New in Iris 1.5.0, we have developed a method to improve location accuracy in areas where there are significant differences between the digital elevation model and actual ground elevation.
To map SAR imagery to geographic locations, a DEM is required. However, publicly available global DEMs such as Shuttle Radar Topography Mission (SRTM) and Copernicus GLO-30 were collected 10-25 years ago and therefore have become outdated in areas where open-pit mining has created large holes in the ground, or tailings walls have been built up significantly. When more recent or high resolution DEMs are provided, we can use these instead, however in some instances, publicly available data is all that is available to use.
Errors in the DEM can be identified with InSAR because they present as a phase contribution that correlates with the component of orbit variations. This allows InSAR to effectively estimate DEM errors. While this known effect has always been computed and accounted for when estimating the deformation time-series, our recent work allows us to improve SAR imagery co-registration based on the estimated DEM error.
Our team has researched the effects that DEM error can have on the precise location of points, and we are thrilled to introduce a novel new repositioned points capability in Iris that allows users to plot their points in the location that has been corrected for DEM errors. We believe this will significantly strengthen the ability to assess the stability of slopes and monitor your assets. In many cases, such as with legacy assets or areas without large topography changes, there will be little to no difference between the DEM-error corrected point locations and the initial point locations, however, in areas where active mining has changed topography, there may be larger differences.
In the example below, Figure 1 is optical satellite imagery showing part of an open-pit mine the surrounding area. Figure 2 shows the typical method for displaying InSAR results using a legacy DEM on a regular grid, while Figure 3 shows the DEM-error corrected point locations. On the left side of the images, outside of the pit, we see there is little to no correction due to estimated DEM-error between the two images, while within the pit itself, which is actively being mined, we do see the points shifting. Notice how many of the shifts coincide with the locations of benches; these did not exist in the original DEM, so this results in a DEM error and hence a co-registration error. Iris now allows users to toggle between the original points derived from legacy DEM standard regular grid and the repositioned points, which allows comparisons and highlights areas that are most affected by DEM-error estimates.
Note that the DEM error estimation relies on InSAR phase that can contain other sources of error, such as transient partial decorrelation. For this reason, successive InSAR analyses may not estimate the DEM error and resulting point relocation completely consistently.
Figure 1. Optical satellite imagery over an active mining operation visualized in Iris.
Figure 2. The typical method for displaying InSAR results in Iris, using a legacy DEM on a regular grid.
Figure 3. The ‘Use repositioned points” function is selected in Iris to show DEM-error corrected point locations. The active pit area on the right of the Iris viewport shows points shifting between Figure 2 and 3 due to improved co-registration capability in Iris.