Converting Geo-referenced TIFs from
Image Files
by Judy Bragg
The IMAGE GEOREFERENCE tool is used to convert image files to geo-referenced TIF
images that can be used as background charts in HYPACK® and SURVEY. You can also
load them to the ENC EDITOR to be used as a guide for building an S57 chart of your project
area.
You will need the following:
• The image files can be from anywhere: screen captures of other chart types, satellite
photos or aerial fly-over shots. The tool accepts TIF, BMP or JPG. (If your image is of a
different file type, it can easily be converted in most graphic programs.)
• The positions of at least two distinct features in your image. These positions are
called ‘ground control points’. For greater accuracy, use more than two points and space
them as far apart as you can.
When you have your image and ground control points, you are ready to geo-reference your
image.
Launch the IMAGE GEOREFERENCE tool by selecting PREPARATION-EDITORSIMAGE GEOREFERENCE.
2. Load your image file.
a. Select IMAGE-LOAD.
b. Set the file type to match your image file type.
c. Select your image file and click [Open].
The image will be drawn to the IMAGE GEOREFERENCE tool.
3. Define your ground control points.
a. With the default cursor, double-click on the image at the first known position. A red
symbol will appear on the image and a dialog will appear.
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FIGURE 1. Entering Ground Control Points
Enter the position information under ‘World X’ and ‘World Y’.
c. Click [OK].
d. Repeat steps a-c for your additional reference points.
4. Calculate the rotation and scale of your image to fit the world coordinates of your
ground control points.
b.
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Converting Geo-referenced TIFs from Image Files
a.
Select IMAGE-WORLD FILE. A dialog appears with the calculated values.
FIGURE 2. The Image Information Dialog shows the calculated scale and
position values.
Apply the calculated scale and rotation to the image by
clicking [Set Values from GCP’s].
c. Click [Apply]. The geo-referenced image will in the
HYPACK® map display.
d. If there are features behind the TIF, adjust the transparency of the image preview to see them through the TIF
file. Click [Transparency] to access the Transparency dialog where you can use a slider to choose the level of
transparency.
e. Close the Image Info dialog.
5. Modify the HYPACK® chart drawing order and tranparencies, if necessary, to optimize the view of all objects in your map window.
The following figure shows a geo-referenced satellite photograph of a segment of the
Chesapeake Bay Bridge-Tunnel displayed with S57 and DWG files.
b.
FIGURE 3. Sample TIF Displays in HYPACK®
The IMAGE GEOREFERENCE tool will be available in the HYPACK® 2009 release.
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