A system, medium, and method measuring a geometric reliability index, e.g., in image compensation, and more particularly, a system, medium, and method measuring a geometric reliability index in image compensation that can measure a geometric model of a projection surface onto which an image is projected. The system include a pattern generating unit generating first and second patterns, each having a plurality of pattern points for image compensation and the pattern points of the first pattern being different from the pattern points of the second pattern, a pattern transmitting unit transmitting the generated first and second patterns to an image projecting system, a pattern acquiring unit acquiring the first and second patterns projected onto a projection surface by the image projecting system, and a reliability index measuring unit measuring a geometric reliability index of the projection surface on the basis of a variation in position between the corresponding pattern points of the acquired first and second patterns.