GeoPDF
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GeoPDF is a published extension (OGC 08-139r1) to the Adobe PDF file format, from TerraGo Technologies. It is used to present GIS and mapping data in a standard Adobe Systems PDF. This extension adds a coordinate transformation matrix and other metadata to allow transformation of PDF coordinates to a projected Cartesian coordinate system. GeoPDFs often include other advanced PDF features such as layers and object data which can add significant GIS functionality to the file, particularly when used with the TerraGo Technologies plug-in to Adobe Reader.
Contents
Methodology
The TerraGo Technologies GeoPDF utilizes a well defined method for embedding cartographic data within an Adobe Systems PDF. It is based on the ability to transform coordinates in PDF space to coordinates in the Cartesian coordinate system which represents a map projection, and from those coordinates to the latitude and longitude of the spheroid described by a named geodetic datum and projection. The reverse transformations are of course possible. These transformations are represented in the diagram below.
PDF -> projection -> geodetic -> [geodetic (datum shift)] -> projection
The data structure for accomplishing these transformations is in the form of a PDF dictionary entry, compatible with the Adobe PDF specification, version 1.3 and higher. The method is generalized such that a single PDF may contain multiple dictionary entries and thus support multiple map frames within a single PDF. This is accomplished by associating a polygon, referred to as a neatline, with each dictionary instance. The identification of map frames and the actual transformations are accomplished by a plug in to the Adobe Acrobat and Reader products, which reads the data from the dictionary entry and performs the required mathematics. Transformation from the Cartesian map coordinates to alternate datums and projections is accomplished using the GEOTRANS library, available through the National Geospatial-Intelligence Agency.
Uses
Given the ability to transform PDF coordinates to geodetic coordinates, and vice versa, it is then possible to enable several useful functions via plug-ins to the Adobe software. The most obvious of these is to report geodetic positions of cursor points. This can be done by either clicking at a point or by observing a dynamic display of the geodetic coordinates of the current mouse position, transforming the PDF coordinates to geodetic as required. The reverse transformation, from geodetic to PDF, is employed to allow the placement and zoom to a marker representing a specific user requested point. The same functionality is used in displaying the position reported by an attached GPS device.
In addition to reporting geodetic positions, the transformation method can also be used to report coordinates in many different projections or datums, not originally considered by the map maker. Notably, GeoPDF supports the display of coordinates in the United States National Grid (USNG). Although the map graphics is not distorted by a datum or projection change, the coordinate display is changed. In fact, with the current MAP2PDF plug-in it is possible to display the coordinates of a single point in three different datum/projection combinations simultaneously.
While the transformation of single points enables the above functions, capturing multiple points allows the calculation of the distances between points, the angles between points and the perimeter and areas of closed polygons. Access to a model of magnetic declinations at various points on the Earth's surface (the [National Geospatial-Intelligence US/UK World Magnetic Model] for 2005–2010) allows the calculation of a magnetic bearing between any two points on a map. Other models of the earth surface (such as a digital terrain model - DTM) could be accessed in a similar fashion to allow the reporting of elevations at any point covered by the DTM.
Given a set of GeoPDFs, each in the same projection and datum, it then becomes possible to concatenate the set into a single GeoPDF. The set might be of maps each covering a different part of a common area, as in a set of USGS quad sheets, in which case the result would be a seamless map of the area. Or the set might be individual themes of the same area, as with a quad sheet and a GeoPDF of a satellite image of the area, in which case the result allows the display of the satellite imagery overlaid on the map.
Used By
U.S. Army Corps of Engineers Topographic Engineering Center to produce Country DVDs of standard National Geospatial-Intelligence Agency (NGA) maps for countries involved in the Global War on Terror. These DVDs will be provided to all warfighters upon request. See Fact Sheetfor more information.
