In the measurement of optical wave front

a large scale area can be measured with an interferometer of small aperture by correlative stitching technique

in which high measurement precision

high spatial resolution and low price can be maintained. In this technique

a way to get high precision phase distributions of sub-apertures and effective stitching method are important. Stitching between two sub-apertures is simple

but its accumulation error may reduce stitching precision greatly. The paper presents an error averaging method

which requires that the sum of the squared differences of all common areas should be minimized simultaneously. This method is a parallel one in essence. A math model and a solution in Cartesian coordinates are presented. Their validity has been examined by computer simulation.