[Liblas-commits] hg: add Math Transform WKT, info about softwares that provide WK...

liblas-commits at liblas.org liblas-commits at liblas.org
Mon Feb 28 11:17:26 EST 2011 

details:   http://hg.liblas.orghg/rev/5cd75fb456fa
changeset: 2894:5cd75fb456fa
user:      Howard Butler <hobu.inc at gmail.com>
date:      Mon Feb 28 10:17:21 2011 -0600
description:
add Math Transform WKT, info about softwares that provide WKT support, and more formatting

diffstat:

 doc/development/wkt.txt |  141 +++++++++++++++++++++++++++++++++++++++--------
 1 files changed, 116 insertions(+), 25 deletions(-)

diffs (181 lines):

diff -r 365e523cb6d0 -r 5cd75fb456fa doc/development/wkt.txt
--- a/doc/development/wkt.txt	Thu Feb 24 15:44:33 2011 -0600
+++ b/doc/development/wkt.txt	Mon Feb 28 10:17:21 2011 -0600
@@ -7,7 +7,7 @@
 
 :Author: Howard Butler
 :Contact: hobu.inc at gmail dot com
-:Date: 2/16/2011
+:Date: 2/28/2011
 
 Summary
 ------------------------------------------------------------------------------
@@ -33,29 +33,92 @@
   development.
 * GeoTIFF does not offer a mechanism to represent transformations between
   datums, while OGC WKT offers the TOWGS84[] mechanism
+* GeoTIFF does not offer the possibility of fitted coordinate systems.
 
-OGC WKT
+WKT
 ------------------------------------------------------------------------------
 
-The `OGC Simple Feature Access`_ specification provides the "Well-known Text
-Representation of Spatial Reference Systems" (WKT) language in section nine.
-WKT is much more expressive than GeoTIFF keys, has an organization providing
-institutional heft and progress behind it, and a number of open source and
-commercial softwares are available to support parsing and interpreting WKT
-output.
+The `OpenGIS Coordinate Transformation Service Implementation Specification`_
+provides the "Well-known Text Representation of Spatial Reference Systems"
+(WKT) language in section seven. WKT is much more expressive than GeoTIFF keys,
+has an organization providing institutional heft and progress behind it, and a
+number of open source and commercial softwares are available to support
+parsing and interpreting WKT output.
 
 There are a few dialects of WKT of which to be aware. The most common dialect
 is the "ESRI WKT" dialect, which does not include TOWGS84 and authority nodes,
 but there are others including "GDAL", "CSMap", "Oracle 9", "Oracle 10", and
-"GeoTools." It should be assumed that all WKT provided in LAS files should be
-of a dialect that is not the "ESRI WKT" dialect, which lacks some important
-features -- notably TOWGS84 and AUTHORITY nodes. If you have questions about
-how to produce WKT, please participate on the `MetaCRS`_ mailing list.
-Examples of WKT may also be found on the http://spatialreference.org website.
+"GeoTools." While the basic form of WKT in these different dialects (other
+than ESRI) is the same, the names used for different items, such as
+``PARAMETER``, can often be spelled differently. It should be assumed that all
+WKT provided in LAS files should be of a dialect that is not the "ESRI WKT"
+dialect, which lacks some important features -- notably TOWGS84 and AUTHORITY
+nodes. 
 
-.. _`OGC Simple Feature Access`: http://www.opengeospatial.org/standards/sfa
+.. _`OpenGIS Coordinate Transformation Service Implementation Specification`: http://www.opengeospatial.org/standards/ct
 .. _`MetaCRS`: http://trac.osgeo.org/metacrs/
 
+
+
+Implementation
+------------------------------------------------------------------------------
+
+WKT can be specified in two optional Variable Length Record (VLR) entries with the 
+``LASF_Projection`` User ID. The `Math Transform WKT`_ can be used to apply 
+transformations to the `Coordinate System WKT`_.
+
+Math Transform WKT
+..............................................................................
+
+Math Transform WKT (section 7.1 of the `OGC Coordinate Transform Specification`_) 
+can also be specified to apply to a ``FITTED_CS``-style coordinate system.  
+
+OGC Math Transform WKT strings shall be provided in a VLR with the
+following attributes:
+
+* User ID: LASF_Projection
+* Record ID: 2111
+
+The following conditions should also apply:
+
+* Because VLRs can only be 65536 bytes size, an OGC Math Transform WKT
+  string in a VLR data block is functionally limited to 64K in size.
+* The OGC Math Transform WKT VLR data shall be a null-terminated string.
+* The OGC Math Transform WKT VLR data shall be considered UTF-8.
+* The OGC Math Transform WKT VLR data shall be considered ``C``
+  locale-based, and no localization of the numeric strings within the WKT
+  should be performed.
+
+.. note::
+    Math Transform WKT's widespread use is not expected, but it is provided
+    for completeness and flexibility.
+
+
+Coordinate System WKT
+..............................................................................
+
+OGC Coordinate System WKT strings shall be provided in a VLR with the
+following attributes:
+
+* User ID: LASF_Projection
+* Record ID: 2112
+
+The following conditions should also apply:
+
+* Because VLRs can only be 65536 bytes size, an OGC Coordinate System WKT
+  string in a VLR data block is functionally limited to 64K in size.
+* The OGC Coordinate System WKT VLR data shall be a null-terminated string.
+* The OGC Coordinate System WKT VLR data shall be considered UTF-8.
+* The OGC Coordinate System WKT VLR data shall be considered ``C``
+  locale-based, and no localization of the numeric strings within the WKT
+  should be performed.
+
+.. note::
+    Though not specifically mentioned in previous LAS specifications,
+    coordinate system information (in GeoTIFF or WKT) applies to the final
+    coordinate data that are produced by transforming the raw coordinates by
+    the scale and offset specified in the header for the given dimension.
+
 Transition
 ------------------------------------------------------------------------------
 
@@ -73,23 +136,51 @@
 have both OGC WKT and GeoTIFF VLRs, the OGC WKT description, which is more
 expressive, should be preferred if possible.
 
-Implementation
+Resources
 ------------------------------------------------------------------------------
 
-OGC WKT strings shall be provided in a VLR with the following attributes:
+The ASPRS LAS specification attempts to offload specification of coordinate
+system description and transformation by standing on the shoulders of other
+specifications in the ecosystem. To that end, there are options for both
+software and support for an LAS developer to take advantage of. Those below
+are provided for information purposes only, and are not expected to be
+canonical. Use your favorite search engine for more details.
 
-* User ID: LASF_Projection
-* Record ID: 2112
+Software
+..............................................................................
 
-The following conditions should also apply:
+As mentioned previously, a number of open source and commercial softwares are
+available to help an implementor of ASPRS LAS utilize Coordinate System and
+Math Transform WKT in their software.
 
-* Because VLRs can only be 65536 bytes size, an OGC WKT string in a VLR data block 
-  is functionally limited to 64K in size.
-* The OGC WKT VLR data shall be a null-terminated string.
-* The OGC WKT VLR data shall be considered UTF-8.
-* The OGC WKT VLR data shall be considered ``C`` locale-based, and no 
-  localization of the numeric strings within the WKT should be performed.
+Some common open source softwares include:
 
+* `GDAL`_'s `OSR`_
+* `CS-Map`_
+* `GeoTools`_/`Geotoolkit`_ 
+
+Some commercial options include:
+
+* `Blue Marble®`_ 
+* `ArcGIS®`_
+
+Questions
+..............................................................................
+
+If you have questions about how to produce WKT, please participate on
+the `MetaCRS`_ mailing list. Examples of WKT may also be found on the
+http://spatialreference.org website.
+
+.. _`OSR`: http://www.gdal.org/ogr/osr_tutorial.html
+.. _`ArcGIS®`: http://www.arcgis.com
+
+.. _`Blue Marble®`: http://www.bluemarblegeo.com/
+.. _`GDAL`: http://www.gdal.org
+.. _`CS-Map`: http://trac.osgeo.org/csmap/
+.. _`GeoTools`: http://www.geotools.org/
+.. _`Geotoolkit`: http://www.geotoolkit.org/
+
+.. _`OGC Coordinate Transform Specification`: http://www.opengeospatial.org/standards/ct
 
 
 .. _`ASPRS LAS`: http://www.asprs.org/society/committees/standards/lidar_exchange_format.html

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