  Visual place recognition is challenging in the urban environment and is
usually viewed as a large scale image retrieval task. The intrinsic challenges
in place recognition exist that the confusing objects such as cars and trees
frequently occur in the complex urban scene, and buildings with repetitive
structures may cause over-counting and the burstiness problem degrading the
image representations. To address these problems, we present an Attention-based
Pyramid Aggregation Network (APANet), which is trained in an end-to-end manner
for place recognition. One main component of APANet, the spatial pyramid
pooling, can effectively encode the multi-size buildings containing
geo-information. The other one, the attention block, is adopted as a region
evaluator for suppressing the confusing regional features while highlighting
the discriminative ones. When testing, we further propose a simple yet
effective PCA power whitening strategy, which significantly improves the widely
used PCA whitening by reasonably limiting the impact of over-counting.
Experimental evaluations demonstrate that the proposed APANet outperforms the
state-of-the-art methods on two place recognition benchmarks, and generalizes
well on standard image retrieval datasets.
