Assessing horizontal positional accuracy of Google Earth imagery in the city of Montreal, Canada
The horizontal positional accuracy of Google Earth is assessed in the city of Montreal, Canada, using the precise coordinates of ten GPS points spatially distributed all over the city. The results show that the positional accuracy varies in the study area between ∼0.1 m in the south to ∼2.7 m in the north. Furthermore, two methods are developed for correcting the observed positional errors: (a) using a set of transformation parameters between true coordinates of the geodetic points and their coordinates in Google Earth, and by (b) interpolating the misfit vectors at the geodetic points. The former method reduces the overall accuracy to ∼67 cm RMSE, whereas the latter one practically removes all the distortion (RMSE = 1 cm). Both methods can be developed for other places in the world subject to availability of appropriate control points. In addition, a displacement problem caused by the topography of the area and the viewing angle of the imaging satellite is discussed, and it is shown that the true positions can be shifted even up to several meters, as a consequence.