## DESCRIPTION

The r.in.lidar module will load and bin LAS LiDAR point clouds into a new raster map. The user may choose from a variety of statistical methods in creating the new raster.

Please note that the current region extents and resolution are used for the import. It is therefore recommended to first use the -s flag to get the extents of the LiDAR point cloud to be imported, then adjust the current region accordingly, and only then proceed with the actual import.

r.in.lidar is designed for processing massive point cloud datasets, for example raw LiDAR or sidescan sonar swath data. It has been tested with datasets as large as tens of billion of points (705GB in a single file).

Available statistics for populating the raster are:

- n | number of points in cell
  min | minimum value of points in cell
  max | maximum value of points in cell
  range | range of points in cell
  sum | sum of points in cell
  mean | average value of points in cell
  stddev | standard deviation of points in cell
  variance | variance of points in cell
  coeff_var | coefficient of variance of points in cell
  median | median value of points in cell
  percentile | pth percentile of points in cell
  skewness | skewness of points in cell
  trimmean | trimmed mean of points in cell

- Variance and derivatives use the biased estimator (n). [subject to change]
- Coefficient of variance is given in percentage and defined as `(stddev/mean)*100`.

## NOTES

### Gridded data

If data is known to be on a regular grid r.in.lidar can reconstruct the map perfectly as long as some care is taken to set up the region correctly and that the data's native map projection is used. A typical method would involve determining the grid resolution either by examining the data's associated documentation or by studying the text file. Next scan the data with r.in.lidar's -s (or -g) flag to find the input data's bounds. GRASS uses the cell-center raster convention where data points fall within the center of a cell, as opposed to the grid-node convention. Therefore you will need to grow the region out by half a cell in all directions beyond what the scan found in the file. After the region bounds and resolution are set correctly with g.region, run r.in.lidar using the n method and verify that n=1 at all places. r.univar can help. Once you are confident that the region exactly matches the data proceed to run r.in.lidar using one of the mean, min, max, or median methods. With n=1 throughout, the result should be identical regardless of which of those methods are used.

### Memory use

While the input file can be arbitrarily large, r.in.lidar will use a large amount of system memory for large raster regions (10000x10000). If the module refuses to start complaining that there isn't enough memory, use the percent parameter to run the module in several passes. In addition using a less precise map format (`CELL` [integer] or `FCELL` [floating point]) will use less memory than a `DCELL` [double precision floating point] output map. Methods such as n, min, max, sum will also use less memory, while stddev, variance, and coeff_var will use more. The aggregate functions median, percentile, skewness and trimmed mean will use even more memory and may not be appropriate for use with arbitrarily large input files.

The default map type=`FCELL` is intended as compromise between preserving data precision and limiting system resource consumption.

### Setting region bounds and resolution

You can use the -s scan flag to find the extent of the input data (and thus point density) before performing the full import. Use g.region to adjust the region bounds to match. The -g shell style flag prints the extent suitable as parameters for g.region. A suitable resolution can be found by dividing the number of input points by the area covered. e.g.

      wc -l inputfile.txt
      g.region -p
      # points_per_cell = n_points / (rows * cols)

      g.region -e
      # UTM location:
      # points_per_sq_m = n_points / (ns_extent * ew_extent)

      # Lat/Lon location:
      # points_per_sq_m = n_points / (ns_extent * ew_extent*cos(lat) * (1852*60)^2)

If you only intend to interpolate the data with r.to.vect and v.surf.rst, then there is little point to setting the region resolution so fine that you only catch one data point per cell -- you might as well use "`v.in.ascii -zbt`" directly.

### Filtering

Points falling outside the current region will be skipped. This includes points falling exactly on the southern region bound. (to capture those adjust the region with "`g.region s=s-0.000001`"; see g.region)

Blank lines and comment lines starting with the hash symbol (`#`) will be skipped.

The zrange parameter may be used for filtering the input data by vertical extent. Example uses might include preparing multiple raster sections to be combined into a 3D raster array with r.to.rast3, or for filtering outliers on relatively flat terrain.

In varied terrain the user may find that min maps make for a good noise filter as most LIDAR noise is from premature hits. The min map may also be useful to find the underlying topography in a forested or urban environment if the cells are over sampled.

The user can use a combination of r.in.lidar output maps to create custom filters. e.g. use r.mapcalc to create a `mean-(2*stddev)` map. [In this example the user may want to include a lower bound filter in r.mapcalc to remove highly variable points (small n) or run r.neighbors to smooth the stddev map before further use.]

### Interpolation into a DEM

The vector engine's topological abilities introduce a finite memory overhead per vector point which will limit the size a vector map relative to available RAM. If you want more, use the r.to.vect -b flag to skip building topology. Without topology, however, all you'll be able to do with the vector map is display with d.vect and interpolate with v.surf.rst. Run r.univar on your raster map to check the number of non-NULL cells and adjust bounds and/or resolution as needed before proceeding.

Typical commands to create a DEM using a regularized spline fit:

      r.univar lidar_min
      r.to.vect -z feature=point in=lidar_min out=lidar_min_pt
      v.surf.rst layer=0 in=lidar_min_pt elev=lidar_min.rst

Typical commands to create a DEM using bsplines:

      r.resamp.bspline in=lidar_min out=lidar_min.bspline

## EXAMPLE

This example is analogous to the example used in the GRASS wiki page for importing LAS as raster DEM.

The sample LAS data are in the file "Serpent Mound Model LAS Data.las", available at appliedimagery.com

      # using v.in.lidar to print file info and to create a new location

      # print LAS file info
      v.in.lidar -p input="Serpent Mound Model LAS Data.las"

      # create location with projection information of the LAS data
      v.in.lidar -i input="Serpent Mound Model LAS Data.las" location=Serpent_Mound

      # quit and restart GRASS in the newly created location "Serpent_Mound"

      # scan the extents of the LAS data
      r.in.lidar -sg input="Serpent Mound Model LAS Data.las"

      # set the region to the extents of the LAS data, align to resolution
      g.region n=4323641.57 s=4320942.61 w=289020.90 e=290106.02 res=1 -ap

      # import as raster DEM
      r.in.lidar input="Serpent Mound Model LAS Data.las" output=Serpent_Mound_Model_LAS_Data method=mean

## TODO

- Support for multiple map output from a single run.
  `method=string[,string,...] output=name[,name,...]`
- Merge with r.in.xyz.

## BUGS

- n map sum can be ever-so-slightly more than ``wc -l`` with e.g. `percent=10` or less.
  Cause unknown.
- n map `percent=100` and `percent=xx` maps differ slightly (point will fall above/below the segmentation line)
  Investigate with "`r.mapcalc diff = bin_n.100 - bin_n.33`" etc.
  Cause unknown.
- "`nan`" can leak into coeff_var maps.
  Cause unknown. Possible work-around: "`r.null setnull=nan`"

If you encounter any problems (or solutions!) please contact the GRASS Development Team.

## SEE ALSO

g.region
m.proj
r.in.xyz
r.fillnulls
r.in.ascii
r.mapcalc
r.neighbors
r.out.xyz
r.to.rast3
r.to.vect
r.univar
v.in.lidar
v.in.ascii
v.surf.rst
v.lidar.correction, v.lidar.edgedetection, v.lidar.growing, v.outlier, v.surf.bspline

pv - The UNIX pipe viewer utility

## AUTHORS

Markus Metz
based on r.in.xyz by Hamish Bowman and Volker Wichmann

Last changed: $Date$