imagery(1)	       Grass User's Manual	       imagery(1)



NNAAMMEE
       iimmaaggeerryy - Description of GRASS image processing functions.

IIMMAAGGEE PPRROOCCEESSSSIINNGG IINN GGRRAASSSS
       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 _G_R_A_S_S
       _T_u_t_o_r_i_a_l_: _I_m_a_g_e _P_r_o_c_e_s_s_i_n_g.

EEXXTTRRAACCTTIINNGG IIMMAAGGEERRYY DDAATTAA IINNTTOO AA GGRRAASSSS DDAATTAABBAASSEE
       Remotely sensed images are captured for computer  process-
       ing  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_._t_a_p_e_._m_s_s),   LANDSAT	 thematic   mapper   (TM)   data,
       (_i_._t_a_p_e_._t_m), and other formats,	such  as  scanned  aerial
       photography  or	SPOT satellite data (_i_._t_a_p_e_._o_t_h_e_r).  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.

UUNNRREEGGIISSTTEERREEDD DDAATTAA
       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  extrac-
       tion  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
       _i_m_a_g_e_r_y	which  is  an x,y database.  New databases can be
       created by users during GRASS  startup.	 See  the  _g_._h_e_l_p
       section	on "Setting Up a GRASS Database" for instructions
       on creating a new database.

CCEELLLL HHEEAADDEERRSS
       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  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_._w_h_e_r_e  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



RREEGGIIOONN AANNDD MMAASSKK
       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_._r_e_g_i_o_n 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_._m_a_s_k  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.

GGRROOUUPPSS
       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  _g_r_o_u_p  data
       structure  accomplishes	this  goal.   The group is essen-
       tially 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 _n_h_a_p.  Then the band files will become  the  raster
       files  _n_h_a_p_._1,  _n_h_a_p_._2, and _n_h_a_p_._3 and the group _n_h_a_p 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_._g_r_o_u_p_.

IIMMAAGGEE RREEGGIISSTTRRAATTIIOONN AANNDD RREECCTTIIFFIICCAATTIIOONN
       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_._p_o_i_n_t_s)  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 individu-
       ally.


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

IIMMAAGGEE CCLLAASSSSIIFFIICCAATTIIOONN
       Image classification methods process all or  a  subset  of
       the  band  files  as  a unit.  Spectral signatures for the
       image are generated (_i_._c_l_u_s_t_e_r) and then used to produce a
       landcover map (_i_._m_a_x_l_i_k).


       The  signatures	must  be  associated only with the raster
       files actually used in the analysis. This means that  with
       a group _s_u_b_g_r_o_u_p_s must be created (_i_._g_r_o_u_p) 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  dif-
       ferent combinations of band files.


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

RREECCTTIIFFIIEEDD VVSS.. UUNNRREECCTTIIFFIIEEDD AANNAALLYYSSIISS
       There  are  two possible routes for processing image data.
       The first is to register the group (_i_._p_o_i_n_t_s), perform the
       analyses on the unrectified band data (_i_._m_a_x_l_i_k), and then
       rectify the results (_i_._r_e_c_t_i_f_y).  The second is to  regis-
       ter  the  group	(_i_._p_o_i_n_t_s), rectify the band data (_i_._r_e_c_-
       _t_i_f_y), then run analyses on the rectified band data in the
       target  location (_i_._r_e_c_t_i_f_y).  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_._r_e_c_t_i_f_y 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.

SSEEEE AALLSSOO
       _G_R_A_S_S _T_u_t_o_r_i_a_l_: _I_m_a_g_e _P_r_o_c_e_s_s_i_n_g


       _d_._w_h_e_r_e

       _g_._r_e_g_i_o_n

       _i_._c_l_u_s_t_e_r

       _i_._g_r_o_u_p

       _i_._m_a_x_l_i_k

       _i_._p_o_i_n_t_s

       _i_._r_e_c_t_i_f_y

       _i_._t_a_p_e_._o_t_h_e_r

       _i_._t_a_p_e_._m_s_s

       _i_._t_a_p_e_._t_m

       _i_._t_a_r_g_e_t

       _r_._m_a_s_k

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

       _L_a_s_t _c_h_a_n_g_e_d_: _$_D_a_t_e_: _2_0_0_2_/_0_1_/_2_5 _0_5_:_4_5_:_3_3 _$



GRASS 5.0					       imagery(1)
