Arc-second Data Research Articles

Vertical accuracy of the USGS 3DEP program data: study cases in Fresno County and in Davis, California

3D Elevation Program (3DEP) aims to generate and disseminate high-resolution topographic elevation data from the source data products including lidar point clouds, original DEMs from which the 3DEP standard DEM datasets were produced, and additional data types produced from IfSAR collections. As such, the accuracy of 3DEP data varies due to the inconsistent quality of the source data. Hence, in order to test vertical accuracy of the current 3DEP data, two precise leveling data sets which are collected in the San Joaquin Experimental Range (SJER) in Fresno County and CalFire site in Davis, California are used as the baselines and the differences are computed. In the earlier studies, assessment of 3DEP data is accomplished using large-area elevation datasets. Nevertheless, these large-area elevation datasets are not as precise as differential (precise) leveling data sets. In this study, two relatively small sites (SJER and CalFire site) are surveyed utilizing precise leveling. These two project sites also differ from each other in terms of terrain relief and land cover. The results show that attainable precision is almost the same for 1/3 arc-second and 1 arc-second data sets. The data sets used for CalFire site are more precise than the data sets used for SJER site. CalFire site data sets are more accurate than SJER data sets. 1 arc-second data provides as good elevation information as 1/3 arc-second data. Terrain relief and land cover are important factors on vertical accuracy coming from 3DEP data.

Geomorphic signatures of active tectonics in Subansiri River Basin, eastern Himalayas

The Subansiri, a major tributary of the Brahmaputra with its catchment area (35763 km2) spreading almost entirely in the Eastern Himalayas across almost all the major and local tectonic features in the area witnesses large numbers of seismic events. Active tectonic indices like relief and slope, drainage pattern, longitudinal profile, valley profile, hypsometry, valley asymmetry factors and transverse topographic symmetry index, stream length gradient, valley floor-height ratio extracted from SRTM 3 arc-second data prove that the evolving basin morphology has substantial contribution from the Himalayan tectonics. Seismic data are incorporated in the study to establish the potentially active tectonic elements in the catchment area. The study shows that the western part of the Subansiri River Basin is profoundly tilted towards north in the upper catchment and towards east in the lower and middle part of the catchment. The predominant tectonic movements in the western part of the basin caused the tilting of the basin towards north in the upstream and towards east in the middle and lower parts.

Overflow probability of the Salt Lake in Hoh Xil Region

After the bursting of Huiten Nor in Hoh Xil Region in September, 2011, the topic on whether the water overflowed from the Salt Lake would enter into the Chumaer River and become the northernmost source of the Yangtze River has aroused wide concern from public and academic field. Based on Landsat TM/ETM+/OLI remote sensing images during 2010–2015, SRTM 1 arc-second data, Google Earth elevation data and the observation data from the Wudaoliang meteorological station, the study initially analyzed the variations of the Salt Lake and its overflowing condition and probability. The results showed that the area of the Salt Lake expanded sharply from October 2011 to April 2013, and then it stepped into a stable expansion period. On October 27, 2015, the area of the Salt Lake had arrived at 151.38 km2, which was about 3.35 times the area of the lake on March 3, 2010. The Salt Lake will overflow when its area reaches the range from 218.90 km2 to 220.63 km2. Due to the differences between SRTM DEM and Google Earth elevation data, the water level of the Salt Lake simulated would be 12 m or 9.6 m higher than the current level when the lake overflowed, and its reservoir capacity would increase by 23.71 km3 or 17.27 km3, respectively. Meanwhile, the overflowed water of the Salt Lake would run into the Qingshui River basin from its eastern part. Although the Salt Lake does not overflow in the coming decade, with watershed expansion of the Salt Lake and the projected precipitation increase in Hoh Xil region, the probability of water overflow from the Salt Lake and becoming a tributary of the Yangtze River will exist in the long term.

Evaluation of the 15 July 2009 Fiordland, New Zealand Tsunami in the Source Region

We undertake detailed near-field numerical modelling of the tsunami generated by the 15 July 2009 earthquake (Mw 7.8) in Fiordland, New Zealand. High resolution bathymetry and topography data at Breaksea and Dusky Sounds, and Chalky and Preservation Inlets are derived mostly from digitised New Zealand nautical charts, Shuttle Radar Topographic Mission (SRTM) 3 arc-second data, and General Bathymetric Chart of the Ocean (GEBCO) 30 s data. A combination of continuous and campaign Global Positioning System (GPS), satellite radar (ALOS/PALSAR InSAR images) and seismology data are used to constrain the seafloor deformation for the initial tsunami condition. This source model, derived independently of DART observations, provides an excellent fit to observed tsunami elevations recorded by DART buoy 55015. The model results in the near field show maximum tsunami elevations in the range 0.5–2.0 m inside the sounds and inlets with maximum flow speeds of 3.0 m/s. Along the open coast, maximum tsunami elevations reach 2.0 m. The high flow speeds through the inlets may change the inlet stratifications and water mass inside the sounds. Media reports and field reconnaissance data show some tsunami evidence at Cormorant Cove, Duck and Goose Coves, and Passage Point.

On the relationship between BL Lacertae objects and Fanaroff-Riley I radio galaxies

The idea that Fanaroff-Riley Class~I Radio Galaxies contain core jets with Lorentz factors of the order of a few and are the parent population for BL~Lac Objects is examined, with particular reference to the data on two FRI Radio Galaxies, NGC~315 and NGC~6251. Conservation laws for an entraining relativistic jet are derived and are used to determine a relationship between Mach number and velocity for initially relativistic jets. One immediate consequence of this relationship is that, if an initially relativistic jet is decelerated to a transonic Mach number between, say 0.5 and 1.5, then its velocity at this point is between 0.3 and 0.7 times the speed of light. Analysis of the milliarcsecond and arcsecond data on NGC~315 and NGC~6251 shows that the Lorentz factors of the core jets in these galaxies can be as high as 2-4 provided that the jet pressures are not much more than an order of magnitude higher than the minimum pressures. Such Lorentz factors are more plausible if the jets are inclined at $\sim 30$ degrees to the line of sight, although larger angles would also be possible. For core Lorentz factors in the range 2-4, the jets are mildly relativistic on the large scale and their velocities are consistent with the jet brightness asymmetries being caused by Doppler beaming. The Lorentz factors required for significant beaming of core jets may be reconciled with subluminal motions of knots in the core if these are reverse shocks advected by the jet. Moderate jet Lorentz factors are possible if the shocks are normal and higher Lorentz factors are possible if the shocks are oblique. This paper provides support for the idea that the transition from two-sided to one-sided jets across the FRI/II break is due to the transition from