By Fred Limp, director, Center for Advanced
Spatial Technologies (www.cast.uark.edu),
University of Arkansas, Fayetteville, Ark.
For several years, Avenza Systems has sold its well-received
MAPublisher product, a plug-in for Adobe Illustrator and
Macromedia’s Freehand that provides exceptional map-making
capabilities. These include the ability to import a range of
vector GIS data formats, along with associated coordinates, as
well as many other capabilities for creating high-quality
cartographic compositions using Illustrator or Freehand. However,
the tools in MAPublisher, Illustrator and Freehand focus on vector
data—not georeferenced imagery.
Now, for imagery users, Avenza’s Geographic Imager offers similar
capabilities, but focuses on raster imagery. Released in the
summer of 2005, the product is a plug-in for Adobe’s Photoshop CS2
image-editing program.
Primary Functionality
Geographic Imager lets users read and write coordinates and
projections for image files, georeference image files, and
automatically mosaic properly located files. Imagery processed
with Geographic Imager then can be input to Adobe Illustrator
MAPublisher products and vice versa, as well as many geospatial
applications. Other basic image-manipulation tasks—editing, color
modification, contrast alternations, etc.—are performed with the
standard Photoshop tools. Such capabilities address two primary
user groups: graphic artists who need to use imagery in their
efforts, and image specialists and cartographers who want to take
advantage of PhotoShop’s extensive image-manipulation tools, but
apply them to geographically accurate images.
File Formats/Importing and Exporting
Geographic Imager can read any image format supported by Adobe
Photoshop and can read and write files with the following geographic
reference formats: TFW, TIFW, WLD, JGW, PGW, SDW, EWW, BLW, DMW and
ECW. It can read the geographic data from GeoTIFFs, Blue Marble’s
reference files and MapInfo’s TAB files. Georeferenced images can be
exported to the various Adobe raster files, along with associated
coordinate files, as well as to GeoTIFF, Blue Marble and MapInfo
formats.
In addition to reading a range of file formats that already have
geographic coordinates, a raster image without coordinates can be
assigned ground control points and geo-located. Users can display an
image and interactively enter control point locations, or they can
be read from a file and then associated with the image locations.
The product presents information on aggregate and individual point
errors.
Re-projection Capabilities
Geographic Imager supports image re-projection using Blue Marble’s
GeoTransform technology, which offers thousands of datums and
projections. The software supports datum shifts, and, when possible,
pixels can be re-sampled to a new size during processing. Geographic
Imager also supports the HARN, NADCON and NTV2 datum shift
capabilities. It’s possible to define new projection parameters,
ellipsoids, angular and linear units and datums, or create new ones
by editing existing parameters. When re-projection involves pixel
re-sampling, a user can select from four different interpolation
methods: nearest neighbor, bilinear, quadratic and cubic.
Automatic Mosaicking
Geographic Imager can read from two to 128 individual geo-referenced
images and automatically create a mosaic. Anyone who has attempted to
create a mosaic using Photoshop’s PhotoMerge photo tool knows this
capability is invaluable. Note that Geographic Imager’s mosaic
capabilities replace the PhotoMerge; using PhotoMerge will strip a file
of its geographic coordinates.
Georeferenced Imagery Considerations
Because it is possible to alter the pixel size and even the shape and
orientation of an image easily within Photoshop, it should be obvious
that in some instances these operations may cause a loss in the location
information when applied to a geographic file, although Avenza has made
every effort to recognize these cases and prevent data loss whenever
possible. Operations that are known to damage georeferencing information
include all the Pallet Function actions. In addition, splitting or
merging channels will result in lost georeferencing information, as will
editing in ImageReady. Most Automate and Script functions will damage
georeferencing information if they modify any of the image geometry,
including saving an image from an Action or Script.
Images that have been georeferenced within Geographic Imager can exist
in either “standard” or “GCP maintenance mode.” Standard mode files are
ones that have been georeferenced via an affine transform. According to
page 17 of the manual, there are limitations in the range of Photoshop
operations that can be applied to images in GCP mode—which ones aren’t
clear, but any that change pixel sixes or orientation would be
problematic. The manual only states, “Some Photoshop operations may
potentially result in either a loss of precision and/or loss of [GCP]
points when operated in GCP mode.”
Finally it should be noted that the use of other Photoshop plug-ins that
alter image geometry will be incompatible with Geographic Imager. It’s
clear that adding georeferencing into the Photoshop data model is a
complex process, and it shouldn’t be surprising that there are several
potential “gotchas.” However, Geographic Imager provides considerable
detail about these potential problems.
Geographic Imager requires Adobe Photoshop CS2. It doesn’t support
earlier versions or Photoshop Elements. Hardware configurations (both
Mac and Windows) are the same as Photoshop’s, with substantial RAM—for
realistic work at least 1GB. Note that this version of Geographic Imager
doesn’t support non-English versions of Photoshop, but the next release
will.
The software comes with a 107-page manual that includes user guide and
tutorial sections. Tutorial data are provided on the distribution CD.
The manual doesn’t have an index, but includes a comprehensive table of
contents.
Bridging a Gap
Avenza’s Geographic Imager can be thought of as a bridge between the
geographic world and the graphic world. A clever ad guy might say, “It
adds the ‘geo’ in geographic.” As such, it plays a key role.
Adobe Photoshop is known for its extraordinary capabilities. But anyone
who as tried to use the program on geographic data knows that what you
gain in graphical power you lose in geography, particularly when there
are two or more images or if there are other geographic vector data.
Geographic Imager brings location awareness into Photoshop and
integrates imagery with vector geographic data already accessible in
MAPublisher.
