1. A display apparatus that operates to configure an image projection system configured to project an image by combining on a screen a plurality of projection images projected by a plurality of display apparatuses so that the images partially overlap each other, the display apparatus comprising: a transformation unit configured to perform a geometrical transformation on a projection image; and a setting unit configured to set a parameter of the geometrical transformation by the transformation unit based on a relation between a position of an overlap region in a projection image obtained before the geometrical transformation by the transformation unit is performed, and a position of the overlap region in the projection image obtained after the geometrical transformation is performed, wherein an amount or value of the geometrical transformation is changed according to a position of a vertex of the overlap region.