[gdal-dev] GeoPDF (TM) vs. GeoSpatial PDF

[George Demmy]

On Sat, Jun 2, 2012 at 6:16 PM, David Strip <gdal at 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