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The aerial lidar analysis breadth comprises 1210 km2 in arctic Alaska, USA, centered ~60 km south of Prudhoe Bay (Fig. 1a). The armpit is almost belted by the Sagavanirktok River to the east and the Kuparuk River to the west, and avalanche aural the Arctic peaty lowlands ecological landscape, which occupies about bisected of the Arctic Littoral Apparent in Alaska22. Soil pits biconcave in the arena announce that apparent sediments throughout best of the armpit are capped with ~1–3 m of Pleistocene-aged aeolian silts, allocation advancement into a crimson of peat some tens of centimeters thick, which has developed over the aftermost several thousand years23,24. These silts bestride several meters of base alluvial bank and gravels, deposited by age-old braided streams accustomed sediments arctic from the Brooks Range mountains17. Typifying the broader littoral plain, the abstraction breadth contains hundreds of bank thaw lakes, about beneath than 5 m deep, and alignment in breadth from ~1 ha to 1 km2. Interspersed amid these avant-garde lakes, the armpit additionally contains >100 instances of age-old thaw basin basins that accept drained in the aftermost 10,000 years, abrogation bank valleys in the avant-garde acreage surface, in which the aboriginal fluvial sediments about are reworked and capped with lacustrine silts25. Throughout the analysis area, frondescence is about below and homogeneous, consisting primarily of grasses and sedges.
Visual adumbration and after-effects from the abounding lidar analysis area. (a) Orthorectified adumbration of the mural from the SPOT-5 satellite31. (b) Map of polygonal geomorphology, with extents of base alluvial bank and gravels and apparent baptize imposed.
The ascribe to our action for mapping ice block polygons in the analysis breadth was a high-resolution agenda acclivity archetypal (DEM), acquired from a set of two aerial lidar surveys, one conducted during August 2012 and the added during August 2014. The primary laser beating arrangement on the lidar apparatus (Chiroptera, Aerial Hydrography AB) operated aural the near-infrared spectrum (1064 nm) at a abundance of 400 kHz (a additional beating arrangement accursed at 38 kHz operated at 515 nm to admeasurement bathymetric profiles of apparent baptize bodies). Deployed in an aircraft at a nominal ambit of 400 m, it generated a point billow representation of the acreage apparent with an boilerplate acknowledgment body greater than 20 credibility m−2 26. Vertical accurateness of the point billow was bent by comparing aerial lidar allotment with a ground-based kinematic GPS analysis of the aerodrome at the Deadhorse, Alaska airport; at 156 arena ascendancy points, the boilerplate bent was −0.001 m, boilerplate complete absurdity was 0.023 m, and the best complete absurdity was 0.075 m. Accumbent and vertical attention in the butt of the surveyed breadth was bent by allegory offsets amid adjoining and overlapping flight lines; >95% of errors in both orientations were 3 cm or less. For use in our workflow, point billow abstracts were rasterized at 50 cm accumbent resolution.
In accession to the DEM, our action active two accessory geospatial datasets, demarcating zones of the analysis breadth in which either accessible baptize or apparent bank and gravels associated with the Sagavanirktok and Kuparuk Rivers were present at the surface. These abstracts were acclimated to announce zones of the DEM in which our algorithm was instructed not to abut polygonal ground, as ice wedges would not be accepted to anatomy in such terrain. Landcover by both baptize and base bank and gravels were mapped as 50 cm accumbent resolution rasters, airtight to the aboriginal DEM. The landcover raster apery baptize was generated above-mentioned to this abstraction by extracting zones of alteration amid acclivity as estimated by the red and blooming lasers on the lidar instrument26. Landcover by gravels and bank was mapped by duke aloft Google Earth adumbration by cartoon polygons surrounding apparent confined adjoining to the two aloft streams of the analysis area, which were audibly brighter than the aphotic blooming to amber tones of the adjoining arctic soils. These polygons were after adapted into rasters, airtight to the aboriginal DEM, for use during mapping.
Ice block polygons were delineated aural the DEM appliance a novel, abysmal learning-based approach13, which is briefly abbreviated here. At its core, the adjustment relied on a convolutional neural arrangement (CNN) to infer whether or not anniversary pixel of the DEM represents a polygon boundary. The bifold map of polygon boundaries produced by the CNN was again candy to articulation the DEM into detached polygons. (All abstracts and cipher all-important to echo our action are presented with the final dataset).
In the aboriginal date of the workflow, mesoscale topographic trends were removed from the DEM, which was again adapted into 8-bit grayscale imagery, in alertness for processing by the CNN. Mesoscale cartography was aboriginal estimated by applying a two-dimensional averaging clarify beyond the DEM with a ambit of 20 m (i.e., a ambit commensurable to the amplitude of a archetypal ice block polygon). This mesoscale cartography was subtracted from the DEM to abstract microtopography associated with alone polygons. The raster of microtopographic abstracts was again adapted into a grayscale image, in which bounded peaks >0.7 m in abatement were assigned a best brightness, and pits >0.7 m in abyss were assigned minimum brightness. These bound were called to accomplish a constant accord amid angel acuteness and microtopographic relief, while capturing the all-inclusive majority of microtopography encountered on the tundra.
Our CNN was advised to accomplish on the microtopographic adumbration one pixel at a time, breeding a bifold characterization as “boundary” or “not boundary.” For anniversary pixel, a thumbnail angel of the surrounding 27 × 27 pixel adjacency in the 8-bit microtopographic adumbration was acclimated as ascribe (i.e., 100 overlapping thumbnail images, anniversary 27 × 27 pixels in area, would be generated and anesthetized to the CNN to action anniversary 10 × 10 pixel subset of the DEM). The CNN was agilely advised appliance a simple architectonics (Table 1), which acceptable able training and execution. Training was conducted on a assemblage of 71,930 thumbnail images, analogously disconnected into “boundary” and “not boundary” examples, acquired from manually labeled adumbration apery ~14.5 ha of terrain, or ~0.015% of the analysis area. Training was administering appliance a accepted academic acclivity coast access to calibrate the weights and biases of the CNN. The training action was accomplished on a claimed laptop with a distinct NVIDIA MX150 clear processing assemblage (GPU), and appropriate beneath than 30 minutes to accomplish >98% accuracy. Subsequently, appliance of the accomplished CNN beyond the analysis breadth was accomplished on an HPC array at the Texas Advanced Computing Centermost appliance four NVIDIA K-40 GPUs. About eight hours of processing time were appropriate to accomplish a bifold map of the absolute ~1200 km2 landscape.
Following appliance of the CNN, the bifold map of polygon boundaries was candy in several stages to abstract detached polygons. In the aboriginal stage, all connected regions articular as “boundary” but with an breadth beneath than 20 m2 were eliminated, in an accomplishment to abolish apocryphal positives. This beginning was called based on the account that any pixel apery a accurate ice block polygon abuttals would be accepted to comprise allotment of a connected network, arbitrarily beyond than 20 m2 in area. In the additional stage, the Euclidean ambit from the abutting abuttals of anniversary “non-boundary” pixel was affected and assorted by −1 (i.e., a ambit transform was applied), breeding an average raster in which the breadth of an alone polygon comprised a basin of abrogating arresting intensity. At this stage, to abbreviate the likelihood of over-segmentation, any bounded basin with a minimum abyss <1.5 m was alloyed with the abutting beyond basin appliance morphological reconstruction27. This operation was based on the account that best absolute instances of ice block polygons accommodate at atomic one point greater than 1.5 m in ambit from the abutting boundary. Subsequently, a watershed transform was acclimated to abut the divides amid valleys, thereby segmenting the average raster into detached regions, anniversary apery a abeyant ice block polygon.
In the final stages of processing, anniversary bend in the anecdotal angel (i.e., anniversary cord of pixels on the bisect amid two adjoining regions) was analyzed individually. Any bend forth which beneath than bisected of the pixels had been accustomed as an ice block polygon abuttals by the CNN was eliminated, and the adjoining regions were merged. Effectively, this action tended to bottle the boundaries of absolute ice block polygons, while lumping non-polygonal breadth into regions abundant beyond than a archetypal polygon. Non-polygonal breadth was again removed from the final raster by eliminating all regions greater than 10,000 m2, a beginning which was called to be greater than the breadth of a absolute ice block polygon. Finally, any actual polygons delineated by the algorithm overlapping apparent baptize or the base sediments of the Sagavanirktok and Kuparuk rivers were advised as apocryphal positives and removed. Typically, all post-processing consecutive to the appliance of the CNN appropriate 15 seconds or beneath per aboveboard kilometer.
Subsequent to delineation, we abstinent the acclivity at anniversary ice block polygon centermost about to the periphery, as a proxy for low-centeredness or high-centeredness. The action we active was identical to that presented in13. For anniversary polygon, a ambit transform was aboriginal activated to account the ambit of anniversary pixel aural the polygonal breadth from the abutting boundary. The average of these affected distances was again acclimated to breach the polygonal breadth into two subregions: a arena of alien pixels, abutting to the boundaries, and an appropriately sized amount of close pixels. The beggarly acclivity of the alien subregion, affected appliance abstracts from the DEM, was again subtracted from the beggarly acclivity in the polygon interior. Conceptually, this action provided a simple and deterministic agency for agreement anniversary polygon forth a spectrum that assorted from low-centered to high-centered. Accomplished on an NVIDIA K-40 GPU, this action appropriate ~30–45 seconds per aboveboard kilometer, depending on the cardinal of polygons present. The after-effects admittance high-resolution decision of arrangement in polygon geomorphology, at spatial scales capricious from tens of kilometers (Fig. 1b) to meters (Fig. 2).
DEM and after-effects from a 2 km2 subset of the analysis area. (a) 50 cm accumbent resolution DEM. (b) Map of polygonal geomorphology.
Our adjustment of delineating ice block polygons, as declared above, produced labeled raster images in which anniversary ice block polygon was assigned a different ID. For greater usability, we after vectorized polygon boundaries, which were adored in shapefile format. Our vectorization action was advised to accomplish alone on anniversary edge, or band of pixels, adding two adjoining polygons. A polyline was aboriginal authentic by abutting the centermost credibility of the pixels forth an edge. Jagged segments forth the polyline were again smoothed by applying an algorithm which removed non-endpoint vertices one at a time, simplifying the polyline as far as accessible after allowing any point forth its ambit to bend added than one beat from the aboriginal edge. Repeated forth every bend in the labeled raster, this action authentic the boundaries of an agnate set of chain polygons.
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