Chris H. Hugenholtz

Small unmanned aircraft systems for remote sensing and Earth science research

To understand and predict Earth-surface dynamics, scientists often rely on access to the latest remote sensing data. Over the past several decades, considerable progress has been made in the development of specialized Earth observation sensors for measuring a wide range of processes and features. Comparatively little progress has been made, however, in the development of new platforms upon which these sensors can be deployed. Conventional platforms are still almost exclusively restricted to piloted aircraft and satellites. For many Earth science research questions and applications these platforms do not yet have the resolution or operational flexibility to provide answers affordably. The most effective remote sensing data match the spatiotemporal scale of the process or feature of interest. An emerging technology comprising unmanned aircraft systems (UAS), also known as unmanned aerial vehicles (UAV), is poised to offer a viable alternative to conventional platforms for acquiring high-resolution remote sensing data with increased operational flexibility, lower cost, and greater versatility (Figure 1).

Subglacial bed form morphology controlled by ice speed and sediment thickness

Subglacial bed forms (drumlins, ribbed moraines, and megascale glacial lineations) are enigmatic repetitive flow-parallel and flow-transverse landforms common in glaciated landscapes. Their evolution and morphology are a potentially powerful constraint for ice sheet modeling, but there is little consensus on bed form dynamics or formative mechanisms. Here we explore shallow sediment bed form dynamics via a simple model that iterates (i) down-flow till flux, (ii) pressure gradient-driven till flux, and (iii) entrainment and deposition of sediment. Under various boundary conditions, replicas of subglacial bed forms readily emerge. Bed form dynamics mirror those in subaqueous and aeolian domains. Transitions between ribbed moraines and elongate flow-parallel bed forms are associated with increasing ice speeds and declining sediment thickness. These simulations provide quantitative flux estimates and suggest that widely observed transitions in shallow sediment subglacial bed forms (e.g., ribbed moraines to drumlinoids to megascale glacial lineations) are manifestations of subtle variations in ice velocity and sediment thickness.

Earthwork Volumetrics with an Unmanned Aerial Vehicle and Softcopy Photogrammetry

AbstractSmall unmanned/uninhabited aerial vehicles (UAVs) are quickly becoming a cost-effective alternative for mapping and volumetrics, particularly for small- to medium-sized earthwork projects. However, the accuracy of photogrammetrically derived digital terrain models (DTMs) from UAV imagery is not extensively tested. This gap is addressed through a case study of stockpile volume estimation. A gravel stockpile was surveyed with a vertical take-off and landing UAV before and after a portion of it was excavated. Softcopy photogrammetry was applied to the UAV images to produce before and after DTMs, each with a resolution of 3.5 cm. The vertical accuracy of the UAV DTMs was estimated with global positioning system (GPS) test points, yielding RMS errors of 0.106 (before) and 0.097 m (after). These errors are similar to, if not lower, than those produced by airborne light detection and ranging (LIDAR), in general, but are higher than for terrestrial laser scanning. The before and after volume of the stockp...

Yardang evolution from maturity to demise

Yardangs are enigmatic wind-parallel ridges sculpted by aeolian processes that are found extensively in arid environments on Earth and Mars. No general theory exists to explain the long-term evolution of yardangs, curtailing modeling of landscape evolution and dynamics of suspended sediment release. We present a hypothesis of yardang evolution using relative rates of sediment flux, interyardang corridor downcutting, yardang denudation, substrate erodibility, and substrate clast content. To develop and sustain yardangs, corridor downcutting must exceed yardang vertical denudation and deflation. However, erosion of substrate yields considerable quantities of sediment that shelters corridors, slowing downcutting. We model the evolution of yardangs through various combinations of rates and substrate compositions, demonstrating the life span, suspended sediment release, and resulting landscape evolution. We find that yardangs have a distinct and predictable evolution, with inevitable demise and unexpectedly dynamic and autogenic erosion rates driven by subtle differences in substrate clast composition.

Estimating aerodynamic roughness (zo ) in mixed grassland prairie with airborne LiDAR

In this research note we show that airborne imaging light detection and ranging (LiDAR) is capable of estimating the aerodynamic roughness height (zo ) in a mixed grassland prairie. This is accomplished by establishing empirical relations between wind profile derived estimates of zo and vegetation height measurements from the airborne LiDAR data. We show that up to 65% of the variation of zo can be explained by LiDAR estimates of vegetation height and that up to 76% of the variation can be explained by the height variability of vegetation. We also show the effects of different filter sizes used to identify the ground surface and the top of the vegetation in the LiDAR point cloud data. Overall, results from this investigation are encouraging for future spaceborne LiDAR missions, especially in terms of the potential for providing new insight on spatial and temporal patterns of zo .

Predictability of dune activity in real dune fields under unidirectional wind regimes

We present an analysis of 10 dune fields to test a model-derived hypothesis of dune field activity. The hypothesis suggests that a quantifiable threshold exists for stabilization in unidirectional wind regimes: active dunes have slipface deposition rates that exceed the vegetation deposition tolerance, and stabilizing dunes have the opposite. We quantified aeolian sand flux, slipface geometry, and vegetation deposition tolerance to directly test the hypothesis at four dune fields (Bigstick, White Sands Stable, White Sands Active, and Cape Cod). We indirectly tested the hypothesis at six additional dune fields with limited vegetation data (Hanford, Ano Nuevo, Skagen Odde, Salton Sea, Oceano Stable, and Oceano Active, “inverse calculation sites”). We used digital topographic data and estimates of aeolian sand flux to approximate the slipface deposition rates prior to stabilization. Results revealed a distinct, quantifiable, and consistent pattern despite diverse environmental conditions: the modal peak of prestabilization slipface deposition rates was 80% of the vegetation deposition tolerance at stabilized or stabilizing dune fields. Results from inverse calculation sites indicate deposition rates at stabilized sites were near a hypothesized maximum vegetation deposition tolerance (1 m a−1), and active sites had slipface deposition rates much higher. Overall, these results confirm the hypothesis and provide evidence of a globally applicable, simple, and previously unidentified predictor for the dynamics of vegetation cover in dune fields under unidirectional wind regimes.

CCDST: A free Canadian climate data scraping tool

In this paper we present a new software tool that automatically fetches, downloads and consolidates climate data from a Web database where the data are contained on multiple Web pages. The tool is called the Canadian Climate Data Scraping Tool (CCDST) and was developed to enhance access and simplify analysis of climate data from Canadas National Climate Data and Information Archive (NCDIA). The CCDST deconstructs a URL for a particular climate station in the NCDIA and then iteratively modifies the date parameters to download large volumes of data, remove individual file headers, and merge data files into one output file. This automated sequence enhances access to climate data by substantially reducing the time needed to manually download data from multiple Web pages. To this end, we present a case study of the temporal dynamics of blowing snow events that resulted in ~3.1 weeks time savings. Without the CCDST, the time involved in manually downloading climate data limits access and restrains researchers and students from exploring climate trends. The tool is coded as a Microsoft Excel macro and is available to researchers and students for free. The main concept and structure of the tool can be modified for other Web databases hosting geophysical data.

Quantifying the effects of terrestrial laser scanner settings and survey configuration on land surface roughness measurement

The application of terrestrial laser scanning (TLS) for measuring Earth surface features is increasing. However, TLS surveys require users to choose and specify certain properties of the scan (i.e., resolution, height, distance, number of scan positions), often with limited understanding of how these properties affect the accuracy of the data. This paper presents results from an experiment that quantifies the effects of different scan settings and survey configurations on the measurement of centimeter-scale surface roughness. The main goal is to provide quantitative evidence to help guide and optimize field-based surface roughness measurements involving TLS data. The experiment involved an array of artificial roughness elements placed on an asphalt surface, similar to the approach of using inverted buckets in boundary layer experiments to simulate a rocky or sparsely vegetated surface with smooth interspaces. The independent variables consisted of laser point spacing, number of scan positions, and the height and distance of the scanner relative to the roughness array. The dependent variables were roughness element height, data occlusion, relative vertical accuracy, the root mean square height of the cup array, and the relative roughness of the asphalt surface. Two roughness patterns were tested, isotropic and anisotropic. Results show that when the laser point spacing was greater than the size of the individual roughness elements, their calculated height was between 32% and 73% below their actual height, but with a smaller spacing the calculated height was either equivalent to their actual height or only slight lower. Therefore, before a TLS survey is undertaken, manual measurements of roughness elements should be used to determine the size of the smallest roughness elements of interest, thus guiding the selection of laser point spacing. Larger point spacing also decreased the vertical accuracy of surfaces interpolated from the point clouds compared to global positioning system points. By combining point clouds from three scan stations arranged in a triangular network around the roughness array, the proportion of data occlusion decreased to as little as 0.24%, but due to error associated with point cloud registration, the accuracy of roughness element height decreased and the roughness of the asphalt surface artificially increased. For the most accurate measurements of the roughness element height and interspace roughness our results suggest that high-resolution point clouds obtained from one vantage point should be used. In regard to scanner height and distance, we measured a doubling of the occluded area when the scanner height decreased and distance to the array increased, thus increasing the angle of incidence. Reducing the angle of incidence decreases occlusion but also limits the areal coverage of each TLS scan and increases the number of scan stations at different vantage points required to cover large areas, which ultimately affects the accuracy of other roughness metrics. Overall, this case study demonstrates that there are trade-offs in that the optimization of one metric (e.g., roughness element height) can have negative effects on another (e.g., data occlusion). The choice of TLS settings and survey configuration, therefore, influences the accuracy of surface roughness measurements.

Restoring Industrial Disturbances with Native Hay in Mixedgrass Prairie in Alberta

Native grassland restorations are often unsuccessful as a consequence of unreliable seed sources and competition from weeds and agronomic species. To improve on conventional approaches, we tested whether native hay can be used as a seed source for restoring native mixedgrass prairie on oil and gas well sites and associated disturbed areas. We also assessed the recovery of grassland that had been cut to harvest native hay. We assessed three wellsites seeded with native hay seven years earlier to evaluate longer term recovery, and they showed significant similarity to controls in adjacent grassland. Additionally, in 2012, five wellsites and an access road were seeded with native hay cut from grassland close to the sites. Grassland cut for native hay recovered within one year, showing similar species composition. In the second year, native hay restored wellsites had significant weedy annual cover; nevertheless, native grasses and forbs germinated, particularly needle grasses, wheatgrasses, and bluegrasses. Three of the native hay restored wellsites were sprayed with a non-selective herbicide in the third year; however, the remaining sites showed good recovery in the third year, with native grasses replacing most of the original weedy species. Collectively, results from this research suggest seeding from native hay is a successful and sustainable technique for restoring native vegetation cover and diversity on industrial disturbances in native grasslands.