On Sat, Jun 2, 2012 at 6:16 PM, David Strip <g...@stripfamily.net> wrote: > Jukka's posting earlier today has made me aware that GDAL will now write a > geospatial PDF. Is it also capable of writing a geoPDF (which is a trademark > of TerraGo, but there is an OGC standard, so possibly it's legal to create > them)?
Hi David, As you mention, GeoPDF is a trademark, but it's increasingly weakly associated with an particular encoding used to georegister the content in a PDF. The method published by OGC as a best practice is the one that was originally used to develop a suite of technologies around using PDF as an geospatial application delivery platform branded as "GeoPDF". There is also a georegistration encoding that was published by Adobe as extensions to ISO 32000, the standard specifying PDF. GeoPDF software can use either, so the encoding has nothing to do with GeoPDFness. Anyone is free to create georegistered PDF using the OGC best practice or with the ISO method. Both specs are freely available on the web. What folks are not supposed to do is sell stuff called GeoPDF if they don't use TerraGo software to cook up the PDFs. I'm not debating rightness, wrongness, or wisdom: just trying to be clear! The choice of which encoding to use is largely a matter of taste, as neither has been subjected to any rigorous scrutiny from a standardization perspective. The OGC encoding was based upon a set of standards promulgated by the US Government and its NATO allies called DIGEST and NGA's GEOTRANS software. More at http://dgiwg.org for DIGEST and http://earth-info.nga.mil/GandG/geotrans/ for GEOTRANS. The ISO spec admits use of certain EPSG codes and WKT. Adobe Reader 9 and later uses a version of Esri's projection engine, so from an implementation perspective it's "whatever PE Reader can reach understands". The geospatial bits of Reader are no longer being actively developed by Adobe, Adobe having left that to TerraGo to provide with its TerraGo Toolbar. > When used in conjunction with the (free) TerraGo toolbar, the geoPDF > provides many more capabilities than a geo-spatial pdf, especially if the > pdf "modify" permission is set. As of version 6 which shipped recently, the TerraGo Toolbar offers continuous display of coordinates and other functionality for any geospatial PDF as long as it can grok the encoding. It supports OGC and ISO and uses proj4 via GDAL under the hood for projections. Like Reader, Toolbar has some more advanced functionality that is accessible if certain permission bits are set. > From the looks of it, the main difference > between a geospatial PDF and a geoPDF is that the latter contains a > dictionary object LGIdict which contains the projection, the coordinate > transform to the page, and that sort of thing. In fact, it will support > multiple data frames on the same page. What you're seeing here is the difference in encoding. The OGC georeferencing uses the LGIDict data structure you mention and the ISO uses something called a measurement dictionary. Both encodings support multiple map frames. There are pointers to the specs on the GDAL geospatial PDF page http://www.gdal.org/frmt_pdf.html. Speaking of which, Even has been doing a knock-out job on the PDF support, IMO! It's a challenge because there are two things that don't jibe well with the GDAL data model. First, PDF is not so much a data format as a presentation/consumption format and as such is conceptually is at a very different level of abstraction than are stuff like shapefiles and GeoTIFF. Second, and related, the distinction between vector and raster hardly makes sense in PDF content, but is a fundamental architectural reality in GDAL/OGR. I mention this mainly to point out some of the battles that Even faces and to provoke some thought about what it means to import or export a PDF from GDAL... Hope this helps! George _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
