m.datum.shift(1)       Grass User's Manual	 m.datum.shift(1)



NNAAMMEE
       mm..ddaattuumm..sshhiifftt  - Datum shift program.
       _(_G_R_A_S_S _D_a_t_a _I_m_p_o_r_t_/_P_r_o_c_e_s_s_i_n_g _P_r_o_g_r_a_m_)

SSYYNNOOPPSSIISS
       mm..ddaattuumm..sshhiifftt llaatt==_d_d_._m_m_._s_s{_n|_s} lloonn==_d_d_._m_m_._s_s{_e|_w} hh==_h_e_i_g_t_h
       _a_b_o_v_e _e_l_l_i_p_s_o_i_d
       iidd==_i_n_p_u_t___d_a_t_u_m oodd==_o_u_t_p_u_t___d_a_t_u_m
       iiss==_i_n_p_u_t___s_p_h_e_r_o_i_d ooss==_o_u_t_p_u_t___s_p_h_e_r_o_i_d  ddxx==_x_s_h_i_f_t	ddyy==_y_s_h_i_f_t
       ddzz==_z_s_h_i_f_t
       --bb _u_s_e _b_l_o_c_k _s_h_i_f_t _m_e_t_h_o_d

       --mm _u_s_e _M_o_l_o_d_e_n_s_k_y _f_o_r_m_u_l_a

       --ww  _u_s_e	_B_u_r_s_a  _W_o_l_f  _7	_p_a_r_a_m_e_t_e_r _t_r_a_n_s_f_o_r_m_a_t_i_o_n _(_n_o_t _y_e_t
       _i_m_p_l_e_m_e_n_t_e_d_)


DDEESSCCRRIIPPTTIIOONN
       _m_._d_a_t_u_m_._s_h_i_f_t returns geographic coordinates  based  on	a
       different datum (and spheroid/ellipsoid) than the one used
       to obtain the original coordinates.


       The input  and  output  spheroids,  _i_s  and  _o_s,  are  the
       spheroids for two different datums.  The input spheroid is
       the one on which the original coordinates are  based.  The
       output spheroid is that on which the resultant coordinates
       will be based.  The  "shifting"	occurs	between  the  two
       datums.	 The shift values, _d_x, _d_y, and _d_z, are constants.
       They indicate the mean differences between points  in  the
       second datum versus the first as measured in meters.


       If both input and output datum _i_d and _o_d are listed in the
       system datum table, it is sufficient to provide input  and
       output datum for the datum shift. The shift values, _d_x, _d_y
       and _d_z are read from the datum table.


       The list of datums and  spheroids  available  is  somewhat
       dynamic.   It  may  not	contain  exactly  the ones listed
       below.	To  determine  the  current  list   of	 possible
       spheroids,  type  in  the  command:  mm..ddaattuumm..sshhiifftt  llaatt==_0_n
       lloonn==_0_w ddxx==_0 ddyy==_0 ddzz==_0 iiss==_h_e_l_p ooss==_h_e_l_p

       A list of available  spheroids  will  be  printed  on  the
       screen.	To determine the current list of possible datums,
       type in the command: mm..ddaattuumm..sshhiifftt llaatt==_0_n  lloonn==_0_w  iidd==_h_e_l_p
       oodd==_h_e_l_p

       If  the	datum or spheroid desired is not on the list, the
       values should  be  added  to  the  system  datum.table  or
       ellipse.table.


       The  height  above  the	ellipsoid is usually not known in
       GRASS. You should approximate this by  zero  (default  for
       _h).  Obviously  the  resulting  height is not a reasonable
       value.

EEXXAAMMPPLLEE
       mm..ddaattuumm..sshhiifftt llaatt==_0_n lloonn==_1_7_5_w iiss==_c_l_a_r_k_6_6  ooss==_w_g_s_7_2  ddxx==_-_2_2
       ddyy==_1_5_7 ddzz==_1_7_6



       Results: 3
	      lat=0.00.05.72999N

	      lon=174.59.55.004133W

	      (h  =  107 [m])



NNOOTTEESS
       Essentially, the program follows these steps for the block
       shift method.  The original point, as defined by  a  lati-
       tude  and  a longitude, is converted to geocentric coordi-
       nates.  The shift values are added to the geocentric coor-
       dinates.  The summed values are then converted to latitude
       and longitude based on the output spheroid.


       The Molodensky method uses a one-step calculation  without
       converting  to  and from geocentric coordinates. The Molo-
       densky formula may be inaccurate for  latitudes	near  the
       poles.  The  coordinate	conversion library will take this
       into account and use the block  shift  formula  for  those
       latitudes.


       Some hints on accuracy:

       Generally  the  accuracy  depends  on  the  transformation
       method used and the accuracy and  spatially  applicability
       of the parameters supplied to the transformation function.
       You always must check if the formula is applicable to your
       problem and supplies the needed accuracy!

       block shift with cartesian coordinates ~ 10 m

       molodensky transformation ~ 5 m

       bursa-wolf transformation ~ 1 m

       3d  similarity  transformation ~ 1 m (needs national simi-
       larity parameters)

       multiple regression equation (MRE) transformation,
       other methods up to 10 cm (generally not needed for GRASS)

       The  transformation parameters in datum.table are meant to
       transform from local datum to wgs84 with the  Block  shift
       method  or  Molodensky function. (reverse the sign for the
       reverse transformation from wgs84  to  the  local  datum).
       All  transformations  need  input and output ellipsoid for
       the calculation of Rm and Rn.
       You can not use the parameters for  the	Block  shift  and
       Molodensky formula with any other datum shift formula. The
       Bursa-Wolf datum transformation needs 7 parameters (3 xyz-
       shift, 3 xyz-rotational, 1 scale factor), which can not be
       used with any other formula.  Specifically do not use  the
       3  xyz-shift parameters for the Bursa- Wolf transformation
       with the Block shift or Molodensky formula, as the parame-
       ters are not independent from another.


       For a brief discussion of spheroids and datums see _m_._l_l_2_u.
       For a  brief  discussion  of  geocentric  coordinates  see
       _m_._l_l_2_g_c.


       This  remains  under  testing and is still an experimental
       program.

SSEEEE AALLSSOO
       _m_._g_c_2_l_l

       _m_._l_l_2_g_c

       _m_._l_l_2_u

       _m_._u_2_l_l

       _s_._i_n_._g_p_s

       _v_._i_n_._g_p_s

       _s_._t_r_a_c_k_._g_p_s

AAUUTTHHOORR
       Michael	 Shapiro,   U.S.Army   Construction   Engineering
       Research Laboratory
       Andreas Lange, andreas.lange@rhein-main.de

       _L_a_s_t _c_h_a_n_g_e_d_: _$_D_a_t_e_: _2_0_0_2_/_0_3_/_1_9 _0_9_:_3_2_:_1_1 _$



GRASS 5.0					 m.datum.shift(1)
