Estimating Control Points for B-Spline Surfaces Using Fully Populated Synthetic Variance–Covariance Matrices for TLS Point Clouds

Jakob Raschhofer,Gabriel Kerekes,Volker Schwieger,Hans Neuner,Corinna Harmening

doi:10.3390/rs13163124

Jakob Raschhofer, Gabriel Kerekes + Show 3 more

Open Access

https://doi.org/10.3390/rs13163124

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Journal: Remote Sensing	Publication Date: Aug 6, 2021
Citations: 5	License type: CC BY 4.0

Affiliation: University of Stuttgart, TU Wien

Abstract

A flexible approach for geometric modelling of point clouds obtained from Terrestrial Laser Scanning (TLS) is by means of B-splines. These functions have gained some popularity in the engineering geodesy as they provide a suitable basis for a spatially continuous and parametric deformation analysis. In the predominant studies on geometric modelling of point clouds by B-splines, uncorrelated and equally weighted measurements are assumed. Trying to overcome this, the elementary errors theory is applied for establishing fully populated covariance matrices of TLS observations that consider correlations in the observed point clouds. In this article, a systematic approach for establishing realistic synthetic variance–covariance matrices (SVCMs) is presented and afterward used to model TLS point clouds by B-splines. Additionally, three criteria are selected to analyze the impact of different SVCMs on the functional and stochastic components of the estimation results. Plausible levels for variances and covariances are obtained using a test specimen of several dm—dimension. It is used to identify the most dominant elementary errors under laboratory conditions. Starting values for the variance level are obtained from a TLS calibration. The impact of SVCMs with different structures and different numeric values are comparatively investigated. Main findings of the paper are that for the analyzed object size and distances, the structure of the covariance matrix does not significantly affect the location of the estimated surface control points, but their precision in terms of the corresponding standard deviations. Regarding the latter, properly setting the main diagonal terms of the SVCM is of superordinate importance compared to setting the off-diagonal ones. The investigation of some individual errors revealed that the influence of their standard deviation on the precision of the estimated parameters is primarily dependent on the scanning distance. When the distance stays the same, one-sided influences on the precision of the estimated control points can be observed with an increase in the standard deviations.

Highlights

Terrestrial Laser Scanning (TLS) point clouds are not the desired end product in an area-wise structural deformation analysis
These methods focus on aspects of functional modelling such as introducing adequate parametrization techniques [5], determining optimal knots vectors [6,7,8] or establishing a suitable number of control points [9]
The primary aim of this article is to describe a systematic approach of establishing realistic synthetic variance–covariance matrices (SVCMs) and to introduce them for modelling measured TLS point clouds from a panoramic scanner by B-spline free-form elements

Summary

Introduction

Terrestrial Laser Scanning (TLS) point clouds are not the desired end product in an area-wise structural deformation analysis. Either geometric primitives or free-form surfaces are estimated based on the aforementioned point cloud in order to reduce the data volume and measurement noise. These can be later used for an area-wise deformation analysis, allowing for a parameter-based and statistically validated deformation analysis. B-spline based methods have gained importance, as they enable the modelling of complex structures [4]. These methods focus on aspects of functional modelling such as introducing adequate parametrization techniques [5], determining optimal knots vectors [6,7,8] or establishing a suitable number of control points [9]

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