# Development of Height Indicators using Omnidirectional Images and Global Appearance Descriptors
## 5 Experiments and Results
### Configuration of the Experiments  
In the previous section, five methods are proposed to estimate the altitude using visual information. On the one hand, the methods 1 and 2 make use of the panoramic image, the method 3 employs the orthographic view, the method 4 can use any of the three projections: panoramic, orthographic or unit sphere projection and finally, the method 5 uses the omnidirectional scene. On the other hand, different appearance descriptors can been used to describe each kind of image projection. Taking this into account, a total of 12 combinations _method + image projection + descriptor_ are considered during the experiments, to test their feasibility. Table 3 shows the configuration of each combination, specifying the height estimation method, the image projection, the description method and the final measurement of topological relative height obtained.  
As far as the choice of the reference and test images is concerned, the following three conditions have been considered in order to broaden the scope of the experiments:  
\begin{table}
\begin{tabular}{l l c c} \hline \hline
**Height Estimation Method** & **Image Projection** & **Descriptor** & **Height Indicator** \\ \hline
**1. Central Cell Correlation** & Panoramic Image & FS & \(d\) (pixels) \\ \cline{3-4}
**2. 2D-DFT Vertical Phase** & Panoramic image & 2D-DFT & \(S\) (_deg_) \\ \hline
**3. Multiscale Analysis** & Orthographic View & FS & \(\Delta fc\) \\ \cline{3-4}
**4. Camera** & Panoramic Image & FS & \(\rho\) \\ \cline{3-4}
**Reference System** & & 2D-DFT & \(\rho\) \\ \cline{3-4}
**Movement** & Orthographic View & FS & \(\rho\) \\ \cline{3-4}  & Unit Sphere Projection & SPT & \(\rho\) \\ \hline
**5. Matching Local Features** & Omnidirectional Scene & SURF & \(d_{1}(pixels)\) \\ \cline{3-4}  & & SURF-RANSAC & \(d_{2}(pixels)\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Combinations of height estimation methods, kind of image projection and description method considered to carry out the experiments. The final topological height measurement is also shown.  
Figure 15: Sample omnidirectional images captured indoors, from the same location, varying only the height of the capture points. (**a**) \(z=125\) cm (\(h=1\)); (**b**) \(z=185\) cm (\(h=5\)) and (**c**) \(z=275\) cm (\(h=11\)).  
* The bottom image of each set (\(h=1\) in Table 2) is considered to be the reference image, and the rest of images of each set are considered as test images. Since each test image presents a different altitude with respect to the reference image in each set, this situation allows us to analyse the linearity of the estimated relative altitude versus the actual relative altitude.
* The image captured at \(h=5\) (intermediate position, equivalent to \(z=185\) cm according to Table 2) is considered to be the reference image, and the rest of images of each set are considered as test images. This permits studying the behaviour of the methods to estimate both positive and negative relative altitudes and analysing the symmetry of the behaviour.
* Different reference images and altitude gaps are considered. This permits assessing the behaviour of the algorithms independently on the image chosen as reference image and on the altitude gap. For each set of images, we carry out as many comparisons as possible considering different images as reference. For example, considering a gap \(\Delta h=2\), equivalent to 30 cm, we compare the first image with the third, the second with the fourth, and so on until carrying out all the experiments that the range of height permits. Table 4 shows the number of experiments for each height gap and data set in this condition. All these experiments are carried out both with positive and negative relative heights.