Optical metrology applied in CCTV inspection in drain and sewer systems

Abstract

This paper addresses the metrological quality of dimensional measurements based on images obtained from CCTV inspections in drain and sewer systems. In this type of indirect visual inspection, a significant number of absolute and relative dimensional quantities can be quantified, contributing to the characterization of the observations and, consequently, to the analysis of performance of drain and sewer systems outside buildings. Unfavorable environmental factors and conditions within the drain or sewer components affect estimation of the quantities of interest and the quality of the recorded images (lighting, lack of reference points, geometric irregularities and subjective assessments, among others). Quantification improvement of the dimensional quantities is a key objective to achieve a better assessment of components condition from these inspections. This study contributes to improve the quality of the dimensional measurements by defining experimental procedures, applicable to the optical systems used in CCTV inspection. The paper describes the European normative framework for these inspection activities and proposes approaches aiming at increasing confidence in the dimensional measurements based on the metrological characterization of the optical systems used, as well as, to establish a traceability chain. Results presented and discussed include the evaluation of measurement uncertainty.