A method for calibrating a powder bed fusion additive manufacturing apparatus comprising: forming a first solidified portion (908, 909) within a first scan region, wherein the first solidified portion (908,909) within the first scan region is formed by irradiating a build material powder while a build unit (302, 400) is moved in a first location; forming a second solidified portion (907, 903) within a second scan region, wherein the second solidified portion (907, 903) within the second scan region is formed by irradiating the build material powder while the build unit (302, 400) is moved in a second location different from said first location, wherein the first (908, 909) and second solidified portions (903, 907) are formed by irradiating a layer of powder so as to solidify, sinter, darken, mark or form marks in the build material; wherein the apparatus is calibrated based on at least one of: a degree of offset of the first and second solidified portions, and a degree of overlap of the first and second solidified portions; and the first solidified portion comprises a first series of solidification lines and the second solidified portion comprises a second series of solidification lines, wherein the first and second series of solidification lines are formed as a Vernier scale; wherein the method further comprises: determining an offset value by detecting the at least one of: a degree of offset of the first (908, 909) and second solidified portion (907, 903), and a degree of overlap of the first and second solidified portions; calibrating the additive manufacturing using the offset value; and applying the offset value to an operation of at least one of a galvanometer and a positioning system for positioning at least one build unit of the additive manufacturing apparatus which build unit is configured to selectively provide the build material and at least partially melt or sinter the build material within a scan region. forming a first solidified portion (908, 909) within a first scan region, wherein the first solidified portion (908,909) within the first scan region is formed by irradiating a build material powder while a build unit (302, 400) is moved in a first location; forming a second solidified portion (907, 903) within a second scan region, wherein the second solidified portion (907, 903) within the second scan region is formed by irradiating the build material powder while the build unit (302, 400) is moved in a second location different from said first location, wherein the first (908, 909) and second solidified portions (903, 907) are formed by irradiating a layer of powder so as to solidify, sinter, darken, mark or form marks in the build material; wherein the apparatus is calibrated based on at least one of: a degree of offset of the first and second solidified portions, and a degree of overlap of the first and second solidified portions; and the first solidified portion comprises a first series of solidification lines and the second solidified portion comprises a second series of solidification lines, wherein the first and second series of solidification lines are formed as a Vernier scale; wherein the method further comprises: determining an offset value by detecting the at least one of: a degree of offset of the first (908, 909) and second solidified portion (907, 903), and a degree of overlap of the first and second solidified portions; calibrating the additive manufacturing using the offset value; and applying the offset value to an operation of at least one of a galvanometer and a positioning system for positioning at least one build unit of the additive manufacturing apparatus which build unit is configured to selectively provide the build material and at least partially melt or sinter the build material within a scan region. determining an offset value by detecting the at least one of: a degree of offset of the first (908, 909) and second solidified portion (907, 903), and a degree of overlap of the first and second solidified portions; calibrating the additive manufacturing using the offset value; and applying the offset value to an operation of at least one of a galvanometer and a positioning system for positioning at least one build unit of the additive manufacturing apparatus which build unit is configured to selectively provide the build material and at least partially melt or sinter the build material within a scan region.