1. Computer-implemented method of generating a pictorial reference database from a plurality of digital images, the method comprising: defining a plurality of distributed geographical sub-areas; selecting, for at least one of the geographical sub-areas, from the plurality of digital images, selected images with a geo-tag located in the respective geographic sub-area; generating, for the selected images, feature vectors for localized visual features of the selected images; determining, for pairs of the selected images, matching features based on the feature vectors; computing, for pairs of the selected images having matching features, in each case, a measure of visual similarity based on different geometric arrangements of the localized visual features; determining clusters of images with matching objects based on the measures of visual similarity; and storing the clusters of images in the pictorial reference database.