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Getting Started with libLAS
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:Author: Howard Butler
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:Contact: hobu.inc at gmail dot com
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libLAS is two things: a library for embedding read and write support of the
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ASPRS LAS format into your own C/C++ applications, and a suite of command-line
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utilities based on :ref:`LASTools <lastools_liblas>` for inspecting, manipulating,
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transforming, and processing LAS LiDAR data.
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This document is an entry point into the world of libLAS, and will give a
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general overview of the types of operations you can do with the :ref:`libLAS utilities <utilities>`
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as well as point you to other information for developing your own applications
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:ref:`download` contains the canonical location for obtaining libLAS in
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both source and binary forms.
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:ref:`osgeo4w_install` contains information how to install libLAS on Windows.
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Packages are available for `DebianGIS`_, but in most other cases you are
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going to have to compile libLAS yourself. :ref:`compilation` provides an
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extensive synopsis of how to do so.
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:ref:`compilation` shows how to compile libLAS for your own use on Windows,
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The libLAS :ref:`command-line utilities <utilities>` provide the bulk of
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user-facing operational software for libLAS, although the underlying libLAS
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library is what powers them. Below is a listing of common operations that
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you might want to do on LAS data, and the utilities and approaches to
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take to complete those tasks.
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Reprojecting an LAS file
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All LAS data are in some sort of coordinate system, even if that coordinate
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system is not described in the LAS file. For terrestrial LAS data, these
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coordinate system descriptions often map to coordinate systems described
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by the `EPSG`_ database. Another source of information about coordinate
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systems in http://spatialreference.org.
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The :ref:`las2las <las2las>` utility is the tool you will want to use to
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reproject LAS data. :ref:`las2las <las2las>` can take advantage of the
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existing coordinate system description that might already be specified in the
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LAS file, or you may override the coordinate system description (or supply
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one if none was specified).
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We're going to use an example file at http://liblas.org/samples/srs.las which
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contains only 10 points and has a coordinate system defined. Please
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download this file if you want to follow along.
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:ref:`las2las <las2las>` is very similar in behavior to another data
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translation utility for raster data -- `gdal_translate`_. To reproject data,
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we must have a description of both the coordinate system we are starting with
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and a description of the coordinate system we are going to. To find out what
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you are starting with, issue a :ref:`lasinfo <lasinfo>` command:
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lasinfo --no-check srs.las
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The --no-check option tells lasinfo to only print the header information
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for the file and to not scan through all of the points. For a 10 point file,
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this of course isn't much of a concern, but with a 50 or 500 million point
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file, it isn't worth waiting for a full scan of the data if all you
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want is header information.
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Our :ref:`lasinfo <lasinfo>` invocation tells us that the ``srs.las`` file
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is in a UTM North Zone 17 coordinate system:
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PROJCS["WGS 84 / UTM zone 17N",
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SPHEROID["WGS 84",6378137,298.257223563,
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AUTHORITY["EPSG","7030"]],
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AUTHORITY["EPSG","6326"]],
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PRIMEM["Greenwich",0],
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UNIT["degree",0.0174532925199433],
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AUTHORITY["EPSG","4326"]],
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PROJECTION["Transverse_Mercator"],
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PARAMETER["latitude_of_origin",0],
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PARAMETER["central_meridian",-81],
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PARAMETER["scale_factor",0.9996],
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PARAMETER["false_easting",500000],
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PARAMETER["false_northing",0],
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AUTHORITY["EPSG","9001"]],
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AUTHORITY["EPSG","32617"]]
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Now that we know our input coordinate system, we can make a decision about
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what to reproject the data to. In our first example, we're going to use
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the venerable plate carrée non-coordinate system, `EPSG:4326`_.
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las2las srs.las --t_srs EPSG:4326
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Our process succeeds, but after a quick inspection of the data with
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``lasinfo output.las`` we see a problem:
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Scale Factor X Y Z: 0.01 0.01 0.01
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Offset X Y Z: -0.00 -0.00 -0.00
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Min X, Y, Z: -83.43, 39.01, 170.58,
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Max X, Y, Z: -83.43, 39.01, 170.76,
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The ``srs.las`` file had a scale of 0.01, or two decimal places of precision
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for its X, Y, and Z coordinates. For UTM data, this is ok, because it implies
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an implicit precision of 1 cm. For decimal degree data of the unprojected
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Plate Carrée coordinate system, it causes us to lose a bunch of precision. We
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need to set our scale values to something that can hold more precision in our
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las2las --t_srs EPSG:4326 srs.las --scale 0.000001 0.000001 0.01
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Another quick inspection with :ref:`lasinfo <lasinfo>` gives us something
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we're more comfortable with:
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Scale Factor X Y Z: 0.000001 0.000001 0.01
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Offset X Y Z: -0.000000 -0.000000 -0.00
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Min X, Y, Z: -83.427598, 39.012599, 170.58
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Max X, Y, Z: -83.427548, 39.012618, 170.76
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Vertical datum transformation of an LAS file
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We're going to continue what we were doing in `Reprojecting an LAS file`_ but
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add a twist -- we want to change the vertical datum on the data from WGS84
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A number of requirements are needed before you can do vertical datum
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transformations with libLAS. The most important is GDAL support, of course,
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but you also need a very current (possibly even unreleased) version of
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`Proj.4`_ and the vertical datum .gtx transformation files. If you're using
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:ref:`osgeo4w_install`, you already have all of this installed when you
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installed libLAS. For Linux or other Unix platforms, you should be aware of
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Assuming you have all of the prerequisites in place, we can do the vertical
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datum transformation quite simply (again, worrying about precision as well):
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las2las srs.las --t_srs EPSG:4326+5703 --scale 0.000001 0.000001 0.01
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The key point there is adding `+5703` to the coordinate system description
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tells the GDAL/Proj.4 machinery to do a vertical transformation. There are
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other ways to have these operations happen using `WKT`_ and even GeoTIFF
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keys, but this is the most simple way to do things via command line.
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Assigning color information to a LAS file
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The following example assumes you are working with the `Autzen_Stadium`_
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example file that is available from the http://liblas.org/sample sample
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Frequent availability of combined terrestrial LiDAR and image captures means
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that its now possible to obtain .las files that you can stylize with RGB
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imagery. The LAS 1.2 specification provides two different point data
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types that allow storing RGB data as 16 bit integers, but the tools to
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do the actual intersection operation have been somewhat limited.
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libLAS 1.6+ allows you to assign color information to a .las file if `GDAL`_
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is linked in at compile-time.
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The :ref:`LAS specifications <specifications>` only allow two different
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point format types to store color information -- point format 2 and point
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format 3. The difference between point format 2 and point format 3 is that
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3 also has time stored on it. Additionally, only LAS 1.2 and 1.3 versions
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support storing color information, but libLAS only can write LAS 1.2 as of
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1. Unzip the `Autzen Stadium <Autzen_Stadium>`_ data.
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$ unzip Autzen_Stadium.zip
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Archive: Autzen_Stadium.zip
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creating: Autzen_Stadium/
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inflating: Autzen_Stadium/image.tif
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inflating: Autzen_Stadium/lidar.las
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2. Issue the :ref:`las2las <las2las>` call
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$ las2las -i lidar.las \
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--color-source image.tif \
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Opening lidar.las to fetch Header
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Setting format to: 1.2
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Setting point format to: 3
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Fetching color from ' image.tif' using bands '1, 2, 3' for R, G, B
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0...10...20...30...40...50...60...70...80...90...100 - done.
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3. Inspect the :ref:`lasinfo <lasinfo>` output and see color information
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Minimum Color: 39 56 56
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Maximum Color: 252 254 251
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:ref:`assign_color` contains more detailed information about this process.
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Compressing an LAS file with `LASzip`_
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libLAS provides the ability to compress data using the fantastic `LASzip`_
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.. _`LASzip`: http://laszip.org
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.. _`CMake`: http://www.cmake.org/
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.. _`CTest`: http://cmake.org/cmake/help/ctest-2-8-docs.html
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.. _`CMake 2.8.0+`: http://www.cmake.org/cmake/help/cmake-2-8-docs.html
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.. _`CDash`: http://www.cdash.org/
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.. _`continuous integration`: http://en.wikipedia.org/wiki/Continuous_integration
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.. _`libLAS CDash`: http://my.cdash.org/index.php?project=libLAS
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.. _`Curses`: http://en.wikipedia.org/wiki/Curses_%28programming_library%29
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.. _`Autoconf`: http://www.gnu.org/software/autoconf/
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.. _`LLVM`: http://llvm.org/
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.. _`OSGeo4W`: http://trac.osgeo.org/osgeo4w/
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.. _`Boost`: http://www.boost.org/
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.. _`DebianGIS`: http://wiki.debian.org/DebianGis
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.. _`gdal_translate`: http://www.gdal.org/gdal_translate.html
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.. _`EPSG`: http://www.epsg-registry.org/
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.. _`EPSG:4326`: http://spatialreference.org/ref/epsg/4326/
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.. _`Proj.4`: http://trac.osgeo.org/proj/
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.. _`WKT`: http://en.wikipedia.org/wiki/Well-known_text#Spatial_reference_systems
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.. _`GDAL`: http://gdal.org
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.. _`Autzen_Stadium`: http://liblas.org/samples/Autzen_Stadium.zip
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doc/start.txt
