järeldada, et kõrged objektid maantee ääres mõjutavad mobiilse laserskaneerimissüsteemiga saadavaid kõrguslikke tulemusi. Suuremat täpsust vajavatel geodeetilistel töödel tuleks kasutada täiendavaid meetode nagu näiteks positsioneerimismärkide tihedam paigutamine.The aim of the current study was to find out, whether the location of the measured area concerning the sky visibility affects mobile laser scanning elevation data and if so, how much. The data used in the study was obtained from the project that was carried out in winter 2016, during mobile laser scanning of the Vastse-Kuuste – Põlva section from the TartuPõlva main road. To accomplish the aim of the study, two test sites were chosen. One test site is located in an open sky area, where the road is surrounded by a field and no high vegetation nor objects are present. Second test site has high forest in both sides of the road. For the elevation data accuracy assessment, the mobile laser scanner data was compared with the checkpoints, that were measured in both test sites using a total station. As a result of the study, it appeared that the elevation data in both test sites were more accurate than one centimeter. The RMSE in the open sky area was 4,69 mm and RMSE 6,85 mm was calculated in the road edged by the forest. The difference of the RMSE in both areas is approximately 2,17 mm. From the results of this study, it could be concluded that the high objects next to the road affect the elevation data gathered with the mobile laser scanner system. Additional methods should be considered for the geodetical works that require higher precision. For example installing ground control targets more densely

Improved Sampling for Terrestrial and Mobile Laser Scanner Point Cloud Data

We introduce and test the performance of two sampling methods that utilize distance distributions of laser point clouds in terrestrial and mobile laser scanning geometries. The methods are leveled histogram sampling and inversely weighted distance sampling. The methods aim to reduce a significant portion of the laser point cloud data while retaining most characteristics of the full point cloud. We test the methods in three case studies in which data were collected using a different terrestrial or mobile laser scanning system in each. Two reference methods, uniform sampling and linear point picking, were used for result comparison. The results demonstrate that correctly selected distance-sensitive sampling techniques allow higher point removal than the references in all the tested case studies

Topographic mapping of rock formations usig GIS methods

Mapování skalních útvarů pomocí geoinformačních metod Abstrakt Tato diplomová práce se zabývá problematikou mapování skalních útvarů z dat pozemního laserového skenování, pozemní fotogrammetrie či UAV fotogrammetrie a automatické filtrace vegetace z nich. Teoretická část práce se zaměřuje na popis fungování a využití těchto metod. Popsána je zde i problematika filtrování 3D bodových mračen. V praktické části práce je popsán postup sběru dat v terénu a jejich následné zpracování. Dále jsou zde použity některé filtrační funkce, které z bodových mračen odstraňují odlehlá měření a vegetaci pomocí vegetačního indexu ExG, klastrovacího algoritmu DBSCAN a Houghovy transformace. Navržený postup je otestována na vybraném skalním útvaru v národním parku České Švýcarsko. Hodnocení použitého filtračního postupu je provedeno na základě porovnání modelů filtrovaných pomocí automatické filtrace s referenčním modely, které byly filtrovány manuálně. Závěrem je vyhodnocena dosažená přesnost modelů pomocí geodetického měření. klíčová slova laserové skenování, fotogrammetrie, UAV, bodové mračno, filtrace datTopographic mapping of rock formations using GIS methods Abstract This thesis deals with issues of creating 3D models of rock formations with data from terrestrial laser scanning, close range photogrammetry and UAV photogrammetry. The theoretical part focuses on explaining functioning and usage of those methods. Beside that there is described issues of 3D point cloud filtering. Practical part of this work describes data collecting and processing procedure. Further there is proposed filtering process which aim to remove noise points from point clouds and remove vegetation with combination of vegetation index ExG, clustering algorithm DBSCAN and Hough Transform. The proposed method is tested on the selected rock formation in Bohemian Switzerland National Park. The evaluation of the proposed method is based on comparison of models filtered with proposed method with reference models, which are filtered manually. Finally, the achieved accuracy of the models is evaluated using geodetic measurements. key words laser scanning, photogrammetry, UAV, point cloud, data filteringDepartment of Applied Geoinformatics and CartographyKatedra aplikované geoinformatiky a kartografiePřírodovědecká fakultaFaculty of Scienc

Spatio-temporal and structural analysis of vegetation dynamics of Lowveld Savanna in South Africa

Savanna vegetation structure parameters are important for assessing the biomes status under various disturbance scenarios. Despite free availability remote sensing data, the use of optical remote sensing data for savanna vegetation structure mapping is limited by sparse and heterogeneous distribution of vegetation canopy. Cloud and aerosol contamination lead to inconsistency in the availability of time series data necessary for continuous vegetation monitoring, especially in the tropics. Long- and medium wavelength microwave data such as synthetic aperture radar (SAR), with their low sensitivity to clouds and atmospheric aerosols, and high temporal and spatial resolution solves these problems. Studies utilising remote sensing data for vegetation monitoring on the other hand, lack quality reference data. This study explores the potential of high-resolution TLS-derived vegetation structure variables as reference to multi-temporal SAR datasets in savanna vegetation monitoring. The overall objectives of this study are: (i) to evaluate the potential of high-resolution TLS-data in extraction of savanna vegetation structure variables; (ii) to estimate landscape-wide aboveground biomass (AGB) and assess changes over four years using multi-temporal L-band SAR within a Lowveld savanna in Kruger National Park; and (iii) to assess interactions between C-band SAR with various savanna vegetation structure variables. Field inventories and TLS campaign were carried out in the wet and dry seasons of 2015 respectively, and provided reference data upon which AGB, CC and cover classes were modelled. L-band SAR modelled AGB was used for change analysis over 4 years, while multitemporal C-band SAR data was used to assess backscatter response to seasonal changes in CC and AGB abundant classes and cover classes. From the AGB change analysis, on average 36 ha of the study area (91 ha) experienced a loss in AGB above 5 t/ha over 4 years. A high backscatter intensity is observed on high abundance AGB, CC classes and large trees as opposed to low CC and AGB abundance classes and small trees. There is high response to all structure variables, with C-band VV showing best polarization in savanna vegetation mapping. Moisture availability in the wet season increases backscatter response from both canopy and background classes