Importing a Non-Georeferenced Image File

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Importing a Non-Georeferenced Image File

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If the data source is a Non-Georeferenced Image file (.gif, .tif, .jpg, .jpeg), there are specific options that you can set, see Figure 3.4:

Figure 3.4 Select Data Sources Pane for Non-Georeferenced Image File

Choose File>Add Map Data to display the Select Data Sources pane.
In the top part of the pane click the gray rectangle to preview the image and use the toolbar to pan, zoom in/out, and fit the image to the view.
By default the Image Calibration pane is displayed in bounds mode. In bounds mode you can set the latitude and longitude of the image manually. Click the row and enter the coordinates. You can also click on the button to interactively select the boundaries.
To switch to control point mode, click the button. This adds two columns: Pixel Column and Pixel Line. In this mode you can set the position of the image manually using the pixel properties or the latitude\longitude properties. In addition, you can drag the red crosses to the point you want; this automatically sets the property values in the Image Calibration pane. The red cross associated with a selected row is contained in a rectangle.
Click the button to add another row and another red cross in the Image pane.
Click the button again to return to bounds mode.
To remove a row from the Image Calibration pane, select the row and click the button.
Since this is a non-georeferenced data source, you must state the coordinate system in which the data source is defined in the Projection pane. The coordinate system in which you finally project the data source can be a different system and should be defined in the Coordinate System pane, see Setting Coordinate Systems.
Click Open.

Setting the Number of Calibration Points

Calibration in the Image Calibration pane uses an IlvFittedTransform class. This class requires a certain number of points to be entered for more complex transformations (that is, a higher degree of interpolation).

You must enter:

For a degree 1 polynomial (linear transformation): 3 calibration points.
For a degree 2 polynomial transformation: 6 calibration points.
For a degree 3 polynomial transformation: 10 calibration points.
For a degree 4 polynomial transformation: 15 calibration points.

Polynomial interpolations are more precise in the area inside the calibration points. If you use this feature, try providing calibration points that cover as large an area as possible. In the first part of Figure 3.5, you can see the initial map of Alaska. Calibration points have been placed around the right side of this image. In the second part of Figure 3.5, you can see the interpolated image, however, the left side is quite skewed.

Figure 3.5 Setting Calibration Points

Predefining the Image Bounds Using World Files

World files provide information for image calibration. A World file has the same filename as the main image file. For example, a file called USA.jgw could accompany an image named USA.jpg, and contain the coordinates of the image in seconds of arc.

An example of a World file content is as follows:

1.50000000000000 (X pixel size) 
0.00000000000000 (translation) 
0.00000000000000 (rotation) 
-1.50000000000000 (Y pixel size) 
1934001.50000000000000 (X coordinate of the upper left pixel)
1187698.50000000000000 (Y coordinate of the upper left pixel)

The Map Builder ignores the translation and rotation parameters.

When loading images in GIF, JPG, PNG, or TIF format, the Map Builder scans the directory to see if a World file exists, and if so, predefines the image bounds accordingly in the importation dialog box.

For GIF, JPG, PNG, and TIF formats, the world file extensions checked are in the form .tifw or .tfw, .gifw or .gfw, .jpgw or .jgw, and .pngw or .pgw respectively.

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