Software review for February 1996 issue of the IGIC newsletter, Coordinated GIS by Paul F. Anderson, Associate Professor of Landscape Architecture and Agronomy 481 College of Design, Iowa State University, Ames, IA 50011 voice 515-294-8943, fax 515-294-2348,

Do you ever need to change the geometry of a map or aerial photograph? If so, Transformer from Blue Marble Geographics could provide you the tools to get the job done. Transformer is a Windows 3.1 package that helps you perform three major functions involved in changing image geometry: (1) selecting control points and entering reference coordinates, (2) modifying (transforming) images, and (3) tiling (mosaicking) transformed images together. All the operations for each of these major functions are located on one tab display panel or page of Transformer. Each display panel has its own visible tab, so it's easy to change from one panel to another with one mouse click on the appropriate tab: Reference, Transform, or Mosaic.

Project application

We used Transformer to change the geometry of 108 aerial photos to match GIS interpretive maps. This allowed us to use image editing software (Adobe Photoshop 3.0) to overlay raster GIS maps on an aerial photo background for presentation. We're helping Golden Hills Resource Conservation and Development (RC&D) in Oakland, Iowa provide GIS interpretive maps to local land owners, managers, and public officials, who use the maps to make decisions about land use and management in floodplains. At the beginning of the project, Golden Hills RCandD staff members Marty Braster and Gregg Hadish interviewed potential local users of GIS information, who preferred that GIS results be presented on an air photo background, similar to USDA county soil survey map atlas sheets. Some people in GIS and digital cartography call these combinations of GIS maps and aerial imagery "image maps."

Image map (air photo/GIS map) of one section of land in Fremont County, IowaPotential local users that we interviewed preferred to have a fast point and click system with pre-stored (canned) GIS interpretive maps that would help them make decisions about floodplain land use and management. Early in our project, this approach came to be called a GIS Reference System. We started creating our GIS Reference System by scanning 108 USDA farm program compliance slides that cover our 188 square mile study area in Fremont County. These 35mm compliance slides were scanned using a Nikon Coolscan 35mm transparency scanner and an HP Vectra Pentium PC. We saved each digital image as a full color 90 DPI compressed TIFF. Transformer will also read BMP, TGA, JPG, and PCX files. Our TIFF file sizes averaged 1.5 MB for images of approximately 478 columns of pixels by 942 rows of pixels. We then converted the images to 256 gray scale at 72 DPI using Photoshop.

Entering control points

Each scanned slide image included approximately two sections (an area of approximately one mile by two miles). Our first job in using Transformer was to select 5 or 6 control points on each aerial photo. Then, for each control point, we entered X and Y coordinates. Most of our control points were section corners.

After loading a raw (untransformed) TIFF image in Transformer's Reference tab display panel, Transformer displays a thumbnail (subsampled) version of the complete image in the Overview window on the left side of the panel. On the right side of the panel, Transformer displays a full size (1:1) view of a portion of the TIFF image in the Point Pick Window. Using the left mouse button, we panned around to view any portion of the complete image at 1:1 or other enlargement. After we moved the cursor over a control point on the right image, we used the right mouse button to mark the control point. This also displayed the image coordinates (pixel column and row) in the center of the panel.

We then input the corresponding X and Y ground coordinates by typing or by cutting and pasting them using the Windows Clipboard from our DBF file which is loaded in Microsoft Excel. One Transformer option we would like to have used but couldn't was importing X and Y coordinates from another georeferenced image file, such as a USGS DRG (Digital Raster Graphic) file. That will be possible in our future projects, after DRGs are available for all of Iowa (currently, only the Des Moines SE quadrangle DRG file is available). However, this Automatic Reference Point Transfer option requires use of a compatible software package, such as Blue Marble's Geographic View, a custom Visual Basic program, ArcView 2, MapInfo, or other compatible software that supports Dynamic Data Exchange (DDE).

After we input X and Y coordinates for each control point, we added these data to our list of control points for the image by clicking the mouse on the "Add" button. This list of control point data appears in the lower part of the panel. After entering all control points for the image, we clicked on the "Solve" button which applies the coordinate transformation algorithm of our choice (Affine, 1st Order Polynomial, and 2nd Order Polynomial). Transformer then displays the X residuals, Y residuals, and combined residuals for each control point. Occasionally, we displayed the optional graph of residuals. If we weren't happy with the resultant residuals, we edited and replaced the offending control point data. Then we clicked the "Solve" button again. We repeated these two steps again, if necessary, until we achieved acceptable residuals. Fortunately, the need for editing and re-solving didn't happen too often (less than 10% of the time).

This Reference tab display panel in Transformer is well designed, which makes it quick and easy to use. We were impressed with the ease of zooming and panning around the image. We were also impressed with the ease in adding, editing, and replacing data in the control point list. Transformer prompted us to save a Reference Settings File (RSF) file before we went on to the next panel. This RSF file is a small ASCII file containing the control point data for the image. It was easy and quick to reload the image and the RSF if we needed to go back and check our control point data. Transformer uses this file next in the Transform step. Alternatively, Transformer can also read ESRI World Files (TFW, WLD).

Modifying (transforming) images

The Transform tab display panel provided data entry boxes for us to enter the name of the (transformed) output image, the pixel resolution (pixel size in ground units), the Border Color Value for background pixels, and coordinates of the image area we wanted to save in the output file. We used a Border Color Value of zero, which on our system represented the color black. We used a pixel resolution of 3.7 meters because when we display our final images at 72 DPI, each pixel will represent an area of approximately 3.7 meters square.

The most difficult input data for us to deal with at first were the coordinates of the image area we wanted to save in our output TIFF file. This feature, called the Transformed Area Definition, gave us the option of including more area or less area than the input image covers, but we had to know the coordinates of the area. For our purposes, we always started by clicking the "Default to Reference Point Extents" button, which displays the extreme north, south, east, and west coordinates based on the control point data entered on the Reference tab display panel. For most of the 108 images, we then changed the default coordinates by adding or subtracting 100 meters. This strategy allowed us to include an output area that was 100 meters larger on each side of the sections.

This panel gave us one other important set of options: the input coordinate system and the output coordinate system. Transformer contains 9 coordinate systems and 22 horizontal distance units from which to choose. (Essentially, this is a specially integrated version of Blue Marble's Geographic Calculator software.) When we finally clicked on the "Transform" button, Transformer prompted us to save a Transform Settings File (TSF) file containing the data and selections we made on this Transform tab display panel for this image.

Then, Transformer saved an output TIFF image. Transformer writes only uncompressed TIFF files. Our 33 Mhz 486 PC took approximately 30 seconds to process and save each output image. For some inexplicable reason, Transformer automatically disappeared from the computer screen at this point. It hid itself (and Excel) behind the Windows Program Manager after it quickly displayed an "all done" dialog box. The first time this happened to us, we thought that Transformer had crashed. We were glad to find out that no data or images had been lost and that Transformer was still running. We quickly redisplayed Transformer and Excel on the computer screen using the Alt-Tab key combination or Ctrl-Esc key combination.

Tiling transformed images

The Mosaic tab display panel provides a way to tile more than one transformed image together in one large image. There are buttons for "Open input," "Add," "Remove," and "Clear" to build a list of image files that Transformer tiles together. Several output parameters include Border Color Value and name of the Mosaic Settings File (MSF). An ESRI World File is created for each completed mosaicked image. During our first tests last fall, we ran into several error messages during this operation. However, because we did not need this mosaic function for our project, we did not take time to further investigate the cause of this error message.

Project application

After we transformed aerial photo images in Transformer, we loaded each one in Photoshop 3.0 to overlay GIS maps. Our first job in Photoshop was to crop each transformed air photo image to just one section (plus approximately 100 meters around the section for visual context). Then we overlayed GIS maps on the air photo layer in Photoshop. The GIS maps were produced in GRASS/GIS, which Golden Hills RCandD is using for their floodplain study, scenic byway study, and other projects. Because georeference coordinates are not used in Photoshop, we needed to visually register the maps over the transformed aerial photos. This was easy to do with the move and scale features in Photoshop. After experimenting in Photoshop, we settled on a opacity setting of 50 percent for each GIS map layer. This provided the best overall color rendition and color contrast in the final images. It also produced the best overall printing results on a 300 DPI laser printer (see accompanying example). If the air photo images were too dark to provide good color contrast, we used Photoshop's image brightness and contrast settings to modify the air photo image. The last step before saving each of the final combination map/photo images was "flattening." Flattening is Photoshop's term for saving a TIFF file by eliminating all layers except one map layer and the air photo background layer.

Overall evaluation

Overlaying GIS map results on a digital air photo background will certainly be a more common way of presenting GIS results in the future. For many years before computer-assisted GIS and image processing, it's been a familiar presentation form for users of USDA soil survey maps. New tools, such as orthophoto quads, along with more options in GIS and image processing software, will make this even easier to do and more commonplace in the future. Until orthophoto quads become available for every study area, stand-alone packages such as Transformer will help get the job done in the mean time.

We originally purchased version 2.02 of Transformer last October for the list price of $795. Before we purchased the package, we sent for a demo disk with an on-screen slide show that described in words and graphics many of Transformer's features. The demo package was somewhat helpful, but it was difficult to evaluate the usefulness of Transformer to our project until we purchased the complete package. Fortunately, Transformer provided the capabilities we needed for our GIS Reference System project.

Transformer adds to the options already available through Arc/Info, Arc/View, ERDAS image processing system, and other larger packages. As a stand-alone option, Transformer is certainly efficient and functional for some needs. It's more visually-based and less costly than some of these other alternative packages. Transformer has more features than we were able to use for this project, so we're interested in learning more through our other project applications in the future.

Transformer software comes on two diskettes and is relatively easy and fast to install. Program files occupy about 4.5 MB of hard drive space. Our 33 Mhz 486 PC (with Windows 3.1 and 24 MB of RAM) provided quite acceptable performance for our project, even with Excel running at the same time.

Early versions of Transformer had an integrated scanner interface that allowed you to scan images directly from a TWAIN compatible scanner. Beginning with version 2.02, the scanner interface was removed. Instead, Blue Marble Geographics now recommends using the application software bundled with scanners.

The hard copy documentation is adequate to get the software installed and to get started experimenting with images. The help file provides much the same information. Neither is as complete as the documentation that comes with some software (Adobe software, for example), but both are more complete than the documentation that comes with other software (Blue Marble's Geographic View, for example).

In addition to Transformer, Blue Marble Geographics also makes Geographic View, Geographic Calculator, and Geographic Tracker (GPS) software. We purchased and tested Geographic View software for our project also. We wanted a viewing package that could meet the needs of our local participants and provide easy point and click access to GIS and aerial photo images. Geographic View provides some of these functions quite well. However, it didn't fit our needs as well as Adobe Acrobat, a multi-platform generic text and graphics viewing and printing system with hyperlinks, bookmarks, annotation notes, and threaded articles.

For more information about Transformer, contact Blue Marble Geographics at 46 Water Street, Gardiner, Maine 04345, voice 207-582-6747, fax 207-582-7001, orders 800-616-2725, bbs 207-588-0054. Also, they advertise regularly in GIS World.

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Last update: 1 December 97