We present the mapping of the plasmonic properties of gold nanoparticles that are embedded in a TiO2 thin film deposited over two different substrates, glass and silicon. An improved electron energy-loss spectroscopy (EELS) imaging technique was used to extract plasmon maps with nanometre resolution. Several representative cases of randomly dispersed NPs have been examined to carefully evaluate surrounding effects on the optical response of such nanostructured material. Data were compared to analytical calculations and showed good agreement. These results validate previous structural and far-field optical results and provide a clear description of the optical phenomena that take place at a nanometre scale in these materials. They are of primary importance for enlightening the way to the fabrication of thin film materials including metallic nanostructures for photovoltaic applications.