# Development of Height Indicators using Omnidirectional Images and Global Appearance Descriptors
## 6 Conclusions  
In this work, five methods to estimate the height of a mobile platform have been proposed and a comparative evaluation has been carried out. All the methods use only omnidirectional images captured by a catadioptric vision sensor mounted on the platform. Four of them are based on global appearance and, also, an additional method based on local features has been included, with comparative purposes. A complete and exhaustive set of experiments has been carried out to test the validity of each approach. Some challenging sets of images captured both indoors and outdoors have been used to carry out the experiments. Next, we enumerate the principal conclusions obtained from the results:  
* All the methods proposed are able to detect the relative height between the capture points of two images captured along a vertical line, dealing successfully with little displacements in the floor plane and small changes in the orientation of the visual system produced during the capture. * Some of the indicators present a quite linear tendency. In general, this linear tendency is clearer when using images captured outdoors. * The sign of the indicators provides information about the direction of the vertical movement. Therefore, a negative sign indicates that the test image is below the reference image. * In some cases, the results present a relatively high standard deviation, mainly when the height gap between the reference and the test images increases. In general, this effect is more clearly noticeable indoors. * Techniques based on the orthographic projection of the omnidirectional images present the most linear behaviour and the lowest deviation, specially with the method based on the Camera Reference System (CRS) movement. This way, a larger working range can be obtained with this method. * The different techniques rely on the movement of the scene objects to estimate the relative height. Since this movement is quantitatively higher indoors, the indicators obtained with this database present, in general, higher absolute values. As the orthographic projection mainly gathers the floor information, the methods based on this projection present less difference between indoor and outdoor scenes. Therefore, the magnitude of the indicators based on this projection is less dependent on the capture environment. This is an additional advantage of this kind of projection, specially when using the CRS method along with the FS. * In the indoor environment, the slope of some indicators tends to decrease as the height increases. It happens mainly in the methods based on multiscale analysis and in CRS movement. Also, the effect is more pronounced when the reference image is \(h_{ref}=1\), what means estimating higher height gaps. The effect shown in Figure 19 may have an influence on this behaviour: the objects in the scene experience movements with different magnitude as the height of the camera changes, and this effect will be more pronounced in the case of the higher height gaps, leading to a loss of linearity in these cases. * When comparing to methods based on local features, only the global-appearance methods that make use of the panoramic image have shown relatively worse results (as they present a higher standard deviation in most cases). The other global appearance-methods prove to be an efficient alternative to local features both considering their computational cost, and the linearity and standard deviation of the results. In the light of the above, this work has demonstrated the possibility of using descriptors based on global appearance to carry out height estimation with accuracy. The results would permit the development of integrated visual navigation systems as future developments. Provided that a robot has 4 DOF (\((x,y,z)\) and change of orientation \(\theta\) around the \(z\)-axis), first, an estimation of the coordinates \(x,y,\) and \(\theta\) could be carried out using the principles presented in the work [26].