  This work focuses on assessing the fidelity of Interferometric Synthetic
Aperture Radar (InSAR) as it relates to subsurface ground motion monitoring, as
well as understanding uncertainty in modeling active landslide scarp
displacement for the case study of the in situ monitored El Forn deep seated
landslide in Canillo, Andorra. We used the available Sentinel 1 data on the
Alaska Satellite Facility (ASF) Vertex platform to create deformation velocity
maps and time series of the El Forn landslide scarp. We compared the
performances of InSAR data from the recently launched European Ground Motion
Service (EGMS) platform and the ASF Vertex Platform in a time series comparison
of displacement in the direction of landslide motion with in situ borehole
based measurements from 2019 to 2021, suggesting that ground motion detected
through InSAR can be used in tandem with field monitoring to provide optimal
information with minimum in situ deployment. While identification of active
landslide scarps may be possible via the use of EGMS platform, the intents and
purposes of this work are in assessment of InSAR as a monitoring tool. Based on
that, geospatial interpolation with statistical analysis was conducted to
better understand the necessary number of in situ observations needed to lower
error on a remote sensing recreation of ground motion over the entirety of a
landslide scarp, suggesting between 20 to 25 total observations provides the
optimal normalized root mean squared error for an ordinarily kriged model of
the El Forn landslide scarp.
