Name: Thermokarst Photos

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Description: GeneralFlight lines were planned using a priori information including known areas of exemplary thermokarst landscapes, existing permafrost study sites, a 2016-2017 Sentinel-2 mosaic of 10-m spatial resolution, and a NWT-wide change detection product based on Landsat Tasseled Cap trend analysis (courtesy of Government of Northwest Territories, NWT Centre for Geomatics). Surveys were planned throughout the Taiga Plains (except for the most southern region), Taiga Cordillera (Mackenzie Valley Foothills, Richardson Plateau, Mackenzie Mountains), Taiga Shield (High Boreal, Low Boreal, Low Subarctic), and the Southern and Northern Arctic.The foci areas were covered to the degree possible given weather, aircraft and fuel supply limitations. The aerial survey spanned a total of 50 suitable flying days within a two-year period. Both Bell 407 and Aerospatiale A-Star helicopters were used for survey work. The airborne transects totaled over 26,500 km. From an regional representation perspective, the transects intersected seven Level II ecoregions in the NWT (Taiga Cordillera/Plains/Shield, Boreal/Tundra Cordillera as well as Southern/Northern Arctic, six mainland Level III ecoregions in the NWT, and 78 Level IV ecoregions (Ecosystem Classification Group, 2009).Survey altitude averaged 250 to 350 m Above Ground Level for all regions, but depended daily on the type, topography and weather conditions of the transect. Surveys were conducted at typical cruise speeds of 190-200 km/h (GPS ground speed). Hard-copy maps and Garmin handheld GPS units (62S, Montana 680) were used for navigation, and transect lines typically deviated from what was planned a prioridue to weather or site prioritization.Along the transects a detailed and large-scale record of thermokarst and permafrost landscape features was captured via digital images, geographic locations (tracklogs) and tabulated observational information using ESRI Survey123. Digital images were captured with a Nikon D5600® 24 megapixel digital single-lens reflex camera with Nikon 18-200mm f/3.5-5.6G ED VR II lens and Nikon GP-1 external GPS unit; a Nikon D750® 24 megapixel digital single-lens reflex camera with various lenses (A Tamron 24-70 f2.8, Nikon 50 mm f1.8, and a Nikon 24mm f2.8), and a Sony RX100 III® 20 megapixel 1” sensor compact camera with F1.8-2.8 Carl Zeiss Vario-Sonnar T* lens. The Nikon camera was predominantly operated behind the permafrost scientist (left passenger seat in cockpit) and primarily acquired observations of mass-wasting, hydrology, and periglacial features and landscapes, whereas the Sony camera was operated on the opposite (pilot) side, acquiring similarly featured photos as well as photos depicting general landscape conditions, disturbances and vegetation cover. Over a two-year period 30,961 images were collected that passed the post-processing sequence. These images have formed an invaluable record of current thermokarst, vegetation and landform conditions that can be used in many subject areas, including monitoring of environmental change.Data governanceData owner and discipline authority: NT Geological Survey.Data custodian: NWT Centre for Geomatics.Data stewards: shared responsibility between NT Geological Survey and NWT Centre for Geomatics.Date of last update: November, 2022.Update cycle: annually.Suggested citation for photo dataset: Van der Sluijs, J., Kokelj, S.V., Rudy, A.C.A, 2022. Geomatics field- and web-methodologies supporting a systematic aerial inventory and characterization of thaw-sensitive permafrost terrain in the Northwest Territories.Post-processing informationSeveral photo acquisition challenges complicated data management of the acquired images, including a relatively slow GPS lock of the Nikon GP-1 leading to missing geo-tags (20-30% of cases in year 2020), non-unique photo filenames as well as the presence of blurry or non-relevant photos (e.g., ground photos, wildlife). A post-processing pipeline was implemented to: screen out blurry or non-relevant photos using freeware program PixelPeeper v1.2 (Textures.com, 2016);Batch resize remaining raw photos to half-scale, conduct auto-colour corrections based on histogram stretching (to overcome haze and absence of circular polarizing or UV filter), Apply GNWT Visual Identity branding and compress photos to 90% quality using freeware program Irfanview (Skiljan, 2017);Automatically rename each photo based on date/time/sensor while preserving original filename for cross-reference to Survey123 photo comments using freeware program Darktable (Darktable Team, 2020);Screen photos for the presence of GPS location in EXIF metadata via a custom script developed using the ESRI ArcPy python 2.7 site package, and geo-tag those photos with missing tags using freeware program GeoSetter and the Garmin handheld GPS-tracklogs (interpolated synchronization up to 60 sec with time adjustments);ESRI ArcGIS® Pro script “GeoTagged Photos to Points” was used to generate a hyperlinked point feature class in a file geodatabase. In total 30,961 geo-referenced digital images were available after the post-processing workflow (Nikon: n=19,266, or 62% of total, Sony: n=11,893, or 38% of total). On average, a geo-referenced image was collected every 1-2 km, or about every 25-40 seconds, albeit it that the actual density varies depending on several factors. The accuracy of the single-frequency GPS geotags (generally; 10 m horizontal; 30 m vertical) is known to decrease with increasing speed of movement. In general users can expect a horizontal accuracy of about 50-100 m for the photo locations, however, the absolute accuracy of the geotags was not tested and is therefore unknown. The Nikon GP-1 geotags, although featuring built-in functionality, were sometimes not available or of suspect quality (e.g., a 150-800 m offset away from the flightline). Missing Nikon geotags were interpolated using handheld GPS tracklogs. Nikon photos >150 m from a flightline were manually adjusted. Location information for the non-geotagged Sony photos was also determined using interpolated time synchronization between the photo timestamp and the handheld GPS tracklogs. Time synchronization was resolved to within several seconds, meaning that data users can expect a horizontal offset due to the high speed of movement (e.g., a ground speed of 200 km/h is synonymous to covering a 555 m distance in a 10-sec window). For the purposes of regional permafrost observations, the offsets were deemed unimportant, yet users should be aware of these offsets for applications requiring more precise positions. Ground elevations (Zground) and Altitudes were approximated in reference to the European Space Agency's Copernicus Global Digital Elevation Model (GLO-90) without corrections for vertical datum differences, thus uncertainties and invalid altitude values may exist. Citation for GLO-90: European Space Agency, Sinergise (2021). Copernicus Global Digital Elevation Model. Distributed by OpenTopography. https://doi.org/10.5069/G9028PQB. Accessed: 2022-06-02.

Copyright Text: The data collection and processing procedures were produced by J. van der Sluijs, S.V. Kokelj and A.C.A. Rudy, Northwest Territories Centre for Geomatics and Northwest Territories Geological Survey as a data product to support the Thermokarst Collective Permafrost Mapping Project.

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