As a digital image measurement method

the sub-pixel accurate location of a corner can be found in digital image by sub-pixel accurate corner locator. The purpose of this paper is to introduce digital image measurement method for measuring triaxial specimen deformation based on sub-pixel accurate corner locator. The distortion target of high precision is used to analyze the precision of the method. The results show that both axial and radial measurement precisions achieve 0.02 pixel and the magnitudes of relative measurement precision achieve 10-4. The high precision distance transducer and sub-pixel accurate corner locator method are simultaneity used to measure the small deformation of dry-specimen in triaxial compression test; and the accuracy and reliability of the method are proved.