It is challenging to access the microscopic distribution of strain in subsurface systems with a lateral resolution better than 1 nm and an accuracy of about 10-3 because such measurements require the detection of displacements by 1 pm at near atomic resolution. Nevertheless, such measurements are commonly desirable, in particular, for the investigations of thin heteroepitaxial films. A particular method developed in the past extracted geometrical information from unit cells in lattice images to perform this task in thin layers of cubic quantum well structures. Strain relaxation in thin TEM samples may falsify such measurements and needs to be controlled. It is the purpose of this paper to describe a quantitative method to measure strain with the required accuracy and resolution in the more complex hexagonal systems. A AxGa1-xN/GaN heterostructure grown on a sapphire substrate was investigated. Local strain comes from compositional variations which alter the size of the unit cells that are shown in Figure 1.