Masonry arch stability criteria definition has been developed by numerical modeling of cracked arch shell structure of Riga Dome cathedral. Settlement of supports induced strains in masonry arch shells that lead to stress redistribution and crack forming in masonry shell structures. Deformation criteria should be developed for safety exploitation of heritage masonry structures. The main task of present research is developing criteria for evaluating of masonry shell construction condition. Three-dimensional scanning of arch structure by means of Leica 3D laser scanners were used to determine geometric model of masonry arch shell. Previously accomplished monitoring data were used and a monitoring program was established to define unequal settlement of supports. Riga Dome cathedral support settlement monitoring and computed modeling to define settlement unequality was used for initial information to compare computer modeled deformed structure shape with existing cracked masonry structure. Geodezic monitoring by optical tools and mechanical tensometers have been used for determination of a crack expansion since 2002 year. Twenty one free of temperature effects Optical Fiber Sensors of SOFO type is used to measure deformations to establish safe exploitation limits now. Multilevel analysis of masonry shell has been fulfilled by finite element software and applied for modeling safe exploitation criteria