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Georeferencing LAS files with LAS 1.4
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:Contact: hobu.inc at gmail dot com
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This document proposes how to handle describing spatial reference systems
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in `ASPRS LAS`_ 1.4. LAS 1.4 prescribes that software developers add and
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prefer to use OGC WKT coordinate system description in the form of VLR
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records in LAS files. Softwares can choose to continue to write GeoTIFF keys
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to support older applications, but they should prefer the OGC WKT if it is
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LAS 1.3 and below have used `GeoTIFF`_ keys as described by the `GeoTIFF
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specification`_, but use of the GeoTIFF keys within LAS has a number of
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* GeoTIFF is typically tied to the `EPSG database`_ and does not provide
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simple mechanisms for extended definitions.
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* Only a few softwares provide support for working with GeoTIFF keys directly.
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* The GeoTIFF specification has no organization backing its continued
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* GeoTIFF does not offer a mechanism to represent transformations between
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datums, while OGC WKT offers the TOWGS84[] mechanism
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* GeoTIFF does not offer the possibility of fitted coordinate systems.
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The `OpenGIS Coordinate Transformation Service Implementation Specification`_
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provides the "Well-known Text Representation of Spatial Reference Systems"
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(WKT) language in section seven. WKT is much more expressive than GeoTIFF keys,
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has an organization providing institutional heft and progress behind it, and a
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number of open source and commercial softwares are available to support
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parsing and interpreting WKT output.
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There are a few dialects of WKT of which to be aware. The most common dialect
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is the "ESRI WKT" dialect, which does not include TOWGS84 and authority nodes,
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but there are others including "GDAL", "CSMap", "Oracle 9", "Oracle 10", and
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"GeoTools." While the basic form of WKT in these different dialects (other
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than ESRI) is the same, the names used for different items, such as
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``PARAMETER``, can often be spelled differently. It should be assumed that all
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WKT provided in LAS files should be of a dialect that is not the "ESRI WKT"
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dialect, which lacks some important features -- notably TOWGS84 and AUTHORITY
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.. _`OpenGIS Coordinate Transformation Service Implementation Specification`: http://www.opengeospatial.org/standards/ct
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.. _`MetaCRS`: http://trac.osgeo.org/metacrs/
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WKT can be specified in two optional Variable Length Record (VLR) entries with the
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``LASF_Projection`` User ID. The `Math Transform WKT`_ can be used to apply
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transformations to the `Coordinate System WKT`_.
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Math Transform WKT (section 7.1 of the `OGC Coordinate Transform Specification`_)
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can also be specified to apply to a ``FITTED_CS``-style coordinate system.
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OGC Math Transform WKT strings shall be provided in a VLR with the
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* User ID: LASF_Projection
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The following conditions should also apply:
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* Because VLRs can only be 65536 bytes size, an OGC Math Transform WKT
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string in a VLR data block is functionally limited to 64K in size.
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* The OGC Math Transform WKT VLR data shall be a null-terminated string.
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* The OGC Math Transform WKT VLR data shall be considered UTF-8.
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* The OGC Math Transform WKT VLR data shall be considered ``C``
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locale-based, and no localization of the numeric strings within the WKT
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Math Transform WKT's widespread use is not expected, but it is provided
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for completeness and flexibility.
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OGC Coordinate System WKT strings shall be provided in a VLR with the
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following attributes:
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* User ID: LASF_Projection
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The following conditions should also apply:
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* Because VLRs can only be 65536 bytes size, an OGC Coordinate System WKT
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string in a VLR data block is functionally limited to 64K in size.
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* The OGC Coordinate System WKT VLR data shall be a null-terminated string.
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* The OGC Coordinate System WKT VLR data shall be considered UTF-8.
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* The OGC Coordinate System WKT VLR data shall be considered ``C``
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locale-based, and no localization of the numeric strings within the WKT
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Though not specifically mentioned in previous LAS specifications,
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coordinate system information (in GeoTIFF or WKT) applies to the final
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coordinate data that are produced by transforming the raw coordinates by
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the scale and offset specified in the header for the given dimension.
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Older softwares may want to continue to use GeoTIFF keys, and it is not
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desirable to have an abrupt discontinuity in the description of coordinate
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systems for LAS data. Additionally, while many softwares may not need the
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features of OGC WKT, its usage represents the strongest case for
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interoperability with other software -- especially software in the GIS and CAD
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domains that take advantage of the OGC WKT specification.
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Softwares that currently write GeoTIFF VLR records into LAS files can continue
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to do so in LAS 1.4. It is acceptable to write *only* GeoTIFF VLR records into
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an LAS file, while softwares that choose to write OGC WKT VLRs must also
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write GeoTIFF VLRs to promote backward compatibility. If a file is found to
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have both OGC WKT and GeoTIFF VLRs, the OGC WKT description, which is more
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expressive, should be preferred if possible.
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The ASPRS LAS specification attempts to offload specification of coordinate
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system description and transformation by standing on the shoulders of other
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specifications in the ecosystem. To that end, there are options for both
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software and support for an LAS developer to take advantage of. Those below
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are provided for information purposes only, and are not expected to be
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canonical. Use your favorite search engine for more details.
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As mentioned previously, a number of open source and commercial softwares are
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available to help an implementor of ASPRS LAS utilize Coordinate System and
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Math Transform WKT in their software.
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Some common open source softwares include:
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* `GeoTools`_/`Geotoolkit`_
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Some commercial options include:
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If you have questions about how to produce WKT, please participate on
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the `MetaCRS`_ mailing list. Examples of WKT may also be found on the
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http://spatialreference.org website.
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.. _`OSR`: http://www.gdal.org/ogr/osr_tutorial.html
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.. _`ArcGIS®`: http://www.arcgis.com
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.. _`Blue Marble®`: http://www.bluemarblegeo.com/
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.. _`GDAL`: http://www.gdal.org
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.. _`CS-Map`: http://trac.osgeo.org/csmap/
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.. _`GeoTools`: http://www.geotools.org/
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.. _`Geotoolkit`: http://www.geotoolkit.org/
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.. _`OGC Coordinate Transform Specification`: http://www.opengeospatial.org/standards/ct
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.. _`ASPRS LAS`: http://www.asprs.org/society/committees/standards/lidar_exchange_format.html
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.. _`GeoTIFF specification`: ftp://ftp.remotesensing.org/pub/geotiff/spec/geotiff.rtf
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.. _`GeoTIFF`: http://trac.osgeo.org/geotiff/
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.. _`EPSG database`: http://www.epsg-registry.org/
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doc/development/wkt.txt
