


imagery              GRASS Reference Manual               imagery



NAME
     imagery - Description of GRASS image processing functions.

IMAGE PROCESSING IN GRASS
     The following discussion is intended to provide a quick
     overview of image processing in GRASS.  Some concepts and
     some hints are provided.  For a more complete discussion and
     description of image processing in GRASS see GRASS Tutorial:
     Image Processing.

EXTRACTING IMAGERY DATA INTO A GRASS DATABASE
     Remotely sensed images are captured for computer processing
     by filtering radiation emanating from the image into various
     electromagnetic wavelength bands, converting the overall
     intensity for each band to digital format, and storing the
     values on computer compatible media such as magnetic tape.

     The GRASS programs which extract image data from magnetic
     tape can read LANDSAT multi-spectral scanner (MSS) data
     (i.tape.mss), LANDSAT thematic mapper (TM) data,
     (i.tape.tm), and other formats, such as scanned aerial
     photography or SPOT satellite data (i.tape.other).  They
     extract the band data into raster files in a GRASS database.
     Each band becomes a separate raster file, with standard
     GRASS map layer support, and can be displayed and analyzed
     just like any other raster file.

UNREGISTERED DATA
     The band data extracted from tapes are assumed to be
     unregistered data.  This means that the GRASS software does
     not know the earth coordinates for pixels in the image.  The
     only coordinates known at the time of extraction are the
     columns and the rows relative to the way the data was stored
     on the tape.

     Data can only be extracted into a database which has an x,y
     coordinate system, and not into a projected database (e.g.,
     a UTM database).  This is to prevent users from mixing the
     unregistered data with registered data in the same database.
     The GRASS system comes with the database imagery which is an
     x,y database.  New databases can be created by users during
     GRASS startup.  See the g.help section on "Setting Up a
     GRASS Database" for instructions on creating a new database.

CELL HEADERS
     The cell headers for the band files in these x,y databases
     are set to reflect the rows and columns of the extracted
     data.  The north-south values represent the rows, and the
     east-west values represent the columns.  The resolution of
     the unregistered data is set to 1.

     Note, however, that while the row numbers increase from 1 to



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     n from north to south, GRASS requires that the values of the
     user's current geographic region decrease from north to
     south.  The solution adopted was to represent the rows with
     negative values (i.e., -1 to -n).  This allows them to
     decrease from north to south and, if the minus sign is
     ignored, to reflect the row numbers.

     The cell headers for the layers in x,y databases are set so
     that the coordinates at the center of each pixel exactly
     reflect the row and column for that pixel.  The northern
     edge is set to 0.5 less than the first row, the southern
     edge 0.5 larger than the last row, the west to 0.5 less than
     the first column, and the east to 0.5 larger than the last
     column.  When the image is displayed on the graphics
     monitor, the d.where command can be used to report row and
     column values.

     For example, suppose rows 100-500 and columns 200-800 are
     extracted.  Then the cell headers for the extracted data
     will be given the following values:

               north:     -99.5
               south:    -500.5
               west:      199.5
               east:      800.5
               ns res:      1.0
               ew res:      1.0

REGION AND MASK
     Since the data layers are given essentially contrived cell
     headers, users must exercise extra care when analyzing or
     displaying unregistered images.  It is very easy for the
     user's GRASS region to have absolutely no relationship to
     the data he is trying to display.  This could happen when
     the region is set for data extracted from one tape, but the
     analysis is attempted on data extracted from another tape.
     A good habit to develop is to set the region to exactly
     match one of the band files.  This can be done using the
     GRASS g.region command.

     Another pitfall is to have a mask set to a band file from
     one data set while trying to read another.  Even if the
     region is set properly, the data will appear to be all no-
     data since the mask will effectively knock out any data.  Be
     sure that the mask is either set to a related data layer or
     not set at all.  See r.mask for information on setting and
     unsetting the mask.

     Please note that the tape extraction routines set your
     database region to match the rows and columns of the data
     that is extracted.




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GROUPS
     Since the band files are individual raster files, it is
     necessary to have a mechanism to maintain a relationship
     between band files from the same image as well as raster
     files derived from the band files.  The GRASS group data
     structure accomplishes this goal.  The group is essentially
     a list of names of raster files that belong in the group.
     For user convenience, groups are also created (and updated)
     by the tape extraction routines.  The tape extraction
     programs ask the user to supply a group name as well as to
     specify the bands to be extracted.  Suppose that the user
     extracts bands 1, 2, and 3 into a group called nhap.  Then
     the band files will become the raster files nhap.1, nhap.2,
     and nhap.3 and the group nhap will list these 3 raster files
     as members of the group.

     Groups can also be created and modified by the user using
     the GRASS command i.group.

IMAGE REGISTRATION AND RECTIFICATION
     Image registration and image rectification is the process of
     associating earth coordinates with pixels on the image and
     then converting the unregistered raster files to raster
     files in a projected database.

     Image registration (i.points) is applied to a group, rather
     than to individual raster files.  The control points are
     stored in the group, allowing all related band files to be
     registered in one step rather than individually.

     Image rectification (i.rectify) is applied to individual
     raster files, with the control points for the group used to
     control the rectification and the group target (i.target)
     used to specify the database where the rectified layer will
     live.

IMAGE CLASSIFICATION
     Image classification methods process all or a subset of the
     band files as a unit. Spectral signatures for the image are
     generated (i.cluster) and then used to produce a landcover
     map (i.maxlik).

     The signatures must be associated only with the raster files
     actually used in the analysis. This means that with a group
     subgroups must be created (i.group) which list the band
     files to be grouped for classification purposes.  The
     signatures are stored with the subgroup.

     Note that multiple subroups can be created within a group.
     This allows different classifications to be run with
     different combinations of band files.




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     Also note that raster files produced by the classification
     process (i.maxlik) are automatically listed as part of the
     group.

RECTIFIED VS. UNRECTIFIED ANALYSIS
     There are two possible routes for processing image data.
     The first is to register the group (i.points), perform the
     analyses on the unrectified band data (i.maxlik), and then
     rectify the results (i.rectify).  The second is to register
     the group (i.points), rectify the band data (i.rectify),
     then run analyses on the rectified band data in the target
     location (i.rectify).  Both routes are permissible in GRASS.
     Users will most likely prefer the first.  The second route
     requires leaving GRASS and re-running GRASS under the target
     location.  It also will require that a group be created to
     hold the rectified band files since i.rectify does not
     create or modify groups.  Also, spatial filtering may not be
     as effective on rectified data since the rectification of
     the data requires resampling the original data.

SEE ALSO
     GRASS Tutorial: Image Processing

     d.where, g.region, i.cluster, i.group, i.maxlik, i.points,
     i.rectify, i.tape.other, i.tape.mss, i.tape.tm, i.target,
     r.mask

AUTHOR
     Michael Shapiro, U.S. Army Construction Engineering Research
     Laboratory

























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