OpenTopography Tools
For datasets hosted by OpenTopography, data access is co-located with tools to enable on-demand data processing, visualization, and analysis. These tools are briefly described below. To apply these functions to a dataset search for data HERE.
DEM Generation
TIN (Triangulated Irregular Network)
The TIN (Triangulated Irregular Network) interpolation algorithm offered by OpenTopography is an implementation of Martin Isenburg's high-performance code for generating raster digital elevation models (DEMs) from mass points via streaming Delaunay triangulation. A TIN model represents the landscape as a surface composed of contiguous and non-overlapping triangles constructed between the cloud of lidar returns. Within each triangle the surface is represented by a plane. A raster DEM is then constructed by sampling the triangulated surface elevation at the DEM nodes. More information on Isenburg's streaming TIN is available at: http://www.cs.unc.edu/~isenburg/tin2dem/. TIN-based DEM generation is offered as part of the LAStools suite developed by Isenburg.
Local Gridding
This local binning algorithm utilizes the elevation information from lidar returns contained within a circular search area (radius defined by the user) to calculate the DEM elevation at each grid cell (resolution specified by the user). Five values can be computed for each node in a grid: 1) the minimum, 2) maximum, 3) mean, and 4) inverse distance weighted (idw) mean of the local points, and 5) the number of points in the search area. If the number of points in the search radius is 0, the grid node is assigned a null value. The null filling option uses a moving window of 3, 5, or 7 pixels if chosen by the user to fill null pixels that in the grid. This algorithm was initial developed by OpenTopography as Points2Grid. The algorithm has since been integrated into the open source Point Data Abstraction Library (PDAL) as writers.gdal which is the implementation OpenTopography is using.
Derivative Products
OpenTopography will generate basic raster derivative products from the DEMs produced using the gridding algorithms described above. Derivatives can not be produced from the point count grid product because it is not a DEM.
Hillshade: Shaded relief grid derived from the DEM. Sun is located at 345 degrees (NW) and is 45 degrees above the horizon.
Slope: Grid of topographic slope derived from the DEM. Values are in degrees.
Visualization
Raster Visualizations
OpenTopography will generate hillshade browse images from DEMs that can be viewed as static imagery, overlain in Google Maps, or as Google Earth image overlays (.kmz). An optional color ramp can also be applied to the imagery.
3D Point Cloud Visualization
The 3D point cloud browser visualization allows users to view and manipulate point cloud data in a Web browser. Currently this capability is tested and working with Google Chrome, Firefox and Safari, but not yet in Internet Explorer browsers. It is expected that most WebGL-capable browsers should be able to support this visualization. 3D point cloud visualization leverages Entwine data organization library (entwine.io), Greyhound point cloud streaming (greyhound.io) and Potree open-source WebGL based point cloud renderer (potree.org).
Hydrologic Terrain Analysis Products (TauDEM)
OpenTopography will run hydrologic terrain analysis functions from TauDEM (http://hydrology.usu.edu/taudem) on the digital elevation model produced from a triangulated irregular network (TIN) generated digital elevation model. TauDEM requires a digital elevation model without gaps which is why grid generation is limited to TIN. TauDEM results are returned as multiple GeoTIFF files with names similar to felp0r0c0.tif, felp1r0c0.tif,... Collectively these comprise a rectangular tiling of files that cover the area computed. Multiple files are used so that the 4 GB limitation of single GeoTIFF files is not limiting for large grids. The file names do not matter, but if you must know, the prefix refers to the TauDEM product being produced and the numbers following p, r, c refer to the processor, row and column in the file tiling produced.More information about TauDEM can be found at: http://hydrology.usu.edu/taudem/taudem5/
Global Solar Irradiation
(currently only available for global raster datasets)
This tool calculates monthly global (beam + diffuse + indirect) solar irradiation, and hours of sunlight based on the DEM or DSM surface. Surface shading effects are turned on. Atmospheric turbidity and surface albedo use default parameters for GRASS 7.2 R.Sun.mp Global_Radiation: Monthly sum of global irradiation (Wh.m-2.day-1) for each pixel. Hours: Monthly sum of hours each pixel receives sunlight. For more details see https://grass.osgeo.org/grass72/manuals/r.sun.html