Getting Started with libLAS¶
hobu.inc at gmail dot com
libLAS is two things: a library for embedding read and write support of the ASPRS LAS format into your own C/C++ applications, and a suite of command-line utilities based on LASTools for inspecting, manipulating, transforming, and processing LAS LiDAR data.
This document is an entry point into the world of libLAS, and will give a general overview of the types of operations you can do with the libLAS utilities as well as point you to other information for developing your own applications with libLAS.
Download contains the canonical location for obtaining libLAS in both source and binary forms.
If you use you Homebrew, you can install libLAS this way:
$ brew install liblas
If you want the latest development version:
$ brew install liblas --HEAD
Otherwise, you have to compile libLAS yourself.
The libLAS command-line utilities provide the bulk of user-facing operational software for libLAS, although the underlying libLAS library is what powers them. Below is a listing of common operations that you might want to do on LAS data, and the utilities and approaches to take to complete those tasks.
Reprojecting an LAS file¶
All LAS data are in some sort of coordinate system, even if that coordinate system is not described in the LAS file. For terrestrial LAS data, these coordinate system descriptions often map to coordinate systems described by the EPSG database. Another source of information about coordinate systems in http://spatialreference.org.
The las2las utility is the tool you will want to use to reproject LAS data. las2las can take advantage of the existing coordinate system description that might already be specified in the LAS file, or you may override the coordinate system description (or supply one if none was specified).
We’re going to use an example test/data/srs.las file available from https://github.com/libLAS/libLAS repository which contains only 10 points and has a coordinate system defined. Please download this file if you want to follow along.
las2las is very similar in behavior to another data translation utility for raster data – gdal_translate. To reproject data, we must have a description of both the coordinate system we are starting with and a description of the coordinate system we are going to. To find out what you are starting with, issue a lasinfo command:
lasinfo --no-check srs.las
The –no-check option tells lasinfo to only print the header information for the file and to not scan through all of the points. For a 10 point file, this of course isn’t much of a concern, but with a 50 or 500 million point file, it isn’t worth waiting for a full scan of the data if all you want is header information.
Our lasinfo invocation tells us that the
is in a UTM North Zone 17 coordinate system:
PROJCS["WGS 84 / UTM zone 17N", GEOGCS["WGS 84", DATUM["WGS_1984", SPHEROID["WGS 84",6378137,298.257223563, AUTHORITY["EPSG","7030"]], AUTHORITY["EPSG","6326"]], PRIMEM["Greenwich",0], UNIT["degree",0.0174532925199433], AUTHORITY["EPSG","4326"]], PROJECTION["Transverse_Mercator"], PARAMETER["latitude_of_origin",0], PARAMETER["central_meridian",-81], PARAMETER["scale_factor",0.9996], PARAMETER["false_easting",500000], PARAMETER["false_northing",0], UNIT["metre",1, AUTHORITY["EPSG","9001"]], AUTHORITY["EPSG","32617"]]
Now that we know our input coordinate system, we can make a decision about what to reproject the data to. In our first example, we’re going to use the venerable plate carrée non-coordinate system, EPSG:4326.
las2las srs.las --t_srs EPSG:4326
Our process succeeds, but after a quick inspection of the data with
lasinfo output.las we see a problem:
... Scale Factor X Y Z: 0.01 0.01 0.01 Offset X Y Z: -0.00 -0.00 -0.00 ... Min X, Y, Z: -83.43, 39.01, 170.58, Max X, Y, Z: -83.43, 39.01, 170.76,
srs.las file had a scale of 0.01, or two decimal places of precision
for its X, Y, and Z coordinates. For UTM data, this is ok, because it implies
an implicit precision of 1 cm. For decimal degree data of the unprojected
Plate Carrée coordinate system, it causes us to lose a bunch of precision. We
need to set our scale values to something that can hold more precision in our
las2las --t_srs EPSG:4326 srs.las --scale 0.000001 0.000001 0.01
Another quick inspection with lasinfo gives us something we’re more comfortable with:
... Scale Factor X Y Z: 0.000001 0.000001 0.01 Offset X Y Z: -0.000000 -0.000000 -0.00 ... Min X, Y, Z: -83.427598, 39.012599, 170.58 Max X, Y, Z: -83.427548, 39.012618, 170.76
Vertical datum transformation of an LAS file¶
We’re going to continue what we were doing in Reprojecting an LAS file but add a twist – we want to change the vertical datum on the data from WGS84 to NAVD88.
Assuming you have all of the prerequisites in place, we can do the vertical datum transformation quite simply (again, worrying about precision as well):
las2las srs.las --t_srs EPSG:4326+5703 --scale 0.000001 0.000001 0.01
The key point there is adding +5703 to the coordinate system description tells the GDAL/Proj.4 machinery to do a vertical transformation. There are other ways to have these operations happen using WKT and even GeoTIFF keys, but this is the most simple way to do things via command line.
Assigning color information to a LAS file¶
Frequent availability of combined terrestrial LiDAR and image captures means that its now possible to obtain .las files that you can stylize with RGB imagery. The LAS 1.2 specification provides two different point data types that allow storing RGB data as 16 bit integers, but the tools to do the actual intersection operation have been somewhat limited.
libLAS 1.6+ allows you to assign color information to a .las file if GDAL is linked in at compile-time.
The LAS specifications only allow two different point format types to store color information – point format 2 and point format 3. The difference between point format 2 and point format 3 is that 3 also has time stored on it. Additionally, only LAS 1.2 and 1.3 versions support storing color information, but libLAS only can write LAS 1.2 as of libLAS 1.6.0.
Unzip the Autzen Stadium data.
$ unzip Autzen_Stadium.zip Archive: Autzen_Stadium.zip creating: Autzen_Stadium/ inflating: Autzen_Stadium/image.tif inflating: Autzen_Stadium/lidar.las
Issue the las2las call
$ las2las -i lidar.las \ --color-source image.tif \ -o output.las \ --file-format 1.2 \ --point-format 3 \ -v
Opening lidar.las to fetch Header Setting format to: 1.2 Setting point format to: 3 Fetching color from ' image.tif' using bands '1, 2, 3' for R, G, B Writing output: - : output.las 0...10...20...30...40...50...60...70...80...90...100 - done.
Inspect the lasinfo output and see color information attached.
lasinfo output.las ... Minimum Color: 39 56 56 Maximum Color: 252 254 251
Adding color from an image contains more detailed information about this process.