Installation

The installation of Geographic Imager is a simple process. After it's done you will have an extra palette in your Photoshop workspace. This is the only interface to all the functionality of Geographic Imager. All other windows and dialogs are called from this one palette. This avoids cluttering up the screen, which means there's more room available for the imagery.

First steps

As soon as a (supported) georeferenced image is opened, Geographic Imager reads that information and displays it in the palette. Among the information presented are the coordinate system, geographic coordinates for the corners of the image, image size in pixels and pixel size in real-world units.

If a coordinate system cannot be determined by reading the source data, it needs to be specified before any other action can be taken. This is where the coordinate system library comes into play. Geographic Imager supports a very impressive number of coordinate systems and datum shifts, which are grouped by type (geodetic or projected) and region for easier access. In addition to being able to pick the right system from the list, you can also specify it as 'same as', picking one of the other files that are open at that moment.

Transforming an image is as simple as selecting the desired target coordinate system and picking a datum shift.

Geographic Imager's main palette, showing coordinate system, geographic coordinates for the corners of the image, image size in pixels and pixel size in real-world units for the active image.

Mosaicking

A somewhat more advanced topic is mosaicking. This requires that the images that are to be mosaicked have the same coordinate system and the same pixel size. Only images that match these requirements are selectable in the mosaicking dialog. One very interesting feature of the mosaicking process is that the separate images are imported into the main image as layers. This allows you to change the layer order manually, which can come in useful when combining satellite imagery or aerial photos, before flattening the image (combining all layers into one). The version which I tested had a limitation on mosaicking: mosaicking of layered images is not supported. If you attempt to do this, Geographic Imager gives a warning and offers to do the flattening automatically. By the time you're reading this, an updated version will be available which doesn't have this limitation.

Selecting images for a mosaic.

Georeferencing

The georeferencing process in Geographic Imager follows the common practice of picking at least 3 control points and specifying ‘real world’ coordinates for them (along with a coordinate system). This is done in a separate window. Placement can be done either by manually placing a point inside the image or entering pixel coordinates and corresponding ‘real world’real-world coordinates for them (along with a coordinate system).

The georeferencing dialog offers a way to georeference a 'normal' image.

Taking it for a spin

After going through the tutorial I decided to try it with some data of my own. I took a couple of large Landsat tiles (~200 Mb each) and ran them through a process of transforming them from their UTM projection to my local Netherlands RD. Then I combined the three tiles and after some cropping I ended up with a nice, nationwide mosaic. Photoshop is quite heavy on CPU memory, especially with large files like I was using. As a result, I had to transform each image individually, crop them, then save and close them. Only for the final step, the mosaicking, did I open all three of them. This turned out to be no problem at all; even after cropping the images, they retained the correct georeference. One rule to keep in mind is that you do have to save them as a supported geo-raster format or export the georeference separately. The speed of these transformations and the mosaicking was very reasonable given the size of these files.

Geographic Imager supports a wide range of coordinate systems, conveniently categorized so that they can be selected with ease.

A second test, using 4 sheets of the 1:24.000 DRG for an area in Minnesota, yielded similar results. Again, the process was opening the files, specifying the coordinate system, transforming them all to a common pixel size and mosaicking them. This was all done in a matter of minutes.

Geographic Imager and Photoshop

One thing to keep in mind when working with the combination of Geographic Imager and Photoshop is that not all Photoshop tools are compatible with Geographic Imager. For example, Geographic Imager will not work with files that use indexed colors. They have to be RGB or Grayscale. If you open a file that is in indexed color mode, Geographic Imager will warn you accordingly. It is a simple one mouse-click operation to change the colour mode using native Photoshop tools.

Conclusion

Geographic Imager doesn't do a whole lot, but for what it does, it does well. For a first version it certainly does what it claims to do and I am sure that future versions will add features and functions along the way. The combination of Photoshop and Geographic Imager has a lot to offer for anybody who is using georeferenced raster images. The speed of processing combined with the ease of use makes this a very useful tool. In addition to that, the fact that all of Photoshop's image editing tools are available as well makes it especially powerful. Many other products offer image transformation and mosaicking, but can't be compared to Photoshop in terms of image manipulation. In short The combination of Adobe Photoshop and Avenza's Geographic Imager brings a whole new dimension to spatial imagery.

For more information about Geographic Imager see http://www.avenza.com/products.geographicimager.html or call 1-800-884-2555 for more information.

About the Author

Hans van der Maarel is the founder and owner of Red Geographics, a cartography and GIS consultancy company from The Netherlands.