important to not neglect these correct matches and increase the tolerance to the maximum value. Consequently, we set
tolerance
d
to 0.9 pixels and tolerance
θ
to 0.15 degree, which maintain the final accuracy to pixel level.
Figure 12. The P
2
↔P’
2
and P
4
↔P’
4
are correct matches in image 2 with respect to image 1 but P
3
↔P’
3
is an incorrect match due to different value of θ
3
.
3.1.2 Homography based filtering approach When there is large error in rotation between two data sets
which is not the case with the test data sets, then the developed outlier filtering technique could not be used.
Moreover, the Homography computed with RANSAC based outlier filtering could not be used directly as the
inliers ratio is very low after descriptor matching. However, if the error in scale is small, then it is possible to
only use the d
n
constraint in the developed filtering approach, an example result is shown in Figure 13, which
contains a small amount of outliers. From the correspondences in Figure 13, an accurate homography
matrix can be computed as the inlier ratio is large. Now, with the obtained homography the outlier correspondences
can be removed directly after the descriptor matching. The result of same example after the outlier removal using
Homography is shown in Figure 14.
Figure 13. Correspondences computed without θ
n
constraint. Blue arrows are pointing toward some visible outlier correspondences.
Figure 14. Homography based outliers filtering.
1
This image can be zoomed to 6400 for a very close observation by using the digital copy of this paper.
4. RESULTS
In this section we will describe the results according to the feature extraction steps.
4.1 Feature detection An example result of the keypoint detection is provided in
Figure 15. The two different zoom levels are provided to visualize the feature detection, one is a normal view
Figure 15, top as well as a zoomed view Figure 15, bottom. Overall, there are lots of clusters of keypoints
detected at the corners of road markings. Many keypoints are also detected at the edges of a traffic light passing over
the zebra crossing in the aerial image patch. Comparatively, more keypoints are detected at the corners
of the road markings in the aerial images than in the MLSPC ortho image. It can be noticed with a close
observation that the correct corresponding keypoints are inside the clusters of keypoints in the both images. For
detailed inspection, it is possible to further zoom-in and analyse the grey values and the detected keypoints in
digital copy. For the keypoint detection from all tiles, the threshold and
the total iterations required to obtain the keypoints are provided in Table 1. The extremely different requirements
of threshold and the number of iterations required for the tile 2 and tile 6 shows the noise difference in the images of
the same sensor.
Figure 15. Adaptive Harris corner feature detection from aerial left and MLSPC right image patch
1
.
Ti le
n o.
Ite rati
on s
[M LS
P C
A er
ial image
] Th
re sh
ol d
[M LS
P C
A er
ial image
] K
eyp oi
n ts
[M LS
P C
A er
ial image
]
1 78285 7.80.83 50985091 2 1151179 11.501.78 50965091
3 1250346 12.493.45 51005100
P
1
P
2
P
3
P
4
d
1
d
2
d
3
Image 1
P
1
P
2
P
4
d
1
d
2
Ima ge 2
θ
2
θ
3
θ
2
θ
1
θ
1
θ
3
P
3
d
3
x y
x y
This contribution has been peer-reviewed. doi:10.5194isprsarchives-XLI-B1-609-2016
614
4 885147 8.841.46 50955099 5 36925 3.680.24
50905074 6 16024 1.590.23
50885078 7 35962 3.580.61
50925060 8 483121 4.821.2 50915100
9 256167 2.551.66 50945100 10 56813 5.670.12 50975100
11 5297 5.280.06
50975100 12 31035 3.090.34 50885053
13 70788 7.060.86 50985098 14 778273 7.772.72 50985092
Table 1: Results of the adaptive Harris keypoint detection.
4.2 Feature matching
2
In this section we will discuss the results of the developed filtering approach.
In first glance, matched features appear as random clusters of the keypoints detected in the image pair Figure 16, top.
A zoomed visualization of the matched keypoints with connecting lines is also crowded due to many matches
Figure 16, middle. Therefore, more interactive visualization of the results with Lego patterns is provided
in Figure 16, bottom. By a close observation of the Lego patterns, it can be realized that the patterns are exactly
same in corresponding images. Which also shows that the features are matched to the pixel level accuracy.
Furthermore, the grey values and edges in context of the road markings are also same with respect to each image.
We also analysed the normal probability function of the error in translation of the obtained correspondence and the
total number of matched correspondences together with mean µ and sigma
σ of X and Y axis for all tiles in Table 2.
Figure 16. Different visualizations of the feature matching with developed method.
1
2
Visualizations of all matching results are provided at: https:www.researchgate.netprofileZille_Hussnain
Ti le
n o.
M a
tc h
es fr
o m
d es
cr ip
tor matc
h in
g M
a tc
h es
fr o
m d
eve lop
ed fi
lte ri
n g te
ch .
µ
x
m µ
y
m σ
x
m σ
y
m
1 4393 140 -2.16 0.87
0.03 0.08
2 7016 930 -2.07 0.95
0.06 0.06
3 12961 1703 -2.02 0.81
0.07 0.05
