What is the order of parameters in +towgs84 attribute within a proj string. I assume it is dx,dy,dz,rx,ry,rz,sf
dx,dy,dz are expressed in meters rx,ry,rz are these in degrees/radians/arc seconds? sf value units as well would be great
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The scale parameter are the parts per millon, difference from 1.
Other parameters are ok (meters and arc seconds).
The 3D rotation matrix assumes that angles are really small (it is the case in datum transformations, but can´t be used in other transformations as an angle). The assumption is that the cosine of a small angle is 1 and its sine is the angle itself.
The TOWGS84 parameter is described here:
#define Dx_BF (defn->datum_params) #define Dy_BF (defn->datum_params) #define Dz_BF (defn->datum_params) #define Rx_BF (defn->datum_params) #define Ry_BF (defn->datum_params) #define Rz_BF (defn->datum_params) #define M_BF (defn->datum_params) x_out = M_BF*( x[io] - Rz_BF*y[io] + Ry_BF*z[io]) + Dx_BF; y_out = M_BF*( Rz_BF*x[io] + y[io] - Rx_BF*z[io]) + Dy_BF; z_out = M_BF*(-Ry_BF*x[io] + Rx_BF*y[io] + z[io]) + Dz_BF;
The numbers are coefficients responsible for translation, rotation and scaling. From the linked page
The three translation parameters are in meters as in the three parameter case. The rotational parameters are in seconds of arc. The scaling is apparently the scale change in parts per million.