libLAS RFC 2: SpatialReference Overhaul

Author:Frank Warmerdam
Contact:warmerdam@pobox.com
Date:11/19/10
Status:Implemented
Version:libLAS 1.6

This RFC addresses several areas of change to the liblas::SpatialReference class and associated services. The objectives are:

  • Minimize the default exposure of libLAS applications to definitions from the GeoTIFF library.
  • Minimize the default exposure of libLAS applications to definitions from the GDAL library.
  • Provide a mechanism to capture coordinate system definitions in a LAS file that cannot be represented via GeoTIFF tags via a liblas vendor proprietary WKT VLR.
  • Ensure that the liblas::SpatialReference class can hold such a non GeoTIFF definition.

Justification for Minimized GeoTIFF / GDAL Include Flow Through

Currently the lasspatialreference.hpp and a few other include files pull in libgeotiff, and GDAL include files somewhat gratuitously if these sublibraries are enabled in the build.

This introduces a lot of cruft into the global namespace. Also due to the “C” style includes of these sublibraries, it introduces a lot of junk that might interfere in surprising ways and in contrast to how the rest of libLAS in very careful to put things into namespaces and otherwise be somewhat sparing of includes.

The cleanup should help avoid conflicts and namespace pollution for applications primarily linking against libLAS and not wanting to use libgeotiff or GDAL services directly. It should also make building somewhat less fragile in some cases.

Justification for WKT Coordinate System Representation

There are a significant number of coordinate systems, and options that cannot be represented in GeoTIFF tags. This includes TOWGS84 parameters, options to use particular horizontal and vertical grid shift files, and support for projection methods that don’t have CT_ definitions for GeoTIFF.

Currently the liblas::SpatialReference class converts passed in WKT coordinate system definitions into a GeoTIFF VLR and future requests for the WKT are created by converting the GeoTIFF VLR back into WKT. This makes it essentially impossible currently for las2las to perform transformations between coordinate systems not representable in GeoTIFF terms even if the source and destination are provided on the command-line in WKT format.

Also, it seems with future versions of LAS format some variation of OGC WKT in a VLR will become a supported, or even the preferred way of representing coordinate systems.

Given these points it seems reasonable to make liblas::SpatialReference less GeoTIFF VLR oriented now.

Include File Overhaul

The basic plan is to remove includes of all GeoTIFF include files from any public liblas include files. For objects, like liblas::SpatialReference, that depend on GeoTIFF definitions like GTIF, ST_TIFF or GTIFDefn we will substitute dummy types if the corresponding GeoTIFF include files have not already been included in advance by the application. This looks something like:

// Fake out the compiler if we don't have libgeotiff includes already
#if !defined(__geotiff_h_)
typedef struct GTIFS *GTIF;
#endif
#if !defined(__geo_simpletags_h_)
typedef struct ST_TIFFS *ST_TIFF;
#endif

Thus libLAS library code, and application code that wants “proper” GTIF and ST_TIFF types will need to include the corresponding libgeotiff include files before including lasspatialreference.hpp. Any attempt to include these libgeotiff include files after including lasspatialreference.hpp will cause conflicts. Yes, this is somewhat unfortunate and will be an unobvious requirement to new folks. Some existing application code will likely need a bit of include reordering before it will work.

It is intended that similar approach might be used for GDAL and OGR dependencies. The primary place where this is a concern seems to be lastransform.hpp.

It is also planned to move the lasversion.hpp function implementations into a .cpp file to avoid having to include libgeotiff and GDAL include files right in lasversion.hpp. This should have the added benefit of not making what lasversion.hpp returns depend on what extra macros (like HAVE_GDAL) are defined when it is included vs. when libLAS itself is compiled and linked.

SpatialReference WKT Updates

Additions to SpatialReference Class:

public
  enum GeoVLRType
  {
      eGeoTIFF = 1,
      eOGRWKT = 2
  };

  void ClearVLRs( GeoVLRType eType );

private:
  std::string m_wkt;

The GeoVLRType enumeration is currently used just to define what information should be wiped in a call to ClearVLRs() but in the future it might also be used for other purposes.

The ClearVLRs() method clears either the WKT VLR + m_vlr, or the GeoTIFF VLRs and the associated m_gtiff, m_tiff objects.

The m_wkt is used internally to cache the current WKT string associated with the spatial reference.

The SpatialReference class is updated to:

  • Generated both GeoTIFF and WKT VLRs in the ResetVLRs() method.
  • GetWKT() now just returns the WKT definition if it is available, otherwise it computes the WKT from the GeoTIFF definition.
  • SetWKT() now caches the WKT as well as creating the GeoTIFF definition from it. Then the WKT and GeoTIFF VLRs are generated.

Note

A liblas::SpatialReference instance that was read/generated from VLRs containing only GeoTIFF keys will continue to only have GeoTIFF keys until you issue a call to SetWKT(). You can simply do the following to WKT-enable a liblas::SpatialReference in that case:

reference.SetWKT(reference.GetWKT());

Warning

If you want only GeoTIFF VLRs you should normally generate the SpatialReference and then call “ClearVLRs(eOGRWKT);” before setting the SpatialReference on the header being written. If you want only a WKT VLR then you should call “ClearVLRs(eGeotIFF);” at the same point.

WKT VLR

Currently the OGR WKT VLR is currently being written with a UserId of “liblas” and with a RecordId of 2112 to distinguish it from the GeoTIFF VLRs.

The data is the OGR WKT string with a terminating zero byte. An exception is thrown if the size of the WKT is larger than a VLR record can hold.

Is there any scheme to assign RecordIds? Perhaps we should pick a value outside the normal legal range for TIFF tags? Or perhaps we should be using a different UserId?

Note

It is anticipated at some point in the future there will be a WKT VLR definition as part of the LAS specification and that it’s contents may be more accurately defined as OGC WKT rather than OGR WKT which can contain a variety of extensions to the specification.