Álvaro Gómez-Gutiérrez

Comparing Two Photo-Reconstruction Methods to Produce High Density Point Clouds and DEMs in the Corral del Veleta Rock Glacier (Sierra Nevada, Spain)

In this paper, two methods based on computer vision are presented in order to produce dense point clouds and high resolution DEMs (digital elevation models) of the Corral del Veleta rock glacier in Sierra Nevada (Spain). The first one is a semi-automatic 3D photo-reconstruction method (SA-3D-PR) based on the Scale-Invariant Feature Transform algorithm and the epipolar geometry theory that uses oblique photographs and camera calibration parameters as input. The second method is fully automatic (FA-3D-PR) and is based on the recently released software 123D-Catch that uses the Structure from Motion and MultiView Stereo algorithms and needs as input oblique photographs and some measurements in order to scale and geo-reference the resulting model. The accuracy of the models was tested using as benchmark a 3D model registered by means of a Terrestrial Laser Scanner (TLS). The results indicate that both methods can be applied to micro-scale study of rock glacier morphologies and processes with average distances to the TLS point cloud of 0.28 m and 0.21 m, for the SA-3D-PR and the FA-3D-PR methods, respectively. The performance of the models was also tested by means of the dimensionless relative precision ratio parameter resulting in figures of 1:1071 and 1:1429 for the SA-3D-PR and the FA-3D-PR methods, respectively. Finally, Digital Elevation Models (DEMs) of the study area were produced and compared with the TLS-derived DEM. The

Does HDR Pre-Processing Improve the Accuracy of 3D Models Obtained by Means of two Conventional SfM-MVS Software Packages? The Case of the Corral del Veleta Rock Glacier

The accuracy of different workflows using Structure-from-Motion and Multi-View-Stereo techniques (SfM-MVS) is tested. Twelve point clouds of the Corral del Veleta rock glacier, in Spain, were produced with two different software packages (123D Catch and Agisoft Photoscan), using Low Dynamic Range images and High Dynamic Range compositions (HDR) for three different years (2011, 2012 and 2014). The accuracy of the resulting point clouds was assessed using benchmark models acquired every year with a Terrestrial Laser Scanner. Three parameters were used to estimate the accuracy of each point cloud: the RMSE, the Cloud-to-Cloud distance (C2C) and the Multiscale-Model-to-Model comparison (M3C2). The M3C2 mean error ranged from 0.084 m (standard deviation of 0.403 m) to 1.451 m (standard deviation of 1.625 m). Agisoft Photoscan overcome 123D Catch, producing more accurate and denser point clouds in 11 out 12 cases, being this work, the first available comparison between both software packages in the literature. No significant improvement was observed using HDR pre-processing. To our knowledge, this is the first time that the geometrical accuracy of 3D models obtained using LDR and HDR compositions are compared. These findings may be of interest for researchers who wish to estimate geomorphic changes using SfM-MVS approaches.

Comparison of two methodologies used to estimate erosion rates in Mediterranean ecosystems: 137Cs and exposed tree roots

The 137Cs deposited in soil and exposed tree roots have been widely applied to estimate medium-term soil erosion rates. However, comparative studies between these methods are scarce. For this purpose, three hillsides in two Mediterranean dehesas (rangeland with disperse tree cover) were selected. Regarding the 137Cs technique, a reference site close to the study areas and with similar altitude and rainfall was selected. In order to reduce uncertainties related to the use of point soil profiles, all those collected in an area were combined to form a representative composite profile. The total inventory was 2790±50Bq/m2, and the relaxation coefficient indicated it was an undisturbed soil. The radiocaesium inventory in the study areas was 14-23% lower than in the reference area. The erosion rates for 137Cs were in the range 20.9-38.1tha-1y-1. The exposed root technique was applied to holm oak trees (age about 90years), and the erosion rates were in the range 22-34tha-1y-1. The ratio between exposed root and 137Cs techniques was 1.02±0.11 (S.D.) within the range 0.89-1.2. Both methods produced very similar results equally with respect to the mean erosion rate as well as the relative difference between the hillslope sections, i.e. displaying the same spatial variation in the study areas. As the accounting time for these two techniques is different, 50 and 90y for 137Cs and exposed roots respectively, results suggest that no change in mid-term erosion rates was implied for these areas for almost a century. The use of 137Cs and exposed roots methodology for the determination of mean erosion rates can be reproduced in other ecosystems, but a careful selection of the reference site for 137Cs is essential.

Surface movement and cascade processes on debris cones in temperate high mountain (Picos de Europa, northern Spain)

Debris talus is a very common landform in the temperate high mountain, so much so that it is the most representative of the periglacial and nival processes. This work studies debris cones in the Picos de Europa, an Atlantic mountain range in the north of the Iberian Peninsula. A detailed geomorphological map was prepared, fieldwork was carried out on the debris cone surface, the ground and air thermal regime was analyzed, and a five-year Terrestrial Laser Scan survey carried out. Annual volume changes on the surface of the debris cones were detected and related to active processes and sediment transfer. Two different behaviors were observed in each cone. Cone A is linear, with equilibrium between accumulation and sediment transfer, while Cone B is concave-convex denoting accumulation processes in the upper part deriving from the greater frequency of snow avalanches. Changes in morphology surpass 50 cm/year with most of the activity taking place in the highest and lowest areas. The presence and action of the ice on the debris slope are moderate or non-existent and freeze-thaw processes are only active on the walls at over 2000 m a.s.l. The main processes on debris cones are debris flow and creep related to snowcover, but sediment transfer on the slopes involves high intensity-low frequency (debris flow, avalanches) and high frequency-low intensity processes (creep, shift, solifluction and wasting).