The internal fields in 2.2nm thick InGaN quantum wells in a GaN LED structure have been investigated by using aberration-corrected off-axis electron holography with a spatial resolution of better than 1nm. To improve the spatial resolution, different types of off-axis electron holography acquisitions have been used, including pi phase shifting and phase shifting holography. A series of electron holograms have been summed up to simultaneously improve the sensitivity in the measurements. A value of 20% of indium concentration in the quantum wells has been obtained by comparing the deformation measured by dark-field electron holography and geometrical phase analysis to finite element simulations. The electrostatic potential has then been measured by off-axis electron holography. The mean inner potential difference between the InGaN quantum wells and the GaN quantum barriers is high compared to the piezoelectric potential. Due to the improved spatial resolution, it is possible to compare the experimental results to simulations and remove the mean inner potential component to provide a quantitative measurement of the piezoelectric potential.