• Digital Terrain Model (4 cm/pixel resolution), NE distal lava flow margin, Holuhraun lava flow-field, Vatnajökull National Park, Iceland

      Hamilton, Christopher W.; Scheidt, Stephen P.; University of Arizona, Department of Planetary Sciences, Lunar and Planetary Laboratory (University of Arizona, Department of Planetary Sciences, Lunar and Planetary Laboratory, 2016-07-28)
      azu_geo_holuhraun_hot_springs: Using a Phantom 3 Pro imaging system, aerial images were acquired over short section of the NE Holuhraun lava flow margin, adjacent interior and adjacent river systems. Images were initially geocoded using the device's onboard GPS and a ground control points (GCPs) network. Final products were coregistered to more precise geocoded image data collected later in 2016. Images were processed into geodetically-controlled digital terrain models (DTMs) and orthomosaics using the commercial software package Pix4Dmapper Pro version 2.0.81. In Pix4Dmapper Pro, data products were geocoded by attributing the 3D coordinates of geocoded image coregistration targets to corresponding pixels in a subset of 2D images. The 2016 image data used for coregistration utilized DGPS measurements (See [DOI OF 2016 DATA]). To estimate the localization accuracy, we used a network of N = 7 coregistration points; the resulting mean of the root mean square error (RMSE) of x, y, and z directions (i.e., latitude, longitude, and elevation) is 2.7 cm. Both the DTM and orthomosaic products have a spatial resolution of 4 cm/pixel, but the DTM has an effective resolution of ~16 cm/pixel. azu_geo_holuhraun_hot_springs_2016: Using the UX5-HP imaging system, aerial images were acquired over the SE and NE Holuhraun lava flow margin, adjacent interior and adjacent river systems. The images were captured in JPEG format using a Sony A7R digital camera with a 36 megapixel high-sensitivity CMOS sensor and a 35 mm focal length lens. Images were geocoded after postprocessing of the device's onboard GNSS GPS receiver and a Trimble R10 Differential Global Positioning System (DGPS) base station. The R10 DGPS is capable of producing survey points with excellent precision (0. 8 cm horizontal and 1.5 cm vertical). This allows calculation of the plane’s exact flight paths. Combined with the exact timing of the camera shutter, the positions of each image acquisition is known during the flight path and are therefore airborne control points. Images were processed into geodetically-controlled digital terrain models (DTMs) and orthomosaics using the commercial software package Trimble Business Center version 3.81. The mean standard deviations (at ±1σ) in the x, y, and z directions (i.e., latitude, longitude, and elevation) of terrain points for 2016 are estimated to be ± 3.0 cm, ± 4.0 cm, and ± 6.4 cm, respectively. The orthomosaic tiles have a spatial resolution of 1 cm/pixel; the DTM has a resolution of ~5 cm/pixel.
    • Long-Term and Inter-annual Mass Changes in the Iceland Ice Cap Determined From GRACE Gravity Using Slepian Functions

      von Hippel, Max; Harig, Christopher; Univ Arizona, Dept Math; Univ Arizona, Dept Geosci (Frontiers Media SA, 2019-07-04)
      The Gravity Recovery and Climate Experiment (GRACE) satellites have measured anomalies in the Earth's time-variable gravity field since 2002, allowing for the measurement of the melting of glaciers due to climate change. Many techniques used with GRACE data have difficulty constraining mass change in small regions, such as Iceland, often requiring broad averaging functions in order to capture trends. These techniques also capture data from nearby regions, causing signal leakage. Alternatively, Slepian functions may solve this problem by optimally concentrating data both in the spatial domain (e.g., Iceland) and spectral domain (i.e., the bandwidth of the data). We use synthetic experiments to show that Slepian functions can capture trends over Iceland without meaningful leakage and influence from ice changes in Greenland. We estimate a mass change over Iceland from GRACE data of approximately -9.3 ± 1.0 Gt/yr between March 2002 and November 2016, with an acceleration of 1.1 ± 0.5 Gt/yr2.
    • Short-term variations of Icelandic ice cap mass inferred from cGPS coordinate time series

      Compton, Kathleen; Bennett, Richard A.; Hreinsdóttir, Sigrún; van Dam, Tonie; Bordoni, Andrea; Barletta, Valentina; Spada, Giorgio; Univ Arizona, Dept Geosci; Department of Geosciences; University of Arizona; Tucson Arizona USA; Department of Geosciences; University of Arizona; Tucson Arizona USA; et al. (AMER GEOPHYSICAL UNION, 2017-06)
      As the global climate changes, understanding short-term variations in water storage is increasingly important. Continuously operating Global Positioning System (cGPS) stations in Iceland record annual periodic motionthe elastic response to winter accumulation and spring melt seasonswith peak-to-peak vertical amplitudes over 20 mm for those sites in the Central Highlands. Here for the first time for Iceland, we demonstrate the utility of these cGPS-measured displacements for estimating seasonal and shorter-term ice cap mass changes. We calculate unit responses to each of the five largest ice caps in central Iceland at each of the 62 cGPS locations using an elastic half-space model and estimate ice mass variations from the cGPS time series using a simple least squares inversion scheme. We utilize all three components of motion, taking advantage of the seasonal motion recorded in the horizontal. We remove secular velocities and accelerations and explore the impact that seasonal motions due to atmospheric, hydrologic, and nontidal ocean loading have on our inversion results. Our results match available summer and winter mass balance measurements well, and we reproduce the seasonal stake-based observations of loading and melting within the 1 sigma confidence bounds of the inversion. We identify nonperiodic ice mass changes associated with interannual variability in precipitation and other processes such as increased melting due to reduced ice surface albedo or decreased melting due to ice cap insulation in response to tephra deposition following volcanic eruptions, processes that are not resolved with once or twice-yearly stake measurements.