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Text File | 1991-06-13 | 18.6 KB | 330 lines

Background information and instructions for the sample digital orthophoto image on floppy diskette. Prepared by: Bob Gurda April 15, 1991 Wis. State Cartographer's office 155 Science Hall University of Wisconsin-Madison Madison, WI 53706-1404 phone (608) 262-6850 Background: Many managers and staff who administer land management programs over large areas have wondered what modern desktop computer technology could do to improve use of aerial photography. Their concerns relate both to convenience and to accuracy. Recently it has become practical to consider the potential development of geometrically accurate digital imagery over large areas. On this disk are files that allow a limited preview of such a image library. As part of the CONSOIL Project (see Wisconsin Mapping Bulletin, July 1988), aerial photography was acquired over all of Dane County, Wisconsin in the summer of 1987. The original photo scale was 1:40,000. This is about 1.5 inches = 1 mile. (More specifically, the acquisition was modelled on specifications for the National Aerial Photography Program --- NAPP). The purpose of the acquisition was to create orthophotomaps, each covering 3.75 minutes of latitude and longitude. Each such map covers one-quarter of the area shown on standard paper printed topographic quadrangle maps (which cover 7.5 X 7.5 minutes, and are produced at a scale of 1:24000 or 1" = 2000 feet). These new image maps were prepared at a scale of 1:12000 or 1" = 1000 feet. About 100 such orthophoto quarter quadrangle (OQQ) maps are needed to cover Dane County; 4600 would cover Wisconsin. "Ortho" means perpendicular, so an orthophoto is a photo that has been geometrically modified in order to produce a view of the ground like a map. That is, at any point on the map, the view appears as though the camera had been directly above that point on the ground. An orthophoto is derived from standard aerial photography that is acquired in overlapping mode. The overlap, coupled with photo- identifiable geodetic control (knowledge of the relative positions of points that can be found in the photos), means that the terrain can be 'reconstructed' in a machine; this reconstruction or simulation is called a stereo model. Finally, with the knowledge of the terrain from the stereo model, various distortions that occur in any aerial photograph can be corrected. The result is an image derived from a distorted aerial photograph, but that has the distortions removed. This orthophoto has a defined and accurate map scale, and can be used for measurements or location determination like a map. Commercial software already exists that can simultaneously display both a digital orthophoto image and other digital information such as field and/or wetland and/or ownership boundaries, or well locations or potential forest timber sales, etc. Widespread availability of moderate resolution digital imagery would help make possible a long list of potential applications. Contents of the diskette (1.2 MB, 5.25" pc type) 1). Image file: BLKSW100.ORT Of the 140 OQQs produced in hard copy form for the Dane County area, 36 were also produced in digital form. These 36 are centered over the Black Earth Creek Watershed, just west of the Madison metropolitan area. From the much larger Black Earth Creek watershed area, an easily manageable piece of digital orthophoto imagery has been selected for this sample. It is from the southwestern quarter of the area covered by the printed 7.5-minute quadrangle map sheet titled 'Black Earth'. The printed OQQ sheet for this area is titled 'Black Earth Southwest'. The digital sample on this disk includes the village of Black Earth, US Highway 14, County Trunk and town roads, a railroad corridor, a section of the creek, agricultural fields, and pasture and wooded lands. This is a large file (about 1 MB) that contains data that codes an image. It is made up of 1 million chunks of information, each of which describes a level of grayness (on a scale between 0-255) for an area on the ground that is 4 X 4 meters. The 1 million chunks, or ground areas, are called pixels (short for picture elements). In the case of this particular image, the image is square. It is built up as an array, or matrix, or grid of 1000 X 1000 (=l million) pixels. The array was created by scanning the original photograph at high resolution --- 100 microns in this case. A micron is one millionth of a meter, or one thousandth of a millimeter. Thus, 100 microns is 1/10th of a millimeter. In inches, this is equivalent to 1/250th of an inch). At 1:40,000 original scale of the photograph, each pixel thus captured by the scanner represents 16 square meters on the ground (4m X 4m). (The best currently available satellite images have pixels of 10-30 meter resolution, corresponding to 100-900 square meters). The entire sample image on the diskette covers 4000 X 4000 meters (or 4 X 4 kilometers --- a little more than 6 square miles) . Had the scanner been set up to collect gray level information every 25 microns (four times as fine as this sample image), each pixel would have represented 1 square meter on the ground. The result would have been a digital image with more information extracted from the original photo --- the on-screen image would look more detailed, provided that the original photography in fact contained more detail than that captured by the coarser resolution scanning. In the case of 1:40,000-scale photography, there is indeed more than enough resolution in the image to justify scanning at 25 microns (or even finer). But the idea of increasingly finer scanning has its limitations. First, the photographic film cannot record an infinite amount of detail. In the case of NAPP specifications, the aircraft is flying at 20,000 feet --- almost 4 miles above the landscape --- and the camera is viewing an area that is almost 6 X 6 miles in size. All of this is being imaged on a piece of film that is 9 X 9 inches. Each type of film has a different limit to its resolving power, but experts say that scanning at anything finer than 10 microns (1 one-hundreth of a millimeter) is going beyond current film capacity. A second limitation is not quite so absolute, but more a matter of physial practicality and cost. At a scanning density of 100 microns from 1:40,000-scale aerial photography, like the image on this disk, and assuming 256 gray levels being discriminated, the combined-size of digital files covering an average Wisconsin county would be large- --perhaps 150 MB. For contrast, a high density PC computer diskette holds 1.2 MB, and today's common hard disks hold 30-150 MB. By the use of emerging technology such as compact disk read-only memory (CD- ROM), where up to 600 MB can be encoded on one small disk, large files can be accommodated. An important trade-off to consider, however, it that for every halving of the scanning resolution (such as 25 instead of 50 microns), the digital file size for the same area on the ground is four times larger. At the fine end of scanning resolutions, 10 microns, the average Wisconsin county file(s) would be 10,000 MB, which could be coded onto 15 CD-ROMS. In this sample, each pixel has a gray value between 0 and 255. This requires one "byte" of computer data space to represent each pixel. But the human eye as well as typical computer monitors today cannot make good use of 256 gray levels all at the same time. To view this file effectively, you should use a VGA adapter card and compatible monitor. Color can be used but is not necessary. VGA capabilities will allow 16 gray levels to be displayed. To accomplish this when the file contains 256 levels, display software collapses these levels into 16 display levels for viewing. Although the IMDISP software (see below) allows use of EGA and CGA video adapter cards and displays, you will probably be disappointed by the results. VGA, which works best with IMDISP, is becoming a fairly standard kind of display for many PC computer users. As the user, you can direct the display software to consider only a certain range of the gray levels encoded in the image file to be displayed across the 16 viewing levels. This technique is usually called "stretching" by people who work with digital imagery on a regular basis. Stretching an image provides flexibility in making (more) visible some of the subtle but useful variations of pattern captured by the scanner in its creation of the image file. 2). Executable file: IMDISP.EXE This is a public domain (free) program that can display various kinds of raster (gridded) image files. It has a somewhat limited set of features, and is not terribly user friendly. But a person familiar with using a PC-type computer should be able to learn its basic features fairly quickly. IMDISP was written at the Jet Propulsion Lab as an easy way to view digital image files sent back from interplanetary space probes. It is also useful for viewing digital images of the earth, whether acquired from satellites or from scanned aerial photography. 3). Documentation files: IMDISP.DOC and README.DOC IMDISP.DOC is a user manual for IMDISP.EXE. It is in ASCII format, so it can be printed from the DOS prompt with a command like 'PRINT IMDISP.DOC". While there is an online 'help' function built into IMDISP, you may benefit from having the manual printed out for reference or study. README.DOC is a shorter ASCII file that explains the process of orthophoto production and some of the characteristics of the image file. 4). Batch files: various DEMO*.BAT These are files written by Bob Gurda of the Wisconsin State Cartographer's Office. They are not DOS batch files, but work only after IMDISP has been started. Depending on their design, a particular batch file may need to have an image file loaded prior to being started. (This is true of DEM04K.BAT) Any one of these files can be started by typing the command 'BATCH' followed by the name of the demo file. Since it is not possible to include comments inside these demo files and still have them function, they are undocumented. These types of batch files can be modified or created with any ASCII editing program starting from the DOS prompt. 5). Palette Files: various PS* The IMDISP display software allows various colors to be defined for the 16 different levels of information being displayed. For convenience, these can be defined in palette files. Like a batch file, these are created or modified with an ASCII editor. Getting started A. Use a hard disk if possible. By transfering the contents of the diskette to a special directory on your hard disk, you will find that the program will display the image much faster than from the diskette. B. Use a fast computer if possible. At a CPU processor speed of 16 Mhz, the display program runs reasonably fast (in concert with the hard disk as mentioned above). At very fast CPU speeds, you may not experience faster drawing to the screen because the limiting factor can become the video card. Also, slow hard disk performance may be alleviated with disk caching software and/or creation of a ramdisk. C. Start the display software. --- Change to the appropriate drive/directory. --- Type "Set IMDISP=VGA" <return> --- Type "IMDISP" <return> (hint: put the above 3 steps in a DOS batch file) --- Type "help" <return> ...this will preview the list of available commands --- Type "File" <return> --- Select the file BLKSW100.ORT by typing the appropriate number<return> --- Respond to the questions by typing 1000 <return> 1000 <return> 8 <return> 0 <return> --- Type "display" <return> ... you will see the orthophoto image being "painted" starting at the top of the screen. Assuming that you have VGA display capabilities, you will see 480 rows and 640 columns of the image (not the entire scene). --- Type "erase" <return> ... the screen will be erased. --- Type "display sub 3" <return> ... now the entire image will fit on the VGA screen, but at a loss of image resolution, since the software is displaying only every third pixel in every third row. --- Type "histogram" <return> ... the computer will work for a while, then display a graph that represents the distribution of gray levels across the entire 1 million pixel image. That is, for each of the 256 gray levels (0-255), the software counts how many pixels are coded for that level. You will notice several things: First, there are no pixels coded for gray levels above about 220. These would be very bright up to pure white. By contrast, there is a good deal of information down at the very dark end of the scale. And the bulk of pixels are bunched in a group that has gray levels between about 40 and 90. --- Type "erase" <return> --- Type "disp" <return> ...to redisplay part of the image --- Type "set dn hi 191" <return> ... --- type "disp" <return> --- Type "set dn lo 40" <return> --- Type "set dn hi 90" <return> ... these last two commands will cause those pixels coded below gray level 40 all to be displayed as black, and all those coded above gray level 90 to be displayed as white. As a result, there are more display gray levels left over to show the patterns represented amongst the pixels coded between 40 and 90. This accomplishes a "stretch" of a range of the image. --- Type "disp" <return> ... to view the result. --- Try running the batch file "demo4k.bat" for a very short introduction to digital orthophotography. Your diskette contains several palette files: "ps*". To access one of these, type "pal load ps5" for example. Then type "pal edit" and the I's" key (repeat). To get out of this edit mode, hit the "RETURN" key. The IMDISP program also contains internal palette files. These are accessed via "pal ps o" for example. Note the space between ps and 0. "pal ps 1" gives the default gray scale palette. All of the color palettes result in "false color". The data file represents gray levels, and the colors merely subtitute for various shades of gray. In many cases, the color effect is more distracting than useful in improving the visibility of features. ***************************************************************************** Instructions for viewing digital orthophoto files created with various scanning resolutions. There are several files on the two 360K 5.25" diskettes "ORT_RES_A" and "ORT_RES_B" (alternatively, the contents of these two diskettes may be provided on one 720K 3.5" diskette). The image files all have an extension of "4bp". This extension stands for "four bits per pixel". Copy all of these files to the same directory on your hard disk that holds the "IMDISP.EXE" program and the "BLKSW100.ORT" file. The demo file named "DEMO-RES.BAT" is run after entering IMDISP as usual. Simply type "Batch demo-res.bat" and follow the information that comes up on the screen. As with the full digital ortho image "BLKSW100.ORT", a VGA monitor is necessary to effectively display these images. Any of the six images prepared to compare various photo scanning resolutions can be viewed separately. Simply type "file", select one of the "4bp" files by number, and then just type "display". You might want to try using different palettes to best bring out the varying levels of detail in these samples. Zooming and centering with "cursor" is also available as usual. Just to review the process of producing digital orthophoto files: Scan the original photo, potentially as fine as 10 microns (about the limit of resolution of the film) . When using 1:40,000-scale NAPP photos, a 25-micron scan produces 1-meter pixels. Resample the scanned pixels, based on knowledge of inherent distortion on the photo derived from the stereo model that is controlled by photo-identifiable geodetic control points. The resulting digital file can then be formatted for viewing with various software packages, and can be printed to a film recorder (high tech) or laser printer (low tech), from which copies can then be made by various means. Notes by Bob Gurda, Wis. State Cartographer's Office April 15, 1991