Image scanning apparatus capable of positioning a scan-starting point

An image scanning apparatus, comprising:

• a casing which comprises:
• a scan-platform for placing a document to be scanned, wherein the scan-platform comprises four margins, one of which is an
• upper wider margin, and
• an inner surface;
• a carriage installed at the scan-platform corresponding to an underneath of the document to be scanned, wherein the carriage returns to a starting line which is parallel to the upper wider margin of the scan-platform after having finished a scanning;
• and
• a piece of calibration sheet, which is fixed into the inner surface of the casing and lies between the starting line and the upper wider margin of the scan-platform, wherein the piece of the calibration sheet comprises:
• a hollowed-out marked block defined by a predefined function set, and
• a reference point at an edge of the marked block, a position of the reference point leading to locating a scan-starting point, wherein, after the carriage has moved to a scan line, the reference point can be positioned according to the intersection points between the marked block and the scan line.