A method of fabricating a three dimensional object is provided. A build layer comprising an area of solidified build material and an area of non-solidified build material is formed. The build layer is formed by depositing a layer of non-solidified build material and treating the build material to form the area of solidified build material. The method also involves determining an edge profile for the build layer. Determining the edge profile involves measuring a variation in height of the build layer across a transition between the area of solidified build material and the area of non-solidified build material.