Recent Changes to Langjökull Icecap, Iceland: An investigation integrating airborne LiDAR and satellite imagery


Langjökull, Iceland’s second largest icecap (~950 km$^{2}$), was the subject of an incomplete airborne LiDAR survey in August 2007. This study investigates and evaluates the application of photoclinometry, which employs visible light imagery (here, Landsat ETM+ band 4) to interpolate unmeasured sections of this fragmented data set. A complete digital elevation model (DEM) of Langjökull was produced, and photoclinometry was determined to be a satisfactory and robust technique for topographic interpolation (RMS error = 3.4 m over a 3 km section). Future applications of photoclinometry can ensure optimal results by focusing on the consistent ability of their imager to accurately represent low contrast surfaces; also, consideration of setting characteristic such as solar azimuth, solar elevation, and moderate surface slope will make photoclinometric interpolation more effective. Photoclinometry it is proven to be a current and valuable technique, it is confirmed as a secondary rather than primary tool, and other possible applications of photoclinometry are considered. Using the completed DEM of Langjökull for summer 2007 and a previously prepared corresponding 1997 data set, Langjökull was found to have a specific annual mass balance of -0.99$\pm$0.1 meters per year of water equivalence (m yr$^{-1}$ w.e.), a number which confirms published predictions that Langjökull will likely disappear in the next 200 years. Comparison of remotely-sensed mass balance values and traditional $\textit{in situ}$ measurements revealed a possible systematic disparity; it is hypothesized that field measurements may not be sufficiently constraining behavior of interior areas and that the signal from strongly receding outlet glaciers may be skewing the $\textit{in situ}$ mass balance value calculated for the entire icecap. An additional DEM of outlet Hagafellsjökull Vestari allowed for calculation of specific mass balances of -2.28 m yr$^{-1}$ w.e. for 1997-2001, -3.86 m yr$^{-1}$ w.e. for 2001-2007, and -3.23 m yr$^{-1}$ w.e. for 1997-2007. Similarly, visual inspection and tracing of Landsat images showed a recession of -3.42.5 km$^{2}$ yr$^{-1}$ from 1994 to 2007. The new 2007 DEM allowed for clear visualization of strong recession on several Langjökull outlets as well as interior mass loss and terminus advance witnessing to the 1998 surge event of outlet Hagafellsjökull Eystri. In addition, slight interior elevation increase and anti-correlated mass loss and terminal retreat potentially indicate a future surge of outlet Hagafellsjökull Vestari. In sum, the technological and glaciological information put forward in this study provides a method for innovative cryospheric research, presents a much needed benchmark and update on the state of Langjökull, and ultimately facilitates and encourages continued monitoring of highly important smaller glaciers and icecaps

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This paper was published in Apollo.

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