Camera Elevation Estimation from a Single Mountain Landscape Photograph

07/12/2016 ∙ by Martin Čadík, et al. ∙ 0

This work addresses the problem of camera elevation estimation from a single photograph in an outdoor environment. We introduce a new benchmark dataset of one-hundred thousand images with annotated camera elevation called Alps100K. We propose and experimentally evaluate two automatic data-driven approaches to camera elevation estimation: one based on convolutional neural networks, the other on local features. To compare the proposed methods to human performance, an experiment with 100 subjects is conducted. The experimental results show that both proposed approaches outperform humans and that the best result is achieved by their combination.



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1 Introduction

In outdoor environments one of the more important and informative attributes is the elevation: the height of a geographic location above the sea level. Estimation of elevation has a long history [Cajori(1929)]. Nowadays, elevation data are important for a number of applications, including earth sciences, global climate change research, hydrology, and outdoor navigation. Traditionally the assessment of elevation was the domain of geodesy, which offered several means to measure altitude. Among the most popular methods are barometric altimeters, trigonometric or leveling measurements, and Global Positioning System (GPS).

A rich natural heritage is captured and expanded everyday in the form of landscape photos and videos with an ever-growing geographic coverage of outdoor areas, see Figs. 1 and 2. Unfortunately, almost all currently available photos and videos lack elevation information. Moreover, a majority of them do not even contain the GPS coordinates. In this paper, we tackle the problem of estimating the elevation from visual data only. Automatic annotation of images with an accurate estimate of the elevation can be exploited in a number of applications ranging from leisure activities and tourism applications to image database exploration, education purposes and geographic games.


In order to evaluate elevation estimation methods, we introduce a new dataset containing approximately 100K images of natural environments accompanied with the GPS and elevation information (Section 3). We propose two methods of elevation estimation from image content: one based on convolutional neural networks (CNN) [Zhou et al.(2014)Zhou, Lapedriza, Xiao, Torralba, and Oliva]

, the other exploiting bag-of-words (BOW) based image retrieval 

[Sivic and Zisserman(2003), Jégou and Chum(2012)]. The proposed methods are compared to a human performance in Section 6. To estimate the human performance on this task, an experiment was conducted counting 100 subjects (Section 4). The proposed automatic elevation estimation methods outperform knowledgeable humans, and, moreover, the hybrid combination of BOW with CNNs results in the best predictions.

[width=0.18]photos/3316589622_daa1f9a0a1_3310_28577026_N02.jpg 217.5m43.7644N, 5.3622E[width=0.18]photos/13779096314_90cfe91bf6_7190_56015675_N02.jpg 529m47.7923N, 12.4608E[width=0.18]photos/2717022996_35f69143bf_3243_28577026_N02.jpg 1479m47.5178N, 11.4628E[width=0.18]photos/9782957594_b20dc6ab91_2833_26214672_N00.jpg 1798m46.4384N, 13.6351E[width=0.18]photos/2739008344_5baafeed42_3188_28577026_N02.jpg 2395m47.4274N, 11.044E[width=0.18]photos/6280863550_81fc2634f1_6106_84554176_N00.jpg 2504.5m45.9449N, 7.6527E[width=0.18]photos/9618450790_6a76e5311b_5494_99429139_N06.jpg 3375.5m47.0709N, 12.7011E[width=0.18]photos/4809613070_96fef53485_4134_55239894_N00.jpg 3503m45.8113N, 6.7909E[width=0.18]photos/13879934063_e353bb154c_2862_24257706_N07.jpg 4158.5m45.8463N, 6.8737E[width=0.18]photos/9331085104_c4b9ecbb31_5542_81023074_N05.jpg 4782m45.8336N, 6.8616E

Figure 1: A sample from the new benchmark dataset Alps100K [Čadík et al.(2015)Čadík, Vašíček, Hradiš, Radenović, and Chum]. Image credits - flickr users: AllieCaulfield, Golf Resort Achental Team, Erik, Guillaume Baviere, Tadas Balčiunas, antoine.pardigon, twiga269, Karim von Orelli.
Figure 2: Left: normalized elevation histogram of the Alps mountain range (red) and the distribution of elevations in the Alps100K dataset (green). Right: geographic coverage of the new dataset.

2 Related work

We are not aware of any attempt to predict the camera elevation from visual information contained in a landscape photograph of a natural environment. However, the related field of research is the one of visual geo-localization. If we were able to localize the position of the camera, the task of elevation estimation would reduce to a simple query into a geo-referenced terrain model. Unfortunately existing geo-localization methods [Hays and Efros(2008), Baboud et al.(2011)Baboud, Čad’ik, Eisemann, and Seidel, Baatz et al.(2012)Baatz, Sauer, Köser, and Pollefeys, Zamir and Shah(2014), Tzeng et al.(2013)Tzeng, Zhai, Clements, Townshend, and Zakhor]

are neither robust nor sufficiently accurate for this task. Conversely, the methods presented in this paper may improve geo-localization techniques by reducing the search space to only probable camera elevations.

Height above the ground is among the most important information for navigation of Unmanned Aerial Vehicles

(UAV). The problem of elevation estimation for UAVs has been attacked using computer vision because it has several advantages: it is a passive system, has low energy consumption, and visual information can be reused for navigation or localization. The proposed solutions are based on artificial ground-located landmarks 

[Saripalli et al.(2002)Saripalli, Montgomery, and Sukhatme, Garcia-Pardo et al.(2001)Garcia-Pardo, Sukhatme, and Montgomery], optical flow [Beyeler et al.(2006)Beyeler, Mattiussi, Zufferey, and Floreano, Srinivasan et al.(2003)Srinivasan, Zhang, Chahl, and Garratt], stereoscopic vision [Jung and Lacroix(2003), Meingast et al.(2004)Meingast, Geyer, and Sastry, Eynard et al.(2010)Eynard, Vasseur, Demonceaux, and Fremont]

, and machine learning 

[Cherian et al.(2009)Cherian, Andersh, Morellas, Papanikolopoulos, and Mettler]. Most of these methods assume that the camera’s viewpoint is oriented to the ground and that the camera parameters are known. In contrast, in this work we aim to assess the absolute elevation of the camera. Moreover, the hereby proposed methods work on ordinary photographs of natural outdoor environments. Our scenario is more challenging, since both camera orientation and calibration information is missing.

The lack of work devoted to elevation estimation may be explained by the absence of a suitable dataset. The recently published Places 205 [Zhou et al.(2014)Zhou, Lapedriza, Xiao, Torralba, and Oliva]

is a dataset for training scene classifiers. It contains 205 scene categories and almost 2.5 million images of which a subset could be selected for our task. Unfortunately, Places 205 contains neither elevation information nor GPS coordinates. IM2GPS dataset 

[Hays and Efros(2008)] does contain GPS coordinates for each image; however, these images are mostly captured in cities with a significant bias towards landmarks like the Eiffel tower or the Sydney opera house. This renders IM2GPS dataset useless for our purpose because we focus on landscape photos of outdoor environments, like the ones shown in Fig. 1.

3 Alps100K: a new dataset

We introduce a new dataset of almost 100K annotated (GPS coordinates, elevation, EXIF if available) outdoor images from mountain environments. The collection covers vast geographic area of the Alps, the highest range in Europe; therefore we name it Alps100K. The images exhibit high variation in elevation as well as in landscape appearance. Furthermore, the collection spans all the seasons of the year. To the best of our knowledge, this is the first dataset of this kind. It contains test sets to evaluate elevation estimation performance (see Section 4 for human performance and Section 6 for results of the proposed automated methods). A large proportion of the dataset serves as a training set for the data-driven approaches.

Dataset acquisition.

First we create a list of all hills and mountain peaks located in the seven Alpine countries (Austria, France, Germany, Italy, Liechtenstein, Slovenia, Switzerland) from the OpenStreetMap database [Curran et al.(2012)Curran, Crumlish, and Fisher]. The list of hill names is used to query the Flickr111 photo hosting service. In order to increase the ratio of outdoor images certain tags, such as wedding, family, indoor, still life, are excluded. Only images containing information about the camera location are kept. Out of 1.2M crawled images, about 400K are unique and inside the Alps region.

To cull irrelevant (non-landscape) images a state-of-the-art scene classifier [Zhou et al.(2014)Zhou, Lapedriza, Xiao, Torralba, and Oliva] is applied. Probabilities of 205 scene categories are assigned to each image. We experimentally select 28 categories (mountain, valley, snowfield, etc.) and keep only those images whose cumulative probability in those categories exceeds 0.5. This step significantly improves the relevance of the dataset at the expense of reducing the number of images to circa 25%.

Finally the elevation of the camera is inferred from the GPS coordinates via the digital elevation model222Available from This model covers the Alps with 24 meter spaced samples. The collection contains 98136 outdoor images that span almost all possible elevations observed in the Alps [0, 4782m]. Fig. 2 left compares the elevation distribution of the Alps surface and the elevations in the dataset, Fig. 2 right shows the spatial distribution of the collected images. Geographically the dataset covers virtually all the regions of the Alps with obvious concentrations in tourist spots (e.g., around Zermatt village in Switzerland). The EXIF information is available for 41364 images, which is 42% of the Alps100K dataset. The the presented dataset along with elevation and GPS meta-data is available at the project webpage [Čadík et al.(2015)Čadík, Vašíček, Hradiš, Radenović, and Chum].

4 Human performance in elevation estimation task

In this section we measure the ability of humans to estimate camera elevation from an image. The achieved accuracy is subsequently compared to results achieved by the methods proposed in this paper.

During the experiment 100 participants were asked to estimate the camera (not the depicted scene) elevation for each of the 50 test images. We utilized a custom web-based interface where the participants assessed the elevation of each test image (for an example see Fig. 1) using a slider (see more details on the experiment in the supplementary material available at the project webpage [Čadík et al.(2015)Čadík, Vašíček, Hradiš, Radenović, and Chum]). The elevation of the test images ranged in [79m, 4463m] (see green crosses in Fig. 3). The images were presented in randomized order, at the resolution of 750500px. After the experiment was finished participants filled out a questionnaire where additional information about their age, experience with the Alps, highest reached elevation, etc., was gathered. The subjects needed 10 minutes on average to complete the experiment.

4.1 Experimental results

Are humans able to estimate the elevation from an image?

We use the analysis of variance (ANOVA) test 

[Bewick et al.(2004)Bewick, Cheek, and Ball] which determines whether there is a systematic effect of an image (i.e.

, the elevation) in human elevation predictions, or whether the predictions are random. Formally, we state the null hypothesis

as follows: there is no significant difference between elevation predictions for the test images. This hypothesis is clearly rejected (), meaning that humans can indeed estimate the camera elevation from the visual information in images.

How well can humans predict the camera elevation?

The predictions for each test image along with the ground truths are plotted in Fig. 3. The overall root-mean-squared error (RMSE) of human elevation predictions is It can be observed that people underestimate high elevations, i.e., elevations above 3000m. In accordance with this, variance of the elevation predictions grows for high altitudes as well.

The effect of human age, sex, experience and other factors collected in post-experiment questionnaire was also analyzed. None of those factors were found statistically significant. However, it is worth noting that our participants were either living in the Alps, had exceptional experience with outdoor sports, or both. Accordingly, the reported average prediction errors should be taken as rather conservative ones.


index to the 50 test images

Figure 3: Comparison of the elevation estimation by humans and the proposed methods (CNN, BOW and combination). Blue boxes show the span of the human predictions: the red mark is the median, the edges of the box are the and

percentiles respectively, the whiskers extend to extreme human guesses that are not considered outliers, and outliers are plotted individually as ’


5 Automatic elevation estimation from landscape photo

In this section we propose two approaches to estimate elevation from the visual content. We use popular convolutional neural networks and methods based on local features and combine them to exploit each approach’s strengths in a single hybrid method.

5.1 Convolutional neural networks (CNN)

The elevation estimation task can be treated as a standard regression problem where the goal is to directly predict the real-valued elevation given the pixel data of a single photo. Convolutional neural networks have proven to be the state-of-the-art in various image-based machine learning tasks including object and scene classification 

[Zhou et al.(2014)Zhou, Lapedriza, Xiao, Torralba, and Oliva], object detection [Girshick et al.(2014)Girshick, Donahue, Darrell, and Malik], semantic segmentation [Long et al.(2014)Long, Shelhamer, and Darrell]

, and facial recognition 

[Taigman et al.(2014)Taigman, Yang, Ranzato, and Wolf]. We build upon the previous successes and apply large convolutional networks to the Alps100K dataset. Considering the relatively small size of the dataset, we initialize the networks from a network previously trained on the Places205 dataset [Zhou et al.(2014)Zhou, Lapedriza, Xiao, Torralba, and Oliva], which includes a rich collection of outdoor scenes among its 2.5 million images. Additionally, we extend the network inputs by EXIF data, which carries information indicative of possible weather conditions and camera settings.

Convolutional network architecture.

The basic network architecture follows previous successful work on image classification. Specifically, it is the same as the one used in the Caffe reference network

333Available from the Caffe package and the Places-CNN network [Zhou et al.(2014)Zhou, Lapedriza, Xiao, Torralba, and Oliva], which are in turn very similar to the network used by Krizhevsky et al. [Krizhevsky et al.(2012)Krizhevsky, Sutskever, and Hinton]

to win the ImageNet challenge in 2012.

The main building blocks of the network (see Tab. 1

) are convolutions followed by Rectified Linear Units (ReLU). First, second, and fifth convolutional layers are followed by max-pooling, each reducing resolution by a factor of two. The activations of the first and second convolutional layers are locally normalized 

[Krizhevsky et al.(2012)Krizhevsky, Sutskever, and Hinton]. The output of the convolutional part of the network is fed into a fully connected layer (fc6) with 4096 units. Weights of this part of the network are initialized from network Places-CNN444Available from [Zhou et al.(2014)Zhou, Lapedriza, Xiao, Torralba, and Oliva].

The final two layers of the network are fully connected and contain 2048 and 1 neurons, respectively. Weights of these layers were initialized from a normal distribution with standard deviation 0.005 and 0.02, respectively. The final activation function is linear and the optimization objective is Mean Squared Error (MSE). The network was trained by mini-batch Stochastic Gradient Descent with momentum.

Layer conv1 pool1 conv2 pool2 conv3 conv4 conv5 pool5 fc6 fc7 fc8
units 96 96 256 256 384 384 256 256 4096 2048 1
kernel - - -
features - - -
Table 1: Architecture of the convolutional network. Layers up to fc6 were initialized from Places-CNN network [Zhou et al.(2014)Zhou, Lapedriza, Xiao, Torralba, and Oliva]. Resolution of the input is pixels.

5.2 Local features

An alternative approach is based on the -NN classifier [Duda et al.(2012)Duda, Hart, and Stork]. Two efficient methods of obtaining nearest neighbours are considered: sparse high-dimensional bag-of-words (BOW) based image retrieval [Sivic and Zisserman(2003)], and image retrieval with compact image representation [Jégou and Chum(2012), Radenović et al.(2015)Radenović, Jégou, and Chum]. In both cases, the number of neighbours used to estimate the elevation is a function of the confidence in the retrieved nearest neighbour.

The BOW approach has been shown to perform well for specific object and place recognition, especially when combined with a spatial verification step [Philbin et al.(2007)Philbin, Chum, Isard, Sivic, and Zisserman]

, while the short-vector image representations 

[Jégou and Chum(2012), Radenović et al.(2015)Radenović, Jégou, and Chum] obtained by a joint dimensionality reduction from multiple vocabularies show certain level of generalization power.

High-dimensional image representation (BOW).

The majority of image retrieval methods based on BOW representation follow the same procedure as introduced in [Sivic and Zisserman(2003)]. First, local features [Mikolajczyk and Schmid(2004)] such as multi-scale Hessian-Affine [Perdoch et al.(2009)Perdoch, Chum, and Matas] are detected and described by an invariant -dimensional descriptor such as SIFT [Lowe(2004)] or RootSIFT [Arandjelovic and Zisserman(2012)] for all images in the dataset. Then the descriptor space is vector-quantized into a visual vocabulary: a -means algorithm is performed on an independent training dataset to create clusters representing the visual words. In our paper, M visual words was used. Finally, a histogram of occurrences of visual words is generated for each image followed by the inverse document frequency weighting (idf), and sparse BOW vectors are obtained with dimensionality .

To estimate the elevation of a photograph the photograph is used to query an elevation-annotated image database (the training part of Alps100K). Efficient retrieval via the inverted file structure [Sivic and Zisserman(2003)] is followed by a spatial verification step [Philbin et al.(2007)Philbin, Chum, Isard, Sivic, and Zisserman] to re-rank the results. If the top ranked image is likely to be from the same location as the query image, i.e., the top ranked image is spatially verified with high confidence (more than features pass the verification test), we use its elevation as the estimate. Otherwise, we use median of elevations of all retrieved -NN images.

Short-vector image representation (mVocab).

Large scale image retrieval with short vectors has recently became popular as a method for reducing high computational or memory costs. In [Jégou and Chum(2012), Radenović et al.(2015)Radenović, Jégou, and Chum] concatenated vocabularies of different origins followed by a joint dimensionality reduction are used as short image descriptors.

In our experiments we follow [Radenović et al.(2015)Radenović, Jégou, and Chum] and combine eight different visual vocabularies of 8K visual words each. The vocabularies are constructed over two different measurement regions and four power-law normalizations of SIFT descriptors. The region sizes are and ( is the scale of the detected feature, and is the standard relative scale change between detected region and the measurement region, as in [Perdoch et al.(2009)Perdoch, Chum, and Matas]). The power-law normalization of SIFT descriptors ranges , where is the original SIFT descriptors, while corresponds to RootSIFT [Arandjelovic and Zisserman(2012)].

Camera elevation of landscape photographs is estimated by finding -NN images from the training dataset and taking the weighted average of their elevations. Weights are calculated using image dissimilarities , , where is the dissimilarity of the top ranked image and is a constant, . The number of retrieved images is fixed, but the number used from that set depends on their similarity to the top ranked image; in addition, we can control the number used from the retrieved set by the appropriate choice of parameter . Specifically, only images that have dissimilarity up to are used to compute weighted average of elevations.

Parameter choice.

We use the training dataset to learn the necessary parameters. For all experiments presented in this paper , and . Slight changes do not significantly influence the presented results.

5.3 Combining the elevation estimators

The BOW representation with spatial verification has been shown to perform well in a specific object or location recognition. Since people take and share photographs from similar locations, a natural approach is to recognize the specific location. On the other hand, BOW does not generalize meaning that it does not perform well on unseen scenes. Therefore, we propose a hybrid method that first tries to estimate the elevation by recognizing the location, and if that fails, i.e., no spatially verified image is retrieved, then by a secondary estimator: either mVocab or CNN.

6 Results

In this section we experimentally evaluate the proposed methods. A test set of 13148 images (13% of the datased) is randomly selected from Alps100K. The rest of the dataset (i.e., 84988 images) is used for training. The selected measure of performance is an overall root-mean-square error (RMSE) of elevation predictions with regards to the known ground truth elevations. The outcome is summarized in Tab. 2, where Baseline denotes a simple elevation predictor that reports the mean elevation of the training dataset for all queries. For completeness we show the best achievable baseline RMSE values as well, i.e., using the mean of the testing elevations (in italics).

All of the proposed methods perform better than the baseline. The best prediction accuracy among the individual methods is achieved by neural networks (CNN); however, this is significantly improved by combining BOW with CNN. For a better understanding of these results, we plot the fraction of correct predictions as a function of the elevation error, see Fig. 4, left. For more than one third of the test images, the BOW approach spatially verifies the query image, which results in extremely low error rate. The specific place recognition performed by the BOW approach seems to work well for this particular task because the geographic distribution of the Alps100K dataset, which corresponds to reality, exhibits many strong peaks at popular places (see Fig. 2, right). CNN, on the other hand, seems to generalize better on previously unseen locations and it surpasses BOW in sparsely covered regions. The combination of BOW and CNN achieves the best RMSE scores, and is more robust as well.

Method test dataset (13148 images) user experiment set (50 images)
Baseline 801.49; 786.42 1383.64; 1154.43
Human - 879.95
CNN 537.11 709.10
BOW 601.63 757.76
mVocab 610.36 811.00
BOW+mVocab 564.14 646.89
BOW+CNN 500.44 531.05
Table 2: Results (overall root-mean-square error in meters)
Figure 4: Left: cumulative elevation prediction accuracy. Right: dependence of prediction bias on image elevation.

The results for a subset of 50 images selected from the test dataset (used in user experiment described in Section 4) are shown in the right column of Tab. 2, which compares the performance of automatic elevation estimation to the performance of humans. Generally, all of the proposed methods achieve better scores than humans. The best RMSE is again obtained by the hybrid combination of BOW and CNN, which is on average significantly better than humans. In Fig. 3 we plot the predictions for each of the 50 images separately. Interestingly, BOW+CNN method exhibits a similar bias as humans; i.e., it tends to underestimate the highest elevations (images #49, 48, 45, 41, 39, 38) and overestimate lowest elevations (#2, 3). This tendency actually holds true for the whole test dataset, as illustrated in Fig. 4 right. We attribute this behavior to the distribution of elevations in Alps100K dataset, which is less populated in both elevation extremes, as shown in Fig 2, left.

CNN+EXIF information.

Visual appearance of photographs is strongly influenced by the season, the time of the day, and camera field of view. We encode time of the day, time of the year, exposure coefficient, and camera field of view as a sparse binary vector and input it to the first fully connected layer of the CNN (fc6). Each of the values is quantized to 16 discrete levels and encoded as 1-of-N. The exposure coefficient () combines relative aperture , exposure time , and sensor sensitivity into a single value that represents “sensitivity of the photograph” to the light in the scene. Assuming that photos are properly exposed, high values imply low-light conditions and low values imply bright conditions. is calculated as . Camera field of view () indicates possible composition of photographs. As most cameras do not store their field of view explicitly in EXIF data, it needs to be computed from sensor size and focal length as .

The combination of CNN with EXIF information was evaluated on a smaller subset of images with EXIF information available (36050 training and 5314 test images respectively). On this subset, CNN+EXIF achieves RMSE=510m, compared to pure CNN with RMSE=550m on the same subset. We conclude that the EXIF data bring a small improvement in elevation predictions.

Video sequences.

We evaluated the BOW+CNN method using three video sequences (#V1-3 available at the project webpage [Čadík et al.(2015)Čadík, Vašíček, Hradiš, Radenović, and Chum]) in Fig. 5. The videos were acquired by a hand-held point-and-shoot camera from spots of constant elevation (green lines), while changing camera yaw and pitch. These videos represent challenging scenarios, in particular, due to the varying pitch and high camera elevations (#V1=2215m, #V2=3088m, #V2=3789m). Moreover, no frame has been spatially verified by BOW in any case, and thus the prediction accuracy depends solely on CNN. Nevertheless, the proposed method achieves decent elevation predictions for #V1 and #V2. The third video (#V3) illustrates the limitations of the current solution: the prediction accuracy for extremely high elevations is low, suffering from a small number of appropriate images in the training set.

[width=0.32]images/Schuhflicker_2_215m [width=0.32]images/StubacherSonnblick_3_088m [width=0.32]images/Grossglockner_3_798m

Figure 5: BOW+CNN elevation predictions (blue) for three video sequences (green) captured using a camera with constant elevation and varying yaw and pitch.

7 Conclusions

Multiple aspects of the camera elevation estimation task were addressed in the paper. A new benchmark dataset of elevation-annotated images Alps100K was collected. Two approaches were proposed to automatically estimate the camera elevation from a single landscape photograph. In an extensive user experiment, human performance on this task was measured. Experimental evaluation showed that the proposed methods outperform human abilities in camera elevation estimation. The best performing method first attempts to recognize specific location via BOW-based image retrieval, and in case of failure, uses CNN to estimate the elevation. In the future, we plan to extend the dataset to different geographic areas and other climate zones.


This work was supported by SoMoPro II (financial contribution from the EU 7 FP People Programme Marie Curie Actions, REA 291782, and from the South Moravian Region), SGS15/155/OHK3/2T/13, and MSMT LL1303 ERC-CZ grants. The content of this article does not reflect the official opinion of the European Union. Responsibility for the information and views expressed therein lies entirely with the authors.


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