Elevation Change and Improved Velocity Retrieval Using Orthorectified Optical Satellite Data from Different Orbits

Abstract

Abstract:Optical satellite products are available at different processing levels. Of these products, terrain corrected (i.e., orthorectified) products are the ones mostly used for glacier displacement estimation. For terrain correction, a digital elevation model (DEM) is used that typically stems from various data sources with variable qualities, from dispersed time instances, or with different spatial resolutions. Consequently, terrain representation used for orthorectifying satellite images is often in disagreement with reality at image acquisition. Normally, the lateral orthoprojection offsets resulting from vertical DEM errors are taken into account in the geolocation error budget of the corrected images, or may even be neglected. The largest offsets of this type are often found over glaciers, as these may show strong elevation changes over time and thus large elevation errors in the reference DEM with respect to image acquisition. The detection and correction of such orthorectification offsets is further complicated by ice flow which adds a second offset component to the displacement vectors between orthorectified data. Vice versa, measurement of glacier flow is complicated by the inherent superposition of ice movement vectors and orthorectification offset vectors. In this study, we try to estimate these orthorectification offsets in the presence of terrain movement and translate them to elevation biases in the reference surface. We demonstrate our method using three different sites which include very dynamic glaciers. For the Oriental Glacier, an outlet of the Southern Patagonian icefield, Landsat 7 and 8 data from different orbits enabled the identification of trends related to elevation change. For the Aletsch Glacier, Swiss Alps, we assess the terrain offsets of both Landsat 8 and Sentinel-2A: a superior DEM appears to be used for Landsat in comparison to Sentinel-2, however a systematic bias is observed in the snow covered areas. Lastly, we demonstrate our methodology in a pipeline structure; displacement estimates for the Helheim-glacier, in Greenland, are mapped and corrected for orthorectification offsets between data from different orbits, which enables a twice as dense a temporal resolution of velocity data, as compared to the standard method of measuring velocities from repeat-orbit data only. In addition, we introduce and implement a novel matching method which uses image triplets. By formulating the three image displacements as a convolution, a geometric constraint can be exploited. Such a constraint enhances the reliability of the displacement estimations. Furthermore the implementation is simple and computationally swift.

MMASTER: Improved ASTER DEMs for Elevation Change Monitoring

“…The improvement of the quality of the DEM compared to the DMO product results in better orthorectification as vertical DEM errors propagate into lateral displacement in the orthoimages [22]. Measurement of seismic displacement using ASTER images has been attempted previously, with varying degrees of success, for instance by [23,24].…”

Circum-Arctic Changes in the Flow of Glaciers and Ice Caps from Satellite SAR Data between the 1990s and 2017

“…Whereas SAR sensors have so far mainly been used to obtain short-term fluctuations and trends in surface velocity for glaciers in polar regions, optical data have been used to obtain decadal trends also in sub-polar and mid-latitude regions [30]. However, the high spatial resolution and repeat interval of optical satellites such as Sentinel 2A/B now also allow investigation of flow velocities on a weekly temporal scale [31,32] and the former temporal distinction has become less clear in this regard. With the increasing overlap of scenes towards the poles, very short observation periods become possible, at least as long as clouds, polar night and shadow (for optical) and the acquisition plans (for SAR) permit.…”

Coseismic displacements of the 14 November 2016 <i>M</i><sub>w</sub> 7.8 Kaikoura, New Zealand, earthquake using the Planet optical cubesat constellation

“…In combination with the high spatial image resolution of around 3 m, details in the displacement field can thus become apparent that are not detected in Sentinel-2 or Landsat 8 data. The envisaged daily repeat by PlanetScope data will further improve the above displacement accuracy by enabling to measure displacements in several image pair combinations and thus exploiting a temporal stack of images and displacements (Dehecq et al, 2015;Kääb et al, 2016;Altena and Kääb, 2017;Stumpf et al, 2017).…”

“…Any orthorectification, no matter how accurate in space, is therefore temporally corrupted by the fact that the ground is a moving target, always changing in time. Typically, ground changes will be small enough to not have a significant effect on orthorectification, but for instance for landslides, major earthquakes, or glaciers the resulting offsets are an inherent problem of orthorectification of monoscopic data Altena and Kääb, 2017). The small field of view of PlanetScope cubesats and the resulting small sensitivity to topographic distortions, the frame geometry of the PlanetScope cameras, and the accessibility of unrectified images all contribute to minimizing and potentially removing topographic distortions.…”

“…Landsat (16 day repeat orbit, 15-30 m resolution), ASTER (16 day repeat orbit, 15 m visible and near infrared resolution) and Sentinel-2 (10 day repeat orbit, 5 day repeat orbit once the Sentinel-2A and 2B constellation is complete, 10-20 m resolution depending on band) are useful for regional displacement fields and provide the approximate horizontal motion components due to their nadir-looking geometry (only ASTER is occasionally pointed in cross-track direction). Landsat and Sentinel-2 data are provided only as orthorectified version (ASTER optionally) so that positions in these orthoimages are potentially contaminated by crosstrack distortions that propagated from errors in the DEM used for orthorectification Altena and Kääb, 2017). Avouac et al (2006) and Girod et al (2015) demonstrated refined sensor models for ASTER that reduce georeference noise significantly, and Avouac et al (2006) developed this approach further to enable measurement of coseismic displacements from ASTER data with an accuracy of a few metres.…”