Dynamic line rating systems provide the system operators information about the actual rating of the lines. The ampacity of the line is determined by the critical span, which is the span with the worst thermal balance of the line. Three different weather monitoring systems have been installed along an overhead power line. These systems allow quantifying the uncertainty due to the spatial variability. The measurements are compared in order to find the critical span and study the uncertainty due to the spatial variability of the weather along a line.