In this paper, we propose to extract a compact yet discriminative visual descriptor directly on the mobile device, which tackles the wireless query transmission latency in mobile landmark search. This descriptor is offline learnt from the location contexts of geo-tagged Web photos from both Flickr and Panoramio with two phrases: First, we segment the landmark photo collections into discrete geographical regions using a Gaussian Mixture Model [Stauffer et al., 2000]. Second, a ranking sensitive vocabulary boosting is introduced to learn a compact codebook within each region. To tackle the locally optimal descriptor learning caused by imprecise geographical segmentation, we further iterate above phrases by feedback an “entropy” based descriptor compactness into a prior distribution to constrain the Gaussian mixture modeling. Consequently, when entering a specific geographical region, the codebook in the mobile device is downstream adapted, which ensures efficient extraction of compact descriptor, its low bit rate transmission, as well as promising discrimination ability. We deploy our descriptor within both HTC and iPhone mobile phones, testing landmark search in typical areas included Beijing, New York, and Barcelona containing one million images. Our learning descriptor outperforms alternative compact descriptors [Chen et al., 2009][Chen et al., 2010][Chandrasekhar et al., 2009a][Chandrasekhar et al., 2009b] with a large margin.