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1991-06-13
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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