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.
- The Hill Shade is a calculated value determining the magnitude of the shading for the scene, based upon the values of the remaining parameters. Zero means no effect is visible, 1 means full effect.
- The Zenith angle is the angle between the upward vertical and the direction of the light source.
- The Azimuth angle is the angle between the x-axis and the projection of a vector directed towards the light source onto the x-y plane.
- The Z Factor governs a scaling of the z-axis units and is taken into account when shading.
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:
- The number of triangles is Triangles, or Triangles plus or minus 1,
- The number of triangles is reduced by the Reduction factor,
- The absolute error is below Absolute,
- The relative error is below Relative.
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:
- Error measure is set to Triangles: number of generated triangles became larger or equal than specified in Triangles parameter
- Error measure is set to Reduction: number of generated triangles became larger or equal than (1 - Reduction) * NumberOfTrianglesInOriginalMesh
- Error measure is set to Absolute: original mesh doesn't deviate from generated one more than Absolute (deviation between two surfaces is maximum distance between any two points with same x-y coordinates)
- Error measure is set to Relative: original mesh doesn't deviate from generated one more than Relative * (diagonal length of bounding box of input surface).
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
- Interpolation: Perform IDW presents the choice between reconstructing the grid from a point cloud, or simply exporting a mesh of individual points. If points are exported as lines, they will have lengths of Relative Length multiplied by the length of the diagonal of the bounding box.
- Interpolation: the Point Cloud option displays the individual points (after subsampling) directly the in 3D view, as a point cloud. When showing the direct point cloud, the points will normally be shown as 2x2 pixel points on screen, either black, or coloured by colormap file data, or by directly loaded RGB colors.
- The Generate Grid (former Clustered) box determines whether the IDW performed is clustered or is performed non-clustered VTK based IDW, or grid is generated using Nearest Neighbor algorithm. Clustering can be more efficient for large point clouds, and may result in noisier appearing surfaces. The Nearest Neighbor algorithm is implemented as fast binning method, so it can be used for gridding of very large point clouds (e.g. 100 milion points and larger).
- The Power sets the value of p as explained above, typically 2 for distance squared.
- Only points within a radius of Radius of a given point are taken into account in the above calculations.
- The Radius Type presents a choice between the absolute radius or a value relative to the length of the diagonal of the bounding box.
- An interpolated point is exported only if the number of sample points within the chosen radius exceeds Min Points.
- Max Points sets a limit on the number of sample values to be taken into account.
Cell Size Parameters
- The Width and Height variables set the number of columns and rows of the output grid.
- The Geospatial Width and Geospatial Height variables set the geospatial width and height of cell of generated grid. Values can be relative or absolute, depending on Metric Type.
- The Metric Type sets wheather to use Width and Height or Geospatial variant for specifying cell's size of output grid. Also selects wheather Geospatial values are relative or absolute.
- The Square Cells sets forcing generating of square cells in output grid. This option is necessary if output grid is exported to ESRI format too.
- The Extend XY Extent is helpful when performing IDW interpolation, such that the resulting grid covers the extend of the original points plus an extrapolated border based on the interpolation radius.
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:
- Export Image as Quad is used where a plane extent image should be placed as-is in the 3D scene, without being interpreted as an elevation grid.
File Parameters:
- A filename for the exported grid can be set with Save Grid to File Possible exported file types are ESRI ArcGIS ASCII Grid (.asc) and Surfer Grid (.grd).
- A filename for the TIN Mesh may be set with Save Decimated TIN Mesh to File. Possible exported file types are:
- xyz - Ascii 3-column CSV file of vertices from Tin
- stl - triangle mesh, without color
- ply - Stanford polygon format
- vtk - VTK/ParaView compatible triangle mesh file
- net - old ArcInfo TIN format (unsupported)
- tri - simple triangle file format
- obj - Wavefront OBJ graphics file, triangle mesh, material and texture exported for 3D Studio Max, Maya, visual simulation software import.
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.