A georeference consists of an ASCII object definition file (.GRF); in case of a georef tiepoints, a georef direct linear or a georef orthophoto, also a binary data file (.GR#) is available. The object definition file contains further references to properties of georeference, namely:
- ♦ the coordinate system that is used by the georeference;
- ♦ in case of a georef tiepoints, georef direct linear or a georef orthophoto, the background map on which tiepoints are positioned;
- ♦ in case of a georef direct linear or a georef orthophoto, the DTM from which height values should be obtained.
System georeference None is available for raster maps that do not have coordinates.
Stores minimum and maximum XY-coordinates, and whether these refer to the corners of the corner pixels or to the centers of the corner pixels.
- Col = a1 X + b1
- Row = a2 Y + b2
Stores a set of tiepoints in RowCol and XY-coordinates and a transformation method. Parameters are found by a least squares method. Height values are not taken into account. Available transformation methods and their formulas are:,br/>
- Conformal; minimum of two tiepoints required:
- Col = aX + bY + c1
- Row = bX - aY + c2
- Affine; recommended for satellite images; minimum of three tiepoints required:
- Col = a11X + a12Y + b1
- Row = a12X + a22Y + b2
- Second order bilinear; minimum of 4 tiepoints required:
- Col = a1 + b1X + c1Y + d1XY
- Row = a2 + b2X + c2Y + d2XY
- Full second order; minimum of 6 tiepoints required:
- Col = a1 + b1X + c1Y + d1XY + e1X2 + f1Y2
- Row = a2 + b2X + c2Y + d2XY + e2X2 + f2Y2
- Third order; minimum of 10 tiepoints required:
- Col = a1 + b1X + c1Y + d1XY + e1X2 + f1Y2 + g1 X3 + h1 X2Y + i1XY2 + j1 Y3
- Row = a2 + b2X + c2Y + d2XY + e2X2 + f2Y2 + g2 X3 + h2 X2Y + i2XY2 + j2 Y3
- Projective; recommended for normal camera, i.e. small format, photographs; conventional rectification; minimum of 4 tiepoints required:
- Col = (aX + bY + c) / (gX + hY +1)
- Row = (dX + eY + f) / (gX + hY + 1)
Recommended for normal camera, i.e. small format, photographs when a DTM is available; also corrects for tilt and relief displacement; Direct Linear Transformation (DLT). Stores a set of tiepoints in RowCol and XYZ-coordinates. Height values can be supplied by the user, otherwise these are obtained from the DTM. Outer orientation parameters, i.e. camera position (X0, Y0, Z0) and camera axis angles (a, b, g) are calculated from the tiepoints. Minimum of 6 tiepoints required; tiepoints must not be co-planar, i.e. the tiepoints should not be on a (tilted) plane.
- Row = (aX + bY + cZ + d) / (eX + fY + gZ +1)
- Col = (hX + iY + jZ + k) / (eX + fY + gZ +1)
Recommended for photogrammetric camera aerial photographs when a DTM is available; also corrects for tilt and relief displacement; Differential rectification. Stores fiducial marks, principal distance, and a set of tiepoints in RowCol and XYZ-coordinates. Height values can be supplied by the user, otherwise these are obtained from the DTM. The user needs to specify fiducial marks and principal distance; from this the principal point is calculated (inner orientation). Camera position (X0, Y0, Z0) and angles (k, f, w) are calculated from the tiepoints (outer orientation). Minimum of 3 tiepoints required.
Stores a number of 3D view parameters: view point, location height, scale height, horizontal and vertical rotation, distance, view angle.
Stores a factor of the pixel size of another raster map and horizontal and vertical offset of another raster map.
Stores whether another georeference is mirrored horizontally, vertically, diagonally or is transposed, or rotated 90, 180, or 270°.
Georeference (1) submap, (2) submapcorners, (3) submapcoords:
- Stores (1) start line and column, and number of lines and columns of input georeference,
- (2) start line and column, and end line and column of input georeference, or
- (3) start coordinate, and end coordinate of input georeference.
Stores lag spacing and number of lags. Origin in the center. Stores in fact differences of coordinates of point pairs or pixel pairs. Internally defined, not available on disk. Uses a coordinate system differential.
Stores for each input map of the stereo pair, the parameters needed for resampling in order to create the output maps of the stereo pair. This includes for each input map: position of principal point, position of transferred principal point, position of optional scaling points; from this the rotation and optional scaling factor for resampling are calculated. Furthermore, the georeference of an input map may be stored.