USGS Washington Water Science Center
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Structure from motion (SfM) photogrammetry is a technique by which many photos of the same scene, taken from different perspectives, can be used to estimate the 3-dimensional structure of that scene. As part of our monitoring efforts of the Oso landslide, we've performed regular overflights in a small plane with a wing-mounted camera taking photos every few seconds. The resulting hundreds of aerial photos are then fed into a SfM software package that extracts an extremely detailed topographic map of the imaged area and then projects the photos onto that surface to produce a seamless, orthorectified aerial image (Figures 1, 2 and 3). Taking photos 500-1000 feet above the land surface, we end up with digital elevation models (DEMs) with 25cm resolution and vertical uncertainties on the order of ~10cm, and aerial imagery with 7cm resolution.
These regular topographic snapshots provide a detailed look at the evolution of the newly formed channel and the landslide surface, allowing us to both monitor the rate and style of change and to better understand the processes at work. By differencing sequential maps, we are also able to estimate the volumes of sediment eroded (Figure 4). This information will provide a measure of the relative contribution of landslide erosion to sediment loads we are monitoring at several gaging stations along both the North Fork and main-stem Stillaguamish (see the maps page for more gage locations), and help inform our hydraulic and sediment transport modeling efforts through these reaches.
Using the Time Machine software program, we've created navigable time lapse animations using currently available shaded relief DEMs and aerial imagery. Zoom in to explore the slowly evolving surface and channel over time.
The aerial images are also available for exploration through gigapan here.
The underlying DEMs and aerial imagery will be available for download as part of a broader publication slated for January 2016. For more immediate access, please email email@example.com.
Figure 1: Screenshot of Agisoft Photoscan Pro, the structure from motion software used in this analysis. Blue squares indicate reconstructed photo location and attitude while flags indicate the location of ground control points used to georeference the data. The visible topography is composed of color-coded xyz points reconstructed from the input photos.
Figure 2: Examples of the output aerial images and shaded relief maps showing Oso landslide on 1/29/15. Extends of figures 3 and 4 are indicated in red and orange outlines, respectively.
Figure 3: Zoomed example of output aerial image and shaded relief map.
Figure 4: Vertical change between 12/1/14 and 1/6/15 along the new channel, as estimated through DEM differencing. Channel widening is evident, as is the progressive filling of a sag pond to the north.