Digitizing of Analogue Data-Charts from Thermohygrographs

Abstract

A thermo-hygrograph is a chart recorder that measures and records both temperature and humidity in an analogue form. Converting the results into digital form is necessary to enhance evaluation of measurements archived on paper charts. Coordinate system of the charts is different than common Cartesian rectangular axis. Temperature and relative humidity axis are curved according to recorder pen holder ray. Different chart types, depending on particular thermohygrograph model, were used so far. In every case, however, the axis grid depicted in the background is geometrically precisely defined. So pattern recognition techniques could be used to find its location automatically. Temperature and relative humidity patterns can be retrieved automatically assuming that the color group of plotted lines is different than the background including the grid. Due to storing conditions, manual descriptions, discoloration and other factors, the plotted pattern cannot be unambiguously identified. Many randomly scattered pixels are interpreted as being part of a plotted line. Among additional measures, like excluding particular areas from analysis, manually removing and including points, a vector analysis of retrieved courses makes digitizing more effective.