A georeference defines the relation between rows and columns in a raster map and 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.

Tip:

For a correct behaviour of a georef direct linear, and a georef orthophoto, it is essential that you have marked the 'Interpolation' check box in the Properties dialog box of your DTM raster map. For more information, refer to the Raster Map Properties dialog box.

Furthermore, system georeference None is available for raster maps that do not have coordinates.

Other types of georeferences are obtained when performing an operation on raster maps:

• 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.

A map window also uses a georeference. When the map window displays a raster map, this is the georeference of the raster map. When a map window only displays vector data, an internal georeference is used that is created from the coordinate system and coordinate boundaries of the vector maps.

Names of georeferences:

In ILWIS 3, object names comply with Windows long file names. Also Universal Naming Convention (UNC) paths are supported. For more information, see How to use long object names.

Tip:

To view which georeference is used by a raster map, check the properties of the raster map.

To create a georeference:

A georeference can be created:

• during rasterization, click the create georeference button in a Rasterize dialog box; the Create Georeference Corners dialog box will appear; you can create a georef corners.
• when a satellite image, a scanned photograph, or a scanned map is displayed in a map window, choose the Create Georeference command from the File menu of the map window; the Create Georeference (in map window) will appear; you can create a georef tiepoints, a georef direct linear or a georef orthophoto.
• when you want to create a 3D view of your study area, start the Display 3D operation, and click the create georeference button in the dialog box; the Create Georeference 3D dialog box will appear; you can create a georef 3D.

Other general ways to create a georeference are:

• by choosing the Create Georeference command on the File menu of the Main window,
• by double-clicking the New Georef item in the Operation-list,
• by opening the Properties dialog box of a raster map: click the Edit button in the Properties dialog box and click the create button in the subsequent Edit Properties dialog box.

For more information, refer to How to create a georeference, How to add coordinates to an image, How to screen digitize, How to monoplot, How to create an orthophoto or How to create a 3D view.

To view or edit a georeference:

You can view and edit a georef corners, a georef tiepoints, a georef directlinear, a georef orthophoto and a georef 3D. The easiest way to edit a georeference is to double-click a georeference in the Catalog. When a raster map which has a georeference is displayed in a map window, you can also open the Edit menu in the map window and choose Georeference.

Depending on the type of georeference you open (or the georeference which is used by the raster map in the map window), a dialog box will appear or an appropriate editor will be opened:

• for a georef corners: the Edit Georeference Corners dialog box will appear;
• for a georef tiepoints, a georef direct linear, or a georef orthophoto: the Tiepoint editor will be opened. When editing a georef orthophoto, the Locate Fiducial Marks dialog box will appear first;
• for a georef 3D: the Display Options - 3D Grid dialog box appears, in which you can specifying how the 3D view should be displayed in the map window. When the 3D view is displayed in a map window, you can select the Georeference command from the Edit menu of the map window to edit the georeference 3D. The Georeference 3D editor is started.

For more information, see How to edit a georeference.

A georef factor, a georef mirrorrotate or a georef submap cannot be edited. Information on these georeferences can be found in their properties dialog box.

Advanced users may wish to open a georef tiepoints, a georef directlinear or a georef orthophoto as a table. For more information, see How to open objects as a table.

Tip:

To have a raster map use another georeference that the current one, perform the Resample operation.

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:

• 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.

These are properties of a georeference.

By viewing the properties of a georeference, you can see for instance the type of the georeference and find out which raster maps are using this georeference. By clicking the Additional Info button in the Properties dialog box of a georef tiepoints, a georef direct linear or a georef orthophoto, you will get extra information on transformation or orientation results.

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:

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)

For more information, see also ILWIS objects : georeference Direct Linear.

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.

See also:

How to create a georeference

How to edit a georeference

Georeference tiepoints editor

Georeference 3D editor