Analyzing magnetic media surfaces for thermal erasures and other characteristics is described. The system includes a drive channel module configured to measure servo automatic gain control values for a magnetic media surface that represent the amount of gain applied by the drive channel module to a preamble signal recorded on the magnetic media surface. The gain control values are then acquired according to certain measurement parameters. The gain control values are then arranged by proximity to each other and organized to generate images that represent changes in the characteristics of the magnetic media surface. Analysis of the images then detects patterns that represent changes in the characteristics of the media surface and determines measurement parameters that coincide with the change in the characteristic.