Introduction
Keeping pace with rapid changes in technology can be both difficult and frustrating. This is especially true when creating and maintaining map data within a digital environment. As the evolutionary path of data creation and updating has moved toward greater use of digital image products, analog aerial photographs have had to be scanned and rectified in order to efficiently transfer critical data into GIS systems.
      This process has been, and remains, one that is necessary to make the interpreted data more useful. Methods and techniques that decrease the time it takes to transfer photo details reduce the cost of labor for map production. As forest mapping has evolved from an analog to a digital process, methods to obtain details from interpreted photographs have also changed. With these changes have come greater complexity and the need for increased knowledge in image scanning, rectification, and image-processing techniques. While some types of digital imagery are used for interpretation, the immense informational content of conventional aerial photographs will secure their place in forest mapping for years to come. Having good stereoviewing capability allows users to see and extract maximum feature content of the imagery. The difficulty in using an analog product in an increasingly digital world has led to the development of a new instrument to digitally compile details directly from photos without the need for scanning.

Traditional Methods
Generally, stand detail transfer is made from a single photographic print or transparency, since the use of more expensive stereoplotting equipment has historically been cost-prohibitive. Traditional methods used to place interpreted features in their correct position on a map have relied upon a planimetrically controlled base. This base could either be constructed by standard photogrammetric processes, or by using available maps such as those supplied by the U.S. Geological Survey. Base map scale must closely approximate photographic scale, or else the distortion inherent in a single photograph is increased when enlarging a small-scale photo to fit a large-scale base map.
      Among the several types of single-photo instruments that use the camera lucida principle, probably the most familiar to foresters are the vertical sketchmaster and the Bausch and Lomb Zoom Transfer Scope. Other such products include reflecting projectors, mirror stereoscopes with plotting arms, and several types of stereo pantographs that are designed to allow scale adjustments of greater magnitude between the photograph and the base map. Confining delineated features to the area of least distortion of each photo (effective area) allowed reasonable adjustments to compensate for slight scale changes, tip or tilt elements, and some relief displacement present on each photograph. The Zoom Transfer Scope allows optical adjustments within the viewing area to help "fit" the photo features to their correct locations by combining scale, stretch and rotation to the viewed image as it is overlaid on the base.

Digital Image Methods
If aerial photo images are to be adjusted to fit a particular map projection as represented by a digital data set, the images must either be georeferenced, rectified, or converted into ortho-images. The choice of which process to use depends upon the accuracy of the base data, the compilation scale, and the final accuracy level desired. In any case, if image data is to be adjusted in digital form it must also exist in digital form, whether scanned or digitally acquired.
      With the exception of using imagery that is digitally acquired, the "digital solution" to using conventional aerial photographs for forest mapping is to scan the photos and rectify them to fit the features on a digital base map. Scanning can be accomplished with a standard desktop scanner, or with a photogrammetric scanner that produces a highly accurate scan without scanner-induced distortion. Both methods can produce digital images, with the amount of detail dependent upon scan resolution levels. However, the closer the scanned images are to the original, the larger the digital file produced.
      A variety of computer systems are available that will display and rectify scanned images to fit base map data. These range from image-processing systems such as Erdas Imagine(r), DVP-GS(tm) and others, to computer versions of analytical stereoplotters such as Autometric's Orthomax(r) and Softplotter(r) software. Making a scanned image fit the base data requires sets of ground control points (GCPs) that can be identified on the scanned images. Finding sufficient GCPs that are spread out enough to allow the image to fit the base data is often difficult in forested landscapes. In many cases there are insufficient objects on a single image, such as road intersections, to allow for an acceptable fit. Also, a problem exists when multiple photos are mosaicked together. In this process, with independent rectification of each image, matching of edges may be a less-than-desirable outcome. If digital orthophoto data exist for a given area and the orthophotos are of recent origin, there may be recognizable features on the orthophoto data that can be used as GCPs on the scanned images. The other alternative, while more costly, may be found with softcopy photogrammetric solutions that take into account camera geometry and image orientation. The creation of orthophotos can alleviate this problem of edge matching, but additional data in the form of digital elevation models and added time increase the cost of this approach. This additional cost may be justified in areas with significant relief, but this is less true for relatively flat terrain.
      The viewing of digital images on a computer screen can be done in stereo for systems that possess the ability to display rectified image pairs, or it can be viewed as a single image. Single images may be adjusted to fit base map data by the use of a differential mono-restitution process that effectively performs a single-image transformation, designed to fit either a planimetric base or a set of GCPs. This process is similar to the manual-optical operation that is used with the Zoom Transfer Scope, except it is wholly a mathematical solution. Unless the images are scanned at sufficiently high resolution to allow the viewing of tree characteristics vital for correct forest-type assignments, this on-screen approach rarely produces more accurate forest-type maps than those derived from original photographs, unless the classification is general in nature.

The Thales Optem Digital Transfer Scope
The Digital Transfer Scope (DTS) manufactured by Thales Optem (Rochester, N.Y.) allows direct digital interpretation and compilation of features from aerial photographs, in either monoscopic or stereoscopic view. This new instrument combines the image-adjustment properties of the older Zoom Transfer Scope, but in digital form. An advanced stereo microscope, designed to function as a stereoscope, allows the viewing of photographic transparencies or contact prints in either monoscopic or stereoscopic modes. Variable magnification is available that is independent for each eyepiece. As a result, photographs of different scales can be viewed in stereo. A built-in computer monitor allows the simultaneous viewing of digital map data superimposed over aerial photographs. A digital input for computer data and an output port for attaching a digital camera are provided as an integral part of the instrument. A film-scanning stage, similar to the viewing stage on the Bausch and Lomb SIS 95 image-interpretation station, is set over variable-intensity illuminators for use with photo positives of differing densities. The scanning stage accommodates standard 9x9 aerial photos, although smaller photos of any size can be used. Illuminators for contact prints are attached to the sides of the instrument. As currently configured, this instrument is fully self-contained and requires only an electrical power source and a computer. Any computer that runs ESRI's ArcView(r) V.3.x can be used. The software that enables direct digital transfer of photo-delineated features is an extension of the ArcView software and is packaged with the instrument.
      Since the images - in this case analog aerial photos - are in hard-copy form, the digital map data shown as an ArcView shape file is adjusted to fit the current aerial photograph being viewed. Map data can be rotated, scaled, stretched, zoomed or warped to fit the same area that is shown on the photograph. When warping, GCPs are selected and an affine transformation is applied to all shape files that are loaded as themes in that particular DTS view. The process is rapid, depending upon the speed of the computer processor and available memory (RAM), and it may be re-run several times to get an acceptable fit. Prior to applying the warp function, a report is produced that lists the RMS errors of each point and the combined effect of all points. Using this control point report, new points can be tested before the transformation is applied. Point, line or polygon features can be compiled and edited by using a mouse, a trackball, or a pen stylus. The compiling is done while viewing the map data superimposed over the current image, either in a point or a freehand (streaming) mode. Once the features have been added, the map data can revert to its original coordinates along with the newly compiled data. As a result, data that are added from the uncontrolled photo will now be located in the controlled map space as a new theme.
      Additional functions provide for image registration and placement, switching of color selections from normal to negative, the calculation of feature geometry (length, area, etc.), and all the attribute and feature-editing capabilities of ArcView.

Comparison of Results
When comparing the use of the DTS to conventional methods for compiling forest stands, roads, water, and other features associated with forest-type mapping, using the DTS cuts processing time by 20 to 49 percent. An even greater savings of time occurs when less-experienced GIS personnel use the DTS. This is probably due to the reduction in the number of steps necessary for complete mapping (production of a planimetric base, transfer, edits, digitizing, etc.), in addition to increased speed in creating point, line or polygon features. Training time varies depending upon the experience of those using the DTS. On average, one's production ability climbs steadily after a few days of practice. Regular use reduces familiarization lag time, and operators who use the DTS from two to three days per week develop a high level of proficiency for production work after only four to six weeks. This time can be reduced even more if an experienced operator is available onsite to coach new users.
       A comparison of the DTS against systems described in the section on Digital Image Methods is a limited one. However, when reviewing all the steps that are necessary to get to the point of delineating features in a digital mode, not having to scan and rectify photos to view them in stereo on a monitor could produce a savings in time of at least 25 percent, depending upon the software and computers involved.

Conclusion
There appears to be a clear difference in the choice of methods to allow digital compilation of data from images. If the images are digital to begin with, then the use of an image processing system to rectify and perform feature delineation may be the most advantageous. If the source of image data is aerial photography (whether contact prints or transparencies), the advantage seems to point toward the DTS instrument. With its excellent optics, the DTS provides a straightforward approach that can be mastered without extensive training in GIS or remote sensing. Users who are familiar with the older technology of the Bausch and Lomb Zoom Transfer Scopes (either mono or stereo) should find the functionality of the DTS reasonably familiar, albeit in a digital context. Users experienced in ArcView should adapt quickly to the DTS.

About the Author:
Roger H. Greene is president of Landmark Applied Technologies (Bucksport, Maine). He may be reached via e-mail at: [email protected].

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