Variation of umbral maximum field strength as a function of radius. Parameters of a linear fit (solid red line) and power law fit (solid blue line) and correlation coefficients are given in Table 1. The dashed lines mark the one-sigma confidence level for each fit. The yellow and purple lines mark the adopted linear fits from Kopp & Rabin (1992) and SP, respectively. The upper x-axis shows the area corresponding to the given radius.