Multi-Pass Grid Creation

Tab on LIDAR point cloud analysis form. 

Set the parameters desired, and then pick one of the four buttons to start processing:

You can use:

  • Noise points: if the noise parts are mostly identified in the point cloud, not using them  can simplify grid creation.
  • Overlap points: if the edge of the overlap occurs within the cell, it could double the return density in part of the cell, potentially skewing the results.  Wihtout overlap the return density within the cell should be more uniform.  This will be increasingly important as the cell size increases.

Min points required: unless the cell has at least this number of returns, it will not be used.  This is designed for cases like bird strikes over water, where the only returns might be unrecognized noise.

Auto thin by cell: if this is checked, when any cell within the block gets to the max number of returns, the block will be recomputed with a thinning factor increased by one.  The computations will use only ever 2d point.  These should be randomly distributed, and with 10,000s of points the statistics should be equivalent.  If thinning of 2 is insufficient, it will be increased again until no cell within the block has too many returns.  This becomes a bigger issue as the cell size increases, or the number of returns per square meter increases.

 

Set the percentile: 100 at the top of the cloud, 50 at the median, and 0 at the base.  In case of unrecognized noise, high or low, pick a percentile like 99 (the best choice depends on the number of points in the tile and the number of noise points).

Block size to use.  This is the side of the block; the number of cells determined in each pass through the data will be the square of this.  This affects the memory that must be allocated.  This determines the number of passes through the data.

Maximum number of returns in a cell.  This affects the memory that must be allocated.  Returns beyond this number will be ignored, which might or might not be an acceptable solution. If the points were randomly distributed in the LAS file, this would probably work, but if they are sorted by elevation, either deliberately or accidently, it might be a problem.  If this is too small, you will be told upon completion, and should consider redoing.  You can avoid this is Auto thin by cell. 

The memory required below suggests when you might adjust the parameters which determine the size (block size and max returns per cell).  If you have lots of other things open, you don't have the free memory shown which is the total on the system.

Grid parameters picked on the Grid params  tab.

Multiple grids control table:

y_res x_res units pixel_is DEM Percentile WKT file_name
1   sec area Mid 5 mid_5_area_1_sec
1   sec point Mid 5 mid_5_point_1_sec
30   m area Mid 5 mid_5_area_30_m
30   m point Mid 5 mid_5_area_30_m
3   sec area Mid 5 mid_5_area_3_sec
3   sec point Mid 5 mid_5_point_3_sec
0.4   sec area Mid 5 mid_5_area_0.4_sec
0.4   sec point Mid 5 mid_5_point_0.4_sec
90   m area Mid 5 mid_5_area_90_m
90   m point Mid 5 mid_5_area_90_m
  1. Create a table in Excel or other program
    • Resolution is the data point spacing
    • Units is sec, m, WKT (in which case it will be projected meters)
    • PixelIsPoint/PixelIsArea  Pick area unless you have a very good reason to test the 1/2 pixel point shifts
    • DEM is DSM (uses just the first returns), Mid (uses all points), DTM (uses just the ground classified points)
    • Percentile: 100 at the top of the cloud, 50 the median, and 0 at the base.  I
    • WKT: name of the projection definition file, which must be in c:\microdem\wkt_proj with a .PRJ extension
    • File Name for the DEM will with constructed with the point cloud name + '_' + contents of this field
  2. Save as CSV
  3. Open database without map, to convert to DBF
  4. LIDAR Point clouds on map
  5. Adjust map area so it covers what you want the lidar-derived grids to cover
  6. Pick the Multiple grids button to start processing
  7. The same control table can be used for muliple point clouds, if each is in its own folder.

Last revision 5/30/2021