«remote sensing ISSN 2072-4292 Article Small-Scale Surface Reconstruction and Volume Calculation of Soil Erosion in ...»
Remote Sens. 2014, 6, 7050-7080; doi:10.3390/rs6087050
Small-Scale Surface Reconstruction and Volume Calculation
of Soil Erosion in Complex Moroccan Gully Morphology Using
Structure from Motion
Andreas Kaiser 1,*, Fabian Neugirg 2, Gilles Rock 3, Christoph Müller 4, Florian Haas 2, Johannes Ries 5 and Jürgen Schmidt 1 Soil and Water Conservation Unit, Technical University Bergakademie Freiberg, D-09599 Freiberg, Germany; E-Mail: email@example.com Department of Physical Geography, Catholic University of Eichstätt-Ingolstadt, D-85072 Eichstätt, Germany; E-Mails: firstname.lastname@example.org (F.N.); email@example.com (F.H.) Department of Environmental Remote Sensing and Geomatics, University of Trier, D-54286 Trier, Germany; E-Mail: firstname.lastname@example.org Department of Management, University Koblenz-Landau, D-56070 Koblenz, Germany;
E-Mail: email@example.com Department of Physical Geography, University of Trier, D-54286 Trier, Germany;
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Received: 27 February 2014; in revised form: 16 July 2014 / Accepted: 21 July 2014 / Published: 29 July 2014 Abstract: This study presents a computer vision application of the structure from motion (SfM) technique in three dimensional high resolution gully monitoring in southern Morocco.
Due to impractical use of terrestrial Light Detection and Ranging (LiDAR) in difficult to access gully systems, the inexpensive SfM is a promising tool for analyzing and monitoring soil loss, gully head retreat and plunge pool development following heavy rain events. Objects with known dimensions were placed around the gully scenes for scaling purposes as a workaround for ground control point (GCP) placement. Additionally, the free scaling with objects was compared to terrestrial laser scanner (TLS) data in a field laboratory in Germany. Results of the latter showed discrepancies of 5.6% in volume difference for erosion and 1.7% for accumulation between SfM and TLS. In the Moroccan research area soil loss varied between 0.58 t in an 18.65 m2 narrowly stretched gully incision and 5.25 t for 17.45 m2 in a widely expanded headcut area following two heavy rain events. Different Remote Sens. 2014, 6 7051 techniques of data preparation were applied and the advantages of SfM for soil erosion monitoring under complex surface conditions were demonstrated.
Keywords: structure from motion; high resolution 3D model; multi-view stereo reconstruction; computer vision; photogrammetry; LiDAR; TLS; Morocco; soil erosion monitoring; gully erosion; volume calculation List of Abbreviations
1. Introduction As gullies represent a major sediment source, especially in arid landscapes, many studies examine gully erosion with different approaches . Therefore, no justification is necessary for further effort in improving existing methods for capturing soil erosion processes and their consequences as the demand for measuring techniques of high precision is apparent . While soil scientific analysis, simulated rainfalls or erosion modeling play a major role in erosion research, imaging techniques to reconstruct surfaces and monitor changes are of major importance. A number of studies refer to soil loss quantification Remote Sens. 2014, 6 7052 by means of remote sensing [3–5]. As computational capacities increase rapidly, recent means in morphological visualization and quantification increasingly complement or replace classical methods.
Since it already proved to be a suitable tool in different sciences such as geoarchaeology , architecture  and robotics , this study aims to apply and validate the structure from motion method in soil erosion research. Castillo et al. (2012) already published an accuracy assessment for different field measuring methods in gullies including structure from motion (SfM) . Few studies utilized the emerging terrestrial approach in geomorphology  or comparable disciplines to generate terrain models .
Especially in difficult to access semi-arid research areas the benefits of the presented method become obvious: logistics and mobility advantages, improved precision compared to established methods and low-cost equipment. While LiDAR represents a major step forward in resolution and accuracy for surface modeling [12,13], it still incorporates inherent disadvantages, e.g., high costs, restricted manageability in rough and inaccessible terrain and required permissions for customs. As the surveyed erosion forms represent a complex and winding morphology, a TLS-system would quickly reach its limits due to either shadowing or a need of too many perspectives .
A fast way for scanning extensive surfaces is the use of an unmanned airborne vehicle (UAV) system which is adequate in a lot of morphological surroundings, but is restricted in gullies. An approach by d’Oleire-Oltmanns et al. (2012)  makes use of small-format aerial photography (SFAP) for generating digital elevation models (DEM) and calculates lost soil volumes by referring the derived models back to the former land surface. Previously published studies map the amount of headcut retreat and sidewall collapses with high precision UAV surveying. Nevertheless, they remain limited in their abilities to picture different shapes inside the gully and undercuts below the surface [3,15].
A time-consuming part of UAV surveying is the distribution of ground control points (GCPs) and their registration with a total station or a differential global positioning system (dGPS). Even though newly available UAV systems are already capable of locating their position with an onboard dGPS unit, they remain costly and entail the above-described restrictions. Thus, precise volume definition from airborne platforms remains hardly feasible as small-scale obstacles, undercuts, pipes and meanders have a great deal of influence on gully dynamics and the behavior of strong channel runoff .
Therefore, it is essential to cover them by 3D surface reproduction.
Over a one-year period with strong rainfall events a measuring campaign to create a time series of surface alteration due to soil erosion was carried out. Images documenting the change on highly degraded soils were used to create high resolution surface models of each time step. In order to further test and demonstrate the abilities of the method for geomorphological issues, to gain information on the precision of SfM measurements and to validate the aforementioned strategy, experiments involving a terrestrial laser scanner (TLS) were facilitated. Hereby, a direct comparison between the scientifically established LiDAR and the ever-expanding structure from motion algorithms and their application in geomorphology was achieved.
The advantages of SfM in geosciences have been demonstrated convincingly by James and Robson (2012) in different scales and applications . Yet, the study presented here demonstrates a non-GCP approach to distinctly reduce working time by putting objects of known sizes into the scene for later scaling of the model and relinquishes UAV data. Thus, the study aims to show the capabilities of terrestrial structure from motion in comparison to TLS as a low-cost and agile tool in Remote Sens. 2014, 6 7053 a geomorphologic complex surrounding with distinct changes in a one-year erosion monitoring setting from March 2012 to March 2013.
2. Research Areas Two research areas will be presented in the following chapter: the Souss Valley in Morocco represents a morphologically active region with immanent and striking gully growth and soil degradation.
These ever-changing soil surfaces were measured and monitored with the here presented SfM technique. The second experimental site in a quarry near Eichstätt served as a field laboratory as it offered appropriate opportunities to validate the results from the Morocco field campaign.
2.1. The Souss Valley, Morocco
The Souss Valley is located in southern Morocco, east of Agadir, framed by the mountain ranges of High Atlas in the north and Anti Atlas to the south. Its triangle shape extends between a 30° and 31° northern latitude and from a 7° to 9° western longitude (Figure 1, ). The plain is described as a transition zone between the Atlantic coastal landscape and the Sahara Desert with the eastern tip around 150 km away from the coast . The research sites Gschechda (GCH) and Hamar (HAM) are situated on a slightly inclined, mainly flat but dissected alluvial fan which drains from the High Atlas mountains in the north to the river Souss in the center of the plain. The fans’ drainage system is highly ephemeral with years without discharge and strong flooding events in the wet season. Due to their alpidic (Cretaceous) overprinting and uplift to above 4100 m (Jebel Toubkal 4167 m), the Palaeozoic, Mesozoic and Cenozoic rocks of the High Atlas Mountains produce vast amounts of sediment. Also, the older Anti Atlas relief, mainly Paleozoic rocks on Precambrian bedrock combined with transgressive sediments, was reactivated by the alpidic uplift causing sedimentation in the southern part of the Souss Valley . The latter again overlay a sequence of older fluvial and lacustrine sediments.
The current surface consists of widespread dissected alluvial fans and terraces that were object to recent research . The deepened base level of erosion is a result of the incised oueds (Arabic term for dry riverbeds that show temporal runoff following heavy rain events), which further increases the dissection of the soil surface as a result of locally increasing relief intensity. The aforementioned research sites are situated in a strongly dissected distal part of an alluvial fan northwest of the city of Taroudannt. The negative water balance (212 mm average annual precipitation, mean temperature of 20 °C) and the long and dry summer put drought stress on the mainly agriculturally used vegetation that requires massive irrigation from aquifer wells. Thus, the wet season between October and March is of major importance for groundwater recharge. Rain events are characterized by a high variability during the mild winter and can produce annual amounts in a few days [22,23]. A high content of soda was shown by our own laboratory analysis, resulting in low aggregate stabilities. Calcium carbonate and gypsum accumulations are allocated throughout the research sites. Further field observations lack clear genetic horizons in the alluvial and colluvial soils supplemented with the aforementioned laboratory experiments, thereby leading to classifying the soils as sodic regosols.
Remote Sens. 2014, 6 7054 Figure 1. Map of the research area in southern Morocco. Testing sites are marked HAM (Hamar) and GCH (Gschechda); image source: , modified.
As a major production site for agro-industrial exports in northwestern Africa, constant high pressure weighs on soil and water resources of the region. Especially soil erosion, as in many semiarid regions worldwide, represents a widespread phenomenon to a striking extent. Gully growth by retreating headcuts and sidewall collapses of existing wadis damage infrastructure, thus affecting both plantation owners and residents. Costly land-levelling measurements are implemented to reclaim arable land which was already lost to water-induced erosion. The success of recapturing degraded badlands or gully systems is highly questionable and leads to an increase in soil loss on the reshaped surface .
Reoccurring, existing or newly developing gully systems and rill structures represent complex morphological appearances with high dynamic development during the wet season in winter. During the measuring period from March 2012 to March 2013, exceptionally strong precipitation events occurred in the Souss Valley causing severe damage to infrastructure (Figure 2).
2.2. Quarry Site, Eichstätt, Germany
A limestone quarry a few kilometers away from Eichstätt, Bavaria serves as a field laboratory (Figure 3). The quarry is easily accessible and equipped with several permanently fixed reflectors, serving as tie points (e.g., for TLS) and a climate station.