Features and matches on buildings

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Hi, I am using SURF with the default parameter to detect features in two images of urban area. Those images were taken with the same camera in different view angles. I plot the images with the key points and I see that the keypoints are on the ground as well as on the building tops. When I match the features between the two images with brute force matcher that picks the two near neighboars and use the ratio test of 0.7 threshold as well as the symmetry test, all the matches that left are on the ground. I loosed the constrain and I am getting more outliers but still all the matches points are on the ground. Do you have any suggestions why this is happening? I am interested to have matching points on the roof tops of the buildings and I am wondering if the parameter choice prompt the matches to be on a flat area. I'll appretiate any feedback.