Magnum One/Magnum One (Mid-American Digital) (Disc Manufacturing).iso/d2/wsortho1.arc/MAPDOCS.TXT

Text File | 1991-06-13 | 19KB | 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 [phone]

 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