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GeoPDF Encoding (TerraGo 2.2), OGC Best Practice

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name GeoPDF (TerraGo) Encoding, Version 2.2, OGC Best Practice 08-139
Description

This description concerns a particular encoding of georegistration information that allows association of an arbitrary coordinate reference system to an illustration on a page of a PDF document. This encoding was developed in the early 2000s at Layton Graphics, Inc. (LGI) and used widely for conversion of printed maps to digital form using products from LGI and later from TerraGo Technologies, Inc. The resulting files were called GeoPDFs. See Notes | History for more detail.

The specification for this georegistration encoding was published in 2009 by the Open Geospatial Consortium (OGC) as Best Practice OGC 08-139r2, entitled "GeoPDF Encoding Best Practice Version 2.2." The intent behind publishing OGC 08-139 was to codify existing practice and to insure that there was official, stable documentation for the georegistration encoding format used in millions of existing GeoPDF files. By this time, GeoPDF was a registered trademark of TerraGo Technologies, Inc. and its use in the OGC document, and particularly in the title, led to confusion. In 2011, OGC published a revision OGC 08-139r3 (still current as of May 2020), with a new title, "PDF Georegistration Encoding Best Practice Version 2.2" and other terminological and administrative changes. OGC 08-139r3 states that GeoPDF "may only be applied to products created under license from TerraGo." For consistency with OGC 08-139r3, this format description uses the terms "PDF georegistration 2.2" and "OGC 08-139" to refer to the georegistration encoding.

PDF georegistration 2.2 is specified as an extension to Adobe PDF as described in PDF Reference 1.7. A primary intended functionality of the encoding is to support calculation of the position in coordinates of any point on a map image and thus to associate PDF annotations and linked files with a particular geospatial location. This location can be expressed in any coordinate reference system, since the encoding includes sufficient information to perform conversions between coordinate systems. The georegistration encoding also supports measurement, such as calculation of distances between points. In particular, this extension supports the functionality in TerraGo's GeoPDF Toolbar and other TerraGo products designed to "deliver complex maps and images from the desks of a few technical experts to the hands of many field operators in a simple to use format."

To quote from the OGC Best Practice document, "Conceptually, PDF georegistration 2.2 specifies map frames associated with a PDF page. A minimal map frame describes a coordinate reference system that is associated with the entire document page. This association provides a collection of tuples that map the page (PDF) coordinate system to the known coordinate reference system." Often, the document consists of one page containing a single map or other georegistered illustration. However, a map frame may be associated with a region on a page; there may be multiple map frames on a given page; and a document may have more than one page containing illustrations with PDF georegistration 2.2 encoding.

The PDF georegistration 2.2 encoding for a page is expressed in a PDF entry with key "LGIDict". This entry is a single map frame or an array of map frames. Each map frame has a mandatory Type entry with the same value of "LGIDict" and one or more of a set of entries to specify the georegistration: CTM (Coordinate Transformation Matrix); Registration array (a sequence of registration points tying map coordinates to PDF page coordinates); Projection dictionary (describes the algorithm and parameters, such as geodetic datum, required to define the coordinate reference system used); and Display dictionary (to specify the coordinate system a consuming application should use to display non-PDF coordinate reference values). OGC 08-139 includes annexes listing supported geodetic datums and map projections. A map frame may also include a Neatline entry to specify the geometric region of the page that is associated with the geospatial coordinate system; this is required if a page includes more than one map frame.

Relationship to other formats
    Used by PDF_family, Portable Document Format. Although PDF georegistration 2.2 was developed with version 1.7 of PDF, in mind, both OGC and the developers believe that it can be used with earlier versions, at least from Version 1.3.
    Has earlier version Earlier versions of PDF georegistration using the name GeoPDF were used in products of Layton Graphics and TerraGo. They are not publicly specified and are not described on this Web site.

Local use Explanation of format description terms

LC experience or existing holdings None.
LC preference The Library of Congress Recommended Formats Statement (RFS) includes GeoPDF as a preferred format for cartographic materials in digital form. The RFS does not specify a version of GeoPDF fle format or of preferred PDF georegistration within GeoPDF files.

Sustainability factors Explanation of format description terms

Disclosure The technique described in OGC 08-139 for embedding georegistration information for maps into a PDF file was developed at Layton Graphics, Inc. and since 2005, maintained by TerraGo Technologies. In 2009, the specification of version 2.2 was approved and published as a best practice by the Open Geospatial Consortium (OGC).
    Documentation

Approved as an OGC Best Practice in February 2009 as OGC 08-139r1. Two revisions of the document have been published, with changes that were editorial and administrative and did not reflect changes in the specification for the encoding.

Adoption

As of September 2010, several U.S. government agencies produced and distributed maps as PDF files with PDF georegistration 2.2 encoding, including the United States Geological Survey (USGS), the U.S. Army Corps of Engineers, and the National Geospatial-Intelligence Agency. See U.S. Army Corps of Engineers 2012 factsheet on Geospatial PDF for information on the use of GeoPDF files for map distribution by the U.S. Army Corps of Engineers. TerraGo Technologies had worked with several GIS vendors, such as ESRI and Intergraph, to develop modules that could create PDF files with PDF georegistration 2.2 encoding and import annotations added in the field back into a GIS system.

In September 2011, USGS published documents about techniques and methods (including formats) for two relatively new map products. Both products, initially used Terrago GeoPDF files as a format for distribution of digital maps for public use. The first produce was a digital replacement for the standard USGS printed maps at 1:24,000. See US Topo Product Standard, Version 1.0 (2011), (as of January 2013, link via Internet Archive). The second product was for digitized versions of USGS Historical Topographic Quadrangle maps, dating back to 1884. See Standards for Scanned U.S. Geological Survey Historical Topographic Quadrangle Collection (2011), (as of January 2013, link via Internet Archive). Both USGS products were later migrated to specifications based on the Adobe encoding for georegistration (see PDF_1_7_geospatial) as introduced with Acrobat 9.0 in June 2008, documented in Adobe Supplement to the ISO 32000, BaseVersion:1.7, ExtensionLevel:3. June 2008, and later incorporated into ISO 32000-2:2017 for PDF 2.0. See PDF_2_0. For the updated specifications from USGS, see http://pubs.usgs.gov/tm/tm11b2/ for the US Topo Product Standard and http://pubs.usgs.gov/tm/11b03/ for the Standards for Scanned U.S. Geological Survey Historical Topographic Quadrangle Collection.

The compilers of this document welcome comments contributing additional information about adoption and/or continued use of PDF georegistration 2.2 as a georegistration encoding.

    Licensing and patents

GeoPDF is a registered trademark of TerraGo Technologies. According to the specification published by OGC, the term GeoPDF may only be applied to products created under license from TerraGo. OGC 08-139r3 introduced a section with more specific patent information. This includes the statement, "This specification is being provided by TerraGo on a reasonable and non-discriminatory, royalty free basis (RAND-RF)."

See PDF_family for patent and licensing information associated with PDF.

Transparency Judging from example files, this georegistration encoding can usually be read by using an ASCII text editor to open the PDF file. In practice, most PDFs have compression filters applied to most of the file content.
Self-documentation The georegistration encoding described here supplies limited geospatial metadata for maps or other georegistered illustrations in a PDF file, metadata sufficient to support measurement. See PDF_family for information on embedding metadata in PDF documents in general.
External dependencies To get full value from a PDF with PDF georegistration 2.2 georegistration encoding as a user, you need the TerraGo Toolbar, an add-on to Adobe Reader. The toolbar is freely downloadable (after registration), but works only on Windows. For mobile devices, there is the TerraGo Edge app; this app is designed for data capture in the field, but can also be used as a viewer for GeoPDF maps. See TerraGo Edge 4 on Apple's App Store and on Google Play.
Technical protection considerations The encoding itself has no means of protection. Encryption or other forms of technical protection may be applied to the file in which the georegistration information is embedded.

Quality and functionality factors Explanation of format description terms

Still Image
Normal rendering The georegistration encoding is independent of image quality and functionality. See PDF for discussion of quality and functionality factors for images in PDF documents.
Clarity (high image resolution) See PDF_family.
Color maintenance See PDF_family.
Support for vector graphics, including graphic effects and typography See PDF_family.
Support for multispectral bands See PDF_family.
Functionality beyond normal rendering See PDF_family.
GIS images and datasets
Normal functionality

The PDF georegistration 2.2 encoding for georegistration of an illustration in a PDF file defines a map frame for the image, by providing data to map the coordinate system of the PDF page to a known coordinate reference system. Three types of coordinate reference systems are supported: geodetic, projected, and unregistered cartesian (for engineering). The map frame specifies the coordinate transformation matrix, all parameters required for a map projection (if used), the reference ellipsoid, and the geodetic datum.

This encoding supports a list predefined geodetic datums and ellipsoids, by means of codes that correspond to names used by the GeoTrans software from the U.S. National Geospatial-Intelligence Agency. Note that OGC 08-139r3 states, "the OGC recognizes that the specification of the datum, ellipsoid, and CRS codes as used in this document do not conform to the EPSG database, the current de-facto normative reference for CRS metadata." The encoding also supports custom geodetic datum definitions. A geodetic datum is comprised of an ellipsoid, a known origin point, and a prime meridian. For PDF georegistration 2.2 encodings, a Greenwich prime meridian is assumed. The description for a custom geodetic datum may also specify a shift operation that converts (at least approximately) the geodetic datum’s coordinates to the widely-used World Geodetic System 1984 geodetic datum (WGS84).

Support for GIS metadata The PDF georegistration 2.2 encoding for georegistration provides specific technical geospatial metadata associated with an image within a document. The encoding specification makes no assumptions or recommendations with respect to GIS metadata for the PDF document as a whole. Version 1.4 of PDF and later support embedding of any metadata in XMP packages. See PDF_family.

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension Not applicable.  PDF georegistration 2.2 does not define a file format, but an encoding for georegistration information that may be associated with an image. In practice, it will be embedded in a file in some version of the PDF format, and that file will likely have the extension pdf.
File signature /LGIDict
A PDF file including PDF georegistration 2.2 encoding will be recognized by the existence of an LGIDict entry associated with at least one page in the PDF.
Pronom PUID See note
No PUID as of May 2020.
Wikidata Title ID Q5533911
See https://www.wikidata.org/wiki/Q5533911.

Notes Explanation of format description terms

General

The PDF georegistration 2.2 encoding uses features of the PDF architecture to embed georegistration information into a page object. It was the first widely adopted technique for supporting georegistration within a PDF. With the release of Acrobat 9.0 and ExtensionLevel 3 to PDF, Version 1.7 in June 2008, Adobe introduced its own schemes for recording georegistration and 3D information in PDF documents. See PDF_1_7_geospatial. In his February 2009 blog post, OGC Approves GeoPDF 2.2 as OGC Best Practice, George Demmy of TerraGo recommended, "If you'd like to roll-your-own geospatial PDF, I'd recommend using Adobe's proposed geospatial extensions to ISO 32000. Our software supports this georegistration technique, and we're already moving in that direction." The compilers of this resource have not attempted to determine the relative adoption of the two approaches to georegistration in PDF documents. Comments welcome.

Georegistration, as described here, supports the viewing and online measurement of maps and geospatial images. The TerraGo product line, using the GeoPDF trademark, also includes tools that use georegistration encoding together with an embedded database to support the collection of information in the field using annotations (markups) associated with a point on the map. Application domains include emergency management, and management of natural resources and remote assets such as utility poles and railway track. GeoPDF files created by or used with these tools can contain data payloads associated with locations on the georegistered image. The detailed encoding for these data payloads is not publicly specified. Comments welcome.

History

The original georegistration encoding used in GeoPDF files was developed at Layton Graphics, Inc. in the early 2000s. The use of the name LGIDict for the encoding object within a PDF file reflects that origin. Version 1 was drafted in 2002; version 2 was introduced in November 2003. The mapping group of Layton Graphics was spun off as TerraGo Technologies in September 2005. See History of GeoPDF: PDF Map Books, LGIView, and LGIDict (May 2009) and Layton Graphics forms TerraGo Technologies (September 2005).

The specification of version 2.2 of the this georegistration encoding was approved as an OGC Best Practice in February 2009 as OGC 08-139r1. See the blog post, OGC Approves GeoPDF 2.2 as OGC Best Practice. Publication as a best practice by the Open Geospatial Consortium (OGC) as OGC 08-139r2, with minor editorial changes, followed in April 2009. In January 2011, a revision was published as PDF Georegistration Encoding Best Practice Version 2.2. OGC 08-139r3, with changes that were editorial and administrative and did not reflect changes in the specification for the encoding.

The TerraGo product line now supports the Adobe geospatial extensions (introduced with Acrobat 9.1 in June 2008 and later incorporated into ISO 32000-2:2017 for PDF 2.0) as well as the encoding specified in OGC 08-139.


Format specifications Explanation of format description terms


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Last Updated: 06/11/2020