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GIS-based risk analysis of ferrous water mains
Published in Mark Knight, Neil Thomson, Underground Infrastructure Research, 2020
K. Dixon, G. Blakey, J. Whiter, V. Hope
For mapping road slopes and elevations, DEM are used. Interpolated spot height data from 1:10,000 scale surveys have been obtained at 10 m centres with a height accuracy of ± 1.5 m in the London region. These data are augmented by data from the TWUL manhole cover elevation data set, which is derived from accurate ground surveys and maintained by the company to facilitate sewer maintenance and drainage area modelling. The DEM point data are converted into raster based grids having a cell size of 10 m square, centred upon each DEM spot height. From the elevation grids, ground slopes are calculated using a 3x3 kernel centred upon each of the cells. The generated data consist of the dip (slope) and the bearing of the dip direction (aspect). Contiguous grid cells having the same slope and aspect values are grouped together and the resulting slope, aspect and elevation grids are converted into polygon coverages (ESRI, 1995). These polygon coverages are overlaid successively on the road centreline coverage to allow the slope, aspect and elevation values to be attached to the centrelines as attributes. To determine gradients along roads, road bearing data are obtained from the National Grid co-ordinates of the start and end nodes of each road arc. It is assumed that, aside from the camber, there is no slope across the width of a road. Using its bearing, the slope along the length of a road is calculated by projecting the area slope onto the road.
Assessment of vertical accuracy of open source 30m resolution space-borne digital elevation models
Published in Geomatics, Natural Hazards and Risk, 2021
Rocky Talchabhadel, Hajime Nakagawa, Kenji Kawaike, Kazuki Yamanoi, Bhesh Raj Thapa
Spot heights are the digital point data published by the DoS, Government of Nepal using the topographic map of 1: 25,000/1: 50,000 scale between 1992 and 2001. The elevation of these spot heights is routinely validated using the Global Positioning System (GPS) during field visits. In the DoS, the topography data are available in two formats: one as contours and another as spot heights. Other available digital data are transportation (roads, trails), land cover, hydrography (river edges), designated area (national parks and protected areas), administrative area, place name, and building (settlement). These databases are based on scanning original maps, vectorization, and several cleaning processes (Chhatkuli 2003). In general, the spot height informs the hill's peak, the average height of flat land, and overall elevation of locally high positions. The distance between adjacent spot heights varies from approximately 200 m to a few km. There has been a continuous updating of topographic base maps using several latest remote sensing techniques. Detailed information could be found in Wagle and Acharya (2020). The river cross-section data were measured on the ground using the total station and level machine. The total station contains a very accurate clinometer which computes elevation differences with accuracy on a cm scale. Relative horizontal distance and elevation differences between the measuring point and targets are calculated based on measured sloping distances and angular differences. The level machine measures a height difference using a highly accurate bubble level, but it can only measure in a horizontal plane. Therefore, it is used to transfer vertical distance from one station to another. And total stations are set on these locations to measure the elevation at several target locations. These data are one of the accurate datasets available for the study area. The same data are used in different studies (Devkota et al. 2020), national documents (ADB 2016), and for operational flood forecasting and early warning systems (DHM 2018).