The impact of lightning is permanently recorded in struck material by a secondary magnetization called lightning induced remanent magnetization (LIRM). We introduce an original rock magnetic method based on alternating field demagnetization to define both the geometry and the intensity of the peak lightning magnetic field. It also provides positive evidence than a material has effectively been struck. On a schoolyard tree struck by lightning, we were able to locate the impact to within a few centimeters, to prove that it was a vertical negative current and to estimate a peak current intensity of 99 ¿ 7 kA. This value is 14 kA higher than the one provided by Meteorage lightning detection network and offers an independent calibration.