14.2.2 Digital Elevation Models
Perhaps the most significant development in geomorphological data collection within the past several decades has been the introduction of widely available surface elevation datasets, derived using various techniques, including traditional photogrammetry, radar altimetry, aerial laser scanning (ALS), and terrestrial laser scanning (TLS) (both varieties of light detection and ranging (LiDAR)), and interferometric synthetic aperture radar (IfSAR, also InSAR), as well as multi- and single-beam sound navigation and ranging (SoNAR), swath bathymetry, and seismic sounding in marine environments (Table 14.2). Digital elevation data are commonly processed into a gridded, regularly sampled dataset, and made available to the end-user as a DEM. Though often (and erroneously) used interchangeably, the term DEM is a superset of the designations ‘digital terrain model’ (DTM) and ‘digital surface model’ (DSM). The latter differ in that DSMs depict the tops of buildings, forest canopies, etc., whereas DTMs represent ‘bare-earth’ models of the ground. DTMs tend to be most suitable for the majority of geomorphological mapping applications, and are now widely regarded as essential tools within the discipline, though require additional processing in order to remove surface clutter and noise (El-Sheimy et al., 2005). Many derivatives can be computed and displayed using DEMs (e.g., slope, fractal, curvature, aspect), however hillshaded or shaded relief surfaces are most intuitive and commonly employed for cartographic and general mapping purposes. The ability of the observer to vary illumination azimuth and sun angle on hillshaded surfaces within GIS environments is a particularly powerful technique and has been demonstrated to improve the detectability of glacial features (Clark and Meehan, 2001; Jansson and Glasser, 2005; Smith and Clark, 2005; Greenwood and Clark, 2008).
Table 14.2. Select Digital Topographic Data Sources With Data Sources Relevant to Glacial Geomorphological Mapping
|Source||Nominal Spatial Resolution||Accuracy|
|Ground survey||Variable, but usually <5 m||Very high vertical and horizontal|
|GPS||Variable, but usually <5 m||Moderate vertical and horizontal, very high vertical and horizontal (dGPS)|
|Photogrammetry (commerical optical sensors)||<1 m||Very high vertical and horizontal|
|LiDAR||<1–3 m||0.15–0.11 m vertical, 1 m horizontal|
|InSAR/IfSAR||2.5–5 m||1–2 m vertical, 2.5–10 m horizontal|
|SRTM, Band C||90 (30) m||16 m vertical, 20 m horizontal|
|SRTM, Band X||30 m||10 m vertical, 6 m horizontal|
|Terra ASTER (GDEM)||30 m||7–50 m vertical, 7–50 m horizontal|
|SPOT 5 (Stereo-Pair Mode)||30 m||10 m vertical, 15 m horizontal|
|TerraSAR-X DSM||10 m||5–10 m vertical, 5–10 m horizontal|
Photogrammetrically derived DEMs rely on the collection and processing of repeat-pass (steerable sensor array), or single-pass (multiple fore/aft sensors) stereographic satellite imagery. Terra ASTER, SPOT 5, and a number of more recently launched commercial VHR sensing systems support stereo-pair acquisition modes. Terra ASTER possesses a second, aft-looking sensor, and has been used to produce a near-global (83 degrees North–83 degrees South) photogrammetric DEM (GDEM and GDEM-2) from along-track, single-pass imagery at 1 arc-second posting interval (~30 m spatial resolution), though use of this product has been somewhat limited in palaeoglaciological applications (e.g., Lytwyn, 2010).
Unlike passive RS systems (e.g., VNIR) that rely on energy emission from the Sun and measurement of target surface reflectance or thermal emission, radar is an active form of RS which emits its own EM energy, and utilizes sensors that collect both wave phase and amplitude from backscattered signals. Signal backscatter information can be combined from different vantage points to construct an interferogram, where phase offsets reflect proportional displacements in surface height (Burgmann et al., 2000; Farr, 2011), thus forming the basis of topographic measurement using InSAR. Interferograms can be acquired in repeat passes with a single antenna, or instantaneously using two antennae. NASA’s Shuttle Radar Topography Mission (SRTM) is a popular example of spaceborne, single-pass InSAR. Flown onboard the Space Shuttle Endeavour over 11 days in February, 2000, SRTM generated a continuous DEM between latitudes 60 degrees North and 56 degrees South at 3 arc-second posting interval (~90 m spatial resolution), and 1 arc-second post spacing (~30 m spatial resolution) for the United States and Australia. These data have been extensively accessed for glacial geomorphological mapping applications, despite their relatively coarse spatial resolution and lack of coverage at high latitudes (e.g., Blundon et al., 2009; Ross et al., 2009; Shaw et al., 2010; Evans et al., 2014). More recently, the commercial collection of airborne InSAR has procured seamless digital elevation products at very high resolution (<5 m) for large areas of the globe. In particular, Intermap’s NextMap series of products provides coverage across most of the United States and Western Europe, and has been used widely in palaeoglaciological mapping applications throughout those areas (e.g., Everest et al., 2005; Smith et al., 2006; Bradwell et al., 2007; Finlayson and Bradwell, 2008; Livingstone et al., 2008, 2010, 2012; Clark et al., 2009, 2012; Evans et al., 2009; Finlayson et al., 2010; Hughes et al., 2010, 2014; Knight, 2010; Phillips et al., 2010; Spagnolo et al., 2011; Margold and Jansson, 2012).
With the ability to generate high-quality digital terrain representations at decimetre-scale spatial resolution, LiDAR has expanded rapidly in recent years and now arguably represents the forefront of terrestrial elevation capture methods. Applications in glacial geomorphology and palaeoglaciology have been substantial and are already too numerous to list, as collection has become widespread, often funded by national or regional survey initiatives. Airborne LiDAR instrumentation, or ALS systems, utilize scanner mounts beneath an aircraft platform that transmit many thousands of light pulses per second. Return times and intensity (sometimes multiples for each pulse) are recorded by a sensor, and the delay between transmission and reception is used to determine elevation (Baltsavias, 1999; Lillesand et al., 2008). A 3D vector point cloud is then generated by integrating positional data from the scanner or platform mount using a differential global positioning system (dGPS), allowing for either direct manipulation within GIS, or subsequent interpolation, using one of several methods, and generation of a gridded DEM (Pfeifer and Mandlburger, 2009). All but final returns within the point cloud can be processed out in order to generate ‘bare-earth’ DTMs with high precision and vertical accuracy, even in heavily forested regions (e.g., Haugerud et al., 2003). The unprecedented detail of LiDAR data comes with the caveat of requiring computer hardware and software capable of effectively handling such large datasets. In certain instances, LiDAR derivatives are resampled to coarser resolutions to reduce computational storage and processing requirements.
Paralleling developments in the terrestrial domain, multibeam echo-sounding has revolutionized palaeoglaciology by providing bathymetric data of the geomorphic imprint produced by the expansive lacustrine and marine-based sectors of former glaciers and ice sheets. With optimal processing, current SoNAR technologies permit cm-scale resolution in shallow waters, and <25 m resolution at depths <1000 m. Integration of marine and terrestrial remotely sensed datasets has the potential to produce more holistic understandings of glacier and ice sheet systems, although synergistic uses have been limited (e.g., Stoker et al., 2009; Freire et al., 2015; Greenwood et al., 2015).