For a raster image to be useful in the map, a number of coordinates must be added to the raster image to be used. By defining fit points, it is decided where in the map the raster image should be located and how it should be rotated. The information about the fit, ie the result of a georeferencing, is stored in a coordinate information file in the World file format. You can choose to georeference a new image or continue working with the pass points in a previously georeferenced image.
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If using images in TIFF format, you need to specify whether the image type is photo or drawing in order for the format to be correct. •Photo refers to camera images or aerial photos and means that the image is saved as JPEG. •Drawing is an image type with a color, adapted for larger areas, which means that the image is saved as PNG. |
1.Navigate to an area on the map that shows approximately the same area as the raster image.
2.Select Window > Split View.
3.Left-click in the map with the mouse pointer to divide the graphic view into two smaller views.
4.Select File > Reference data > Georeference image...
5.Press Add image.
6.Select a reference file of JPEG, PNG or TIFF format from the destination folder.
7.Press Open.
8. Press Add control points.
9. Select a known and prominent point in the raster image.
10. Press on the corresponding point in the active view.
11. Add at least 2 more fit points by repeating steps 11-13.
12. Press Preview to get an overview of how the image has been placed.
13. Press Detach to continue georeferencing the raster image.
14. Check that the residual value shows an acceptable error.
15. Press Save to world file.
a) Check the Save image pyramid check box in case the image is large.
A zip file containing a file hierarchy and metadata describing an image pyramid of the raster image will be saved.
16. Select the destination folder where you want to save the file.
17. Press Save.
18. Press Delete Image to disconnect the raster image from the graphic view.
19. Press Finish.
Settings
Drawing order |
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Preview |
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Detach |
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Save to world file |
More information
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•Try to get a good spread on the control points in the image. To get a good fit, avoid creating 3 control points that together form a straight line. •You can zoom in the graphical view using the scroll wheel on the mouse. •Press the SHIFT key and move the mouse pointer to pan the view. |
Residual value
A residual value is only displayed when you have created 3 contorl points in a georeferencing. Residual error is a residual error that shows the difference in meters between a specified value and the correct value. A residual error of 0.125 means that a control point is incorrect with about 12.5 cm. If the raster image covers a large area of, for example, 100 meters across, then it is an acceptable error. A value above 1 is reasonably a significant error and should therefore be corrected.
When you have created 4 control points, a square mean error is calculated, RMS. Root Mean Square is a measure of the mean value of residual errors. If the residual value is too high, you can delete existing control points and then create new ones in the georeferenced image. In the Georeferge Image window, the Active column, you can check which points have gone wrong and if the residual error is too large. You can check in or out the control points to be used in the calculation. When the line is unchecked, residual errors are calculated automatically on the other lines. Several lines are possible to uncheck. You can choose to delete control points completely by pressing Delete control points. You can also choose to restore deleted control points by pressing Undo Deleted.