How to create a georeference

A georeference stores the relation between rows and columns of a raster map (row,col) and real world coordinates (X,Y) or (X,Y,Z). A georeference is needed for raster maps. A georeference uses a coordinate system.

It is advised that raster maps of the same area use the same georeference, because raster operations in which raster maps are combined will only make sense if the pixels in the maps refer to the same area on the ground.

There are basically five 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 photograph while using a DTM and camera parameters;
georeference 3D: to create a three dimensional view of maps.

General ways to create a georeference:

To create any kind of georeference, you can:

open the File menu of the Main window and choose Create Georeference, or
open the Create item in the Operation-tree, and double-click the New Georef item,
double-click the New Georef item in the Operation-list.

The Create Georeference dialog box will appear. You can create a georef corners, a georef tiepoints, a georef direct linear, a georef orthophoto or a georef 3D.

When an image, a scanned map or a scanned photograph is displayed in a map window, you can open the File menu in the map window, choose Create Georeference. The Create Georeference (in map window) dialog box will appear. You can create a georef tiepoints, a georef direct linear or a georef orthophoto.

Finally, also through the Edit Properties dialog box of a raster map, you create a new georeference by clicking the create georeference button. The Create Georeference (through Properties) dialog box will appear. You can create a georef corners, a georef tiepoints, a georef direct linear or a georef orthophoto.

For more information, see How to view and edit the properties of an object.

To create a georeference corners during a Rasterize operation:

When rasterizing vector maps, you can directly create a georeference corners for the output raster map by clicking the create georeference button in the dialog box of the Rasterize operation. The Create Georeference (during Rasterize) dialog box appears in which you can specify the pixel size and the boundaries of the map.

When rasterizing maps of the same area, it is usually sufficient to only once create a georeference corners; for the other maps to be rasterized, select this same georeference.

To add tiepoints to an image, a scanned photograph or a scanned map:

To add tiepoints to a satellite image, a scanned photograph or a scanned map, first display the image, photo or map in a map window. Then, open the File menu in the map window, and choose the Create Georeference. The Create Georeference (in map window) dialog box appears.

Choose whether you want to create a georef tiepoints, a georef direct linear or a georef orthophoto.

In case of creating a georef tiepoints or a georef direct linear, the Tiepoint editor will be opened.

In case of creating a georef orthophoto, first the Locate Fiducial Marks dialog box will be opened in which you can specify the principle distance of the camera and in which you can reference the fiducial marks of the scanned aerial photograph. After that, the Tiepoint editor will be opened.

In the Tiepoint editor, you can insert tiepoints, also called ground control points, which establish relationships between the map pixels (row,col) and XY- or XYZ-coordinates.

Tip:

When you want to use a created georef tiepoints for all bands of a satellite image: after you have finished adding tiepoints to a background map in the Tiepoint editor, you have to add the created georef tiepoints to all other bands. This can be done by opening the Properties dialog boxes of the other bands, click the Edit Properties button, then select the correct georef tiepoints.

For more information and tips, refer to the Create Georeference dialog boxes mentioned above or to How to add coordinates to a satellite image, How to screen digitize, How to monoplot, How to create an orthophoto.

To create a georeference 3D:

To obtain a three-dimensional view of your study area:

select Visualization, Display 3D on the Operations menu in the Main window, or
open the Visualization item in the Operation-tree, and double-click the Display 3D item,
double-click the Display 3D item in the Operation-list.

In the appearing Display 3D dialog box, click the create georeference button .

The Create Georeference 3D dialog box appears.

Then, after the Display Options 3D Grid dialog box in which you can specify whether you want to see your 3D view with grid lines with or without a raster map drape, a map window is opened with an initial georeference 3D.

To modify the 3D parameters, select Georeference from the Edit menu in the map window and the Georeference 3D editor is started. Change the 3D parameters as you wish and exit the editor. The last set of parameters is stored in the georeference 3D.

The 3D view is displayed in a map window. If you like, you can add any other vector map or annotation to this map window. You can save the 3D view in the map window as a map view. To permanently apply a georeference 3D to a raster map, use the Apply 3D operation.

For more information, see How to create 3D views.