The use of finite elements in smooth domains leads naturally to polyhedral
or piecewise polynomial approximations of the boundary. Hence the approxima
tion error consists of two parts: the geometric part and the finite element
part. We propose to exploit this decomposition in the error analysis by in
troducing an auxiliary problem defined in a polygonal domain approximating
the original smooth domain. The finite element part of the error can be tre
ated in the standard way. To estimate the geometric part of the error, we n
eed quantitative estimates related to perturbation of the geometry. WE deri
ve such estimates using the techniques developed for shape sensitivity anal
ysis.