Create objects: Create georeference

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Georeference corner Georeference tiepoints Georeference direct linear Georeference orthophoto Georeference 3D
Create georeference

A georeference defines the relation between rows and columns in a raster map and its XY-coordinates. The location of pixels in a raster map is thus defined by a georeference. It is advised that raster maps of the same area use the same georeference. A georeference uses a coordinate system which may contain projection information. Polygon, segment and point maps merely use a coordinate system. A georeference is a service object, usually for several raster maps.
Georeference types
There are five main types of georeferences:

  • Georeference corners: a North-oriented georeference to be used during rasterization of vector data or as the North-oriented georeference to which you want to resample maps;
  • Georeference tiepoints : a non-North-oriented georeference to add coordinates to a satellite image or to a scanned photograph, a scanned map, etc. without using a DTM;
  • Georeference direct linear: to add coordinates to a scanned photograph while using a DTM;
  • Georeference orthophoto: to add coordinates to a scanned aerial photograph while using a DTM and camera parameters;
  • Georeference 3D: to create a three dimensional view of maps.

Furthermore, system georeference None is available for raster maps that do not have coordinates. There are other types of georeferences that are obtained when performing an operation on raster maps,namely:

  • georeference factor: created by the Aggregate map and the Densify operation;
  • georeference mirrorrotate: created by the MirrorRotate operation;
  • georeference submap, submapcorners or submapcoords: created by the Sub-map of raster map and the Glue raster maps operation;
  • georeference differential: created by the Variogram surface operation (defined internally, i.e. not available on disk);
  • georeference scale_rotate: created by the Epipolar stereo pair operation for the output maps of the stereo pair.


General use of georeferences
When a raster map has a georeference, you can:

♦ inspect coordinates at the position of the mouse pointer in a map window, at the status bar;
♦ retrieve information at the position of the mouse pointer in a map window from other maps, i.e. functionality of the pixel information window;
♦ inspect the pixel size of raster maps in meters;
♦ check if raster maps fit on top of each other (requirement for MapCalc and Cross);
♦ overlay vector data on raster maps;
♦ rasterize vector data to raster maps;
♦ resample raster data which uses a particular georeference to another georeference; create an orthophoto from a scanned aerial photograph which has a georef orthophoto by resampling it to a georef corners;
♦ screen digitize on satellite images or on scanned photographs which have a georef tiepoints; screen digitize on scanned photographs which have a georef direct linear; monoplot on scanned aerial photographs which have a georef orthophoto.
Additional information

Technical information
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.


Georeference None
System georeference None is available for raster maps that do not have coordinates.
Georeference corners 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


Georeference tiepoints
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)


Georeference DirectLinear
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)


Georeference OrthoPhoto
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.

Georeference 3D
Stores a number of 3D view parameters: view point, location height, scale height, horizontal and vertical rotation, distance, view angle.

Georeference factor
Stores a factor of the pixel size of another raster map and horizontal and vertical offset of another raster map.

Georeference mirrorrotate
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.


Georeference differential
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.

Georeference scale-rotate
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.