the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Near-continuous observation of soil surface changes at single slopes with high spatial resolution via an automated SfM photogrammetric mapping approach
Abstract. Soil erosion represents a major global threat, necessitating a detailed understanding of its spatial and temporal dynamics. Advanced geospatial technologies such as time-lapse structure-from-motion (SfM) photogrammetry provide high-resolution monitoring of surface changes. This study presents a novel event-driven approach for near-continuous monitoring of hillslope surface dynamics over a multi-annual period. The system employed synchronized DSLR (digital single-lens reflex) cameras at three slope stations, triggered by a rain gauge and a daily timer. Ground control points (GCPs) were surveyed with millimeter accuracy to ensure precise georeferencing.
An automated Python-based workflow was developed to synchronize images, detect GCPs using a convolutional neural network (CNN), generate daily digital 3D surface models via SfM, and compute 3D surface models of difference (DoDs). The absolute accuracy of SfM point clouds ranged between 8 mm and 12 mm on average, primarily due to registration errors, with lower deviations (< 5 mm) in central areas after height adjustment. Relative accuracy decreased concentrically with distance from the cameras, with level of detection (LoD) values between 5 mm and 25 mm depending on distance and location.
Time series analysis revealed surface changes driven by rainfall, snowmelt, and agricultural activity. The most significant changes often occurred shortly after tillage, even with minimal rainfall, indicating both erosional and non-erosional processes. A strong negative correlation between rainfall and elevation loss was especially evident within the first seven days following tillage. Seasonal surface lowering of 3–5 cm during winter and occasional positive changes due to frost or vegetation growth were also observed. The monitoring system and workflow are transferable, and the resulting high-resolution datasets are valuable for analyzing erosion dynamics and validating soil erosion models.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2291', Anonymous Referee #1, 18 Jun 2025
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AC2: 'Reply on RC1', Anette Eltner, 14 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2291/egusphere-2025-2291-AC2-supplement.pdf
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AC2: 'Reply on RC1', Anette Eltner, 14 Aug 2025
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RC2: 'Comment on egusphere-2025-2291', Anonymous Referee #2, 16 Jul 2025
In this paper, a new approach for an almost-continuous monitoring of erosion at the hillslope scale is presented. The idea of conducting a continuous survey is very innovative and challenging, and the methods presented are very promising. However, the paper in its present form presents some criticisms and necessitates some revisions.
- In the “abstract”, a lot of acronyms (SfM, GCP, CNN, DoD, LoD) are presented. However, most of them are not useful for the “abstract” section and could be removed.
- The “introduction” section is too simplistic. At least, the papers dealing with the use of SfM for monitoring hillslope erosion at the event/run scale should be presented.
- Line 72: The start of the sentence is a bit twisted. Please, revise.
- Line 77: It is not clear if the investigated field was hydraulically delimited or not. This implies significant differences in terms of runoff generation and sediment transport dynamics.
- Line 79: I think that the “e” should be deleted.
- Some acronyms are not defined in the text.
- The sequence of tenses is not always optimal. Please, revise the text.
- Lines 101-102: The sentence is quite confusing, and some words are repeated.
- The quality of the legends in Figure 3 is too low.
- Line 432: there is a point after mm that should be deleted.
- How do you discern the elevation changes due to vegetation or post-tillage settlement from those due to erosion and deposition phenomena? I believe that resolving this aspect is crucial for the satisfactory application of the presented methodology. Even if briefly discussed, it remains incomplete.
- Have you thought about ways to increase the percentage of total usable days? Is it possible to further protect the setup and avoid big gaps in data collection?
Citation: https://doi.org/10.5194/egusphere-2025-2291-RC2 -
AC1: 'Reply on RC2', Anette Eltner, 14 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2291/egusphere-2025-2291-AC1-supplement.pdf
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AC3: 'Reply on RC2', Anette Eltner, 14 Aug 2025
Publisher’s note: this comment is a copy of AC1 and its content was therefore removed on 15 August 2025.
Citation: https://doi.org/10.5194/egusphere-2025-2291-AC3
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- 1
The manuscript introduces a novel, largely automated structure-from-motion (SfM) photogrammetric system for high-resolution monitoring of soil surface change over 3.5 years on agricultural hillslopes. Synchronized DSLR cameras (triggered by rainfall events and timers) capture daily imagery, which a custom Python workflow processes: it time-synchronizes photos, applies a convolutional neural network to detect ground control points under varying conditions, and runs Agisoft Metashape SfM to reconstruct daily 3D soil-surface point clouds. From these, daily digital surface models and change-of-surface (DoD) maps are derived at millimeter-scale resolution. The method is validated against terrestrial laser scanning (TLS) and UAV photogrammetry. The data from a freshly tilled loess field demonstrates detailed topographic changes following tillage and rainfall. Overall, this approach is innovative and promising for tracking erosion dynamics at high spatial and temporal resolution.
Major Comments
Minor Comments
Line numbering should be continuous throughout for ease of review reference.
Some abbreviations are introduced without definition (e.g. RTC, IoT, LoD, M3C2). Define all acronyms at first use.
The discussion of transferability would benefit from concrete guidance: for instance, recommended mounting improvements (e.g. sturdier rigs, solar power redundancy), or software alternatives (since Agisoft Metashape is proprietary, the authors might suggest open-source SfM tools for reproducibility).