engleski

Application of ‘Structure-from-Motion’ photogrammetry for erosion processes monitoring, Mošćenička Draga example

In last decades we all witnessed a technological revolution in terrestrial scanning, ground surveying, data analysis, and creating more quality of digital elevation models (DEMs). High- resolution topographic surveying is traditionally associated with high capital and logistic costs. Low-cost photogrammetric method named “Structure- form-Motion” (SfM) is ideally suited for low- budget research and application in remote areas (James, 2012 ; Westoby, 2013). SfM method was developed in the 1990s and operates under the same basic tenets as stereoscopic photogrammetry, namely that 3D structure can be resolved from a series of overlapping, offset images. The aim of this paper is to present abilities of SfM photogrammetry and its application in geomorphological terrain analysis. The focus is to show 3D point clouds and digital elevation model delivered from an image sets. Images were acquired by low-cost digital camera on terrain, and later were transformed into point cloud using Autodesk software ReCap. 3D point cloud model was geo- referenced and processed with a Matlab algorithm. As for each terrain photo-series the geo- referenced DEM was made, the changes were compared on the basis of two 3D point clouds. Investigated sight is located in Mošćenička Draga torrential flow catchment, approx. 2km NE from Mošćenička Draga settlement, 15 km SW from Rijeka. Sight is app. 40 m wide and 40 m height. Slopes are very steep to vertical and formed in talus breccias. This sediment type has less resistance from erosion. The catchment area of Mošćenička Draga is one of research locations in Croatian- Japanese project “Risk identification and land-use planning for disaster mitigation of landslides and floods in Croatia”. Figure 1 shows investigated sight, its 3D point cloud delivered from a SfM photogrammetry and three sight profiles. Images for SfM reconstruction were taken on several occasions between 2011 and 2013. To assess the quality of the SfM 3D point cloud, results from the survey delivered in November 2013 were compared with RTK-GPS acquired points. SfM point cloud was geo-referenced using linear transformation based on clearly visible Ground Control Points (GCP) relative (image) and absolute coordinates. GCP were marked before image acquisition, and their coordinates were measured with RTK-GPS. Inside interpolated area (area inside GCP) differences between SfM 3D point cloud and RTK-GPS acquired points were less than 3 cm, while extrapolated points (area outside GCP) had differences up to 10 cm. Average SfM 3D point cloud density about 2000 points per square meter provides sufficient detail of the feature's surface. We used standard ReCap 3D model mesh size. It is possible to use even higher mesh size if higher density is required. The presented 3D model delivered from an image set has adequate quality for valuable use in visualizing and quantifying slopes morphological change. Presented technique is convenient for frequent acquisition of high-resolution DEM at a fraction of the time and cost of alternative approaches. One of the application of Structure- from-Motion photogrammetry method is monitoring of landslide movements. So method utilization in real-time landslide monitoring is one of the future plans.

structure-from-motion; photogrammetry; erosion; monitoring processes

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