1. A processor-based method for constructing an error map for a lithography process, the method comprising the steps of: using the processor, constructing a first error map using spatial error data, including lens signature data, compiled on a lithography tool used in the lithography process, using the processor, constructing a second error map using spatial error data, including pattern placement data, compiled on a mask used in the lithograph process, and combining the first error map and the second error map to produce an overall error vector map for the lithography process.