Gridding Settings

The Geospatial tab allows for conversion of gridded elevation or non-gridded XYZ point cloud data The GRID interface is able to create an elevation mesh display from grid or scatter data, with annotations such as legend, compass, and 3D axes display.

The Grid settings are related and used in conjunction with the Geospatial tab and Interface Settings Color Mapping and Elevation tab.

Below is an overview of the Gridding tab user interface:

Gridding Parameters

  • Subsampling: Before a loaded grid is decimated or the loaded point cloud is processed, the mesh can be reduced by a factor of Subsampling and the number of sampled points within a point cloud is reduced by the same factor. As illustration, a 100x100 input grid, with Subsampling=2, will skip every 2nd row and column, to create a 50x50 grid, containing 1/4 the original data. In the case of a point cloud, every Nth input point will be used.
  • Skip Full Grid is important for large grids. When Subsampling is employed, normally the original high resolution grid data is directly converted to a large texture image, using the loaded colormap file. This may generate a very large image, draped onto a reduced resolution geometric grid. To further reduce memory requirements, processing time and file size, enable Skip Full Grid so that the texture map is generated at the reduced Subsampling grid size, rather than the original input file grid size.

  • Skip Full Grid controls the resolution of the texture generated for the overlay colormap. The default mode is that the original high resolution input grid dimensions are preserved, and a full colour image texture is generated. However, of very large grids, say 2k x 2k or larger, this image may take a large amount of memory. In this case, enable Skip Full Grid, so the image is at the subsampled resolution, rather than the full original resolution.
  • Gridding Operation

    Just show the Points: To work with Point Cloud data and show the points directly without gridding, be sure to select Interpolation = Point Cloud rather than the default IDW interpolation mode.

    Because this interface starts with numerical data rather than a 3D geometric structure, it operates with slightly different assumptions to other interfaces. If you load a data file in one of the supported formats for the GRID interface, then background is always white, and a legend is placed on the right side of the plot area if you include a colormap for elevation. Animation over multiple files in sequence are available GRID style plots.

    Controls

    Hill Shade Parameters: Add light or shade to the scene as would occur if a light were shone from a prescribed source with prescribed bearing, infinitely far away. All angles are in degrees. A flat earth is assumed, with no curvature.

    For more information on HillShade, please see:

    Arcgis desktop 9.2 How Hillshade Works

    Tin Parameters: Set parameters for the simplification (or decimation) algorithm. The Error Measure defines the stopping criteria for the process, and the process terminates when the first of the following occurs:

    A Tin is an optimized triangle network, a class of a Vector GIS data structure, representing the original grid only with fewer triangles. In areas where the surface is relatively flat, large triangles are used, and where there are changing slope smaller more refined triangles are used. If Tin is not enabled, the original full grid is used for display.

    Algorithm for generating TIN mesh is iterative and its stopping criteria is defined by Error Measure parameter. It works by approximating original mesh with 2 triangles and continues by subdividing these triangles into smaller, to get better approximation, until:

    Which error measure (stopping criteria) will be used depends on what user wants to achieve (TIN with specific number of triangles (first two parameters), TIN which looks like original surface (second two parameters).

    For more information on TIN, see: http://en.wikipedia.org/wiki/Triangulated_irregular_network

     

    IDW Parameters: The Inverse Distance Weighting algorithm uses Shepard's method to produce an interpolated surface from a discrete set of points. Given a set of points { xk } with corresponding sample values { u ( xk) }, the interpolated function is given by:

    where

    and p > 0 is a power. The larger the value of p, the sharper the peaks at the interpolated points become.

    Point Cloud Parameters

    Cell Size Parameters

    For more information on IDW please see:

    Wikipedia on Inverse_distance_weighting

     

    : North Compass -A North direction indicator is available on the Scene Axes tab.

    Additional controls are on the GRD Interface tab:

     

    File Parameters:

     

    Workflow Guidance

    Note that in the case of XYZ point cloud data, or any form of GRID data as input, subsampling may be used to eliminate a portion of the data from consideration, right at the start. This may be helpful if you have a large data set and wish to start with a simple overview.

    Any IDW data preparation is performed to produce a grid data structure with uniform size cells. Only after such a grid is produced, can an image overlay be applied, or an optimized or decimated TIN be generated. There is currently no direct path from point cloud to triangle surface, without first creating a grid.

    Default view generation is recommended to be set to "Geospatial" rather than "CAD". This will also allow the inclusion of a geographic Compass rather than simple axes at the corner of the view.