The true roughness of highly granular perpendicular media is measured by forming an inverse replica of the surface of the media. The invention enables AFM measurements of granular media valley depth to more consistently predict the corrosion performance of the media. A liquid resist is used to first replicate the media topography and form the inverse replica. The narrow valleys in the original media are precisely modeled as sharp peaks on the replica. The height of the peaks are readily measured with an AFM tip. The resulting image is a negative of the original surface.