Electron holography at 200 kV in a field emission transmission electron microscope has been used to profile the piezoelectric field across a GaN/1.5 nm In0.52Ga0.48N/GaN single quantum well structure. By using cross-sectional samples under conditions where surface relaxation effects were negligible, a local decrease in potential of 0.6±0.2 V was measured across the InGaN layer in the [0001] direction, implying a local piezoelectric field E[0001̄] of 4 MV cm-1, in agreement with bulk estimates. The potential of the technique for profiling non-uniform fields and other GaN based layers is assessed.