Georegistering an Image to Known Coordinates

At times we have an image that we would like to georegister where we do not have any known good components available for use as a reference component to which the image may be registered. If we have locations within the image for which we know the coordinates (say, as recorded using a GPS device taken to that location) we can nonetheless georegister that image.

 

Georegistering an Image to Coordinates

 

1. Open the image to be georegistered. From the Control Points pane use the New Control Point button to place at least three (and preferably many more) control points in the image at locations for which the coordinates are known.

2. Click on the project pane and insert a new drawing. This will be our target drawing.

3. Open the drawing. It will be blank. Re-project this drawing into Latitude / Longitude projection if it is not already in Latitude / Longitude projection.

4. In the Control Points pane use the New Blank Control Point button to add blank control points to the drawing that correspond to the control points placed in the image. For each control point, double click into the X / Longitude and Y / Latitude boxes to enter the known coordinates for the location of that control point.

5. After all of the control points are entered, re-project the drawing into whatever projection is desired as the end projection for the image.

6. Click on the image to make it the active window and choose Register in the Control Points pane. Manifold will present a dialog asking which component to use as the reference component. The new drawing created in step 3 above will be listed in the list box of available reference components.

7. Chose the new drawing for use as the reference. Manifold will georegister and re-project the image to match the control points placed in the drawing. After the georegistration process, the image will be in the same projection as the target drawing.

 

The above process is often made faster if we have used a GPS device to measure locations in the image and have stored each location as a waypoint in the GPS device. We can then download the waypoints into a drawing and use those points as our control points without the need to manually enter coordinates for each.

 

Another use of this process is to georegister a scanned image of a paper map. Paper maps often have graticules printed on them so we know the exact latitude and longitude coordinates at the intersection of the graticule lines. We can place control points at the intersections of the latitude and longitude lines in the image and then use these coordinates for the corresponding control points in the target drawing.

 

Example

 

Suppose we have a Landsat 7 image of the San Francisco Bay area. We would like to use this image to create new digital maps; however, we do not have any current digital maps to use as reference components. We do have latitude / longitude coordinates for two locations provided by friends with GPS devices who live in the Bay area. The first location is at the southern foot of the Golden Gate Bridge. The second location is at the south end of the Oakland International Airport.

 

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We begin by using the Control Points pane's New Control Point command to mark the known locations on the image with the mouse.

 

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We then insert a new drawing into the project. Make sure this new drawing is in Latitude/Longitude projection so that we can enter control points into it using latitude and longitude coordinates. To do this, first create a new drawing in the project pane, open the drawing and then use Edit - Change Projection to change the projection to Latitude / Longitude. It opens as a blank drawing.

 

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images\btn_new_blank_ctrlpt.gif In the Control Points pane we click New Blank Control Point to add a blank control point to the pane.

 

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The new control point will appear in the blank drawing.

 

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We double click into the X / Longitude field to change it.

 

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We now type in the coordinate to use for Longitude.

 

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When we push Enter the new coordinate is entered.

 

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We can repeat this procedure with the Latitude corodinate.

 

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Clicking New Blank Control Point we add another line to the Control Points pane.

 

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We can manually add the coordinates for this new control point.

 

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We can click Zoom to Fit in the drawing to show both control points in the drawing.

 

We can now click on the image to make it the active window and then press Register in the Control Points pane to register it in the usual way, using the new drawing in which we have created two control points. This example uses only two control points to save space. In "real life" we must use three or more control points. See the Georegistration topic and the Control Points pane topics for details in using these dialogs.

 

See the Georegister a Scanned Paper Map example for a detailed, step-by-step process to georegister an image to known coordinates.

 

Notes

 

As we edit the coordinates for control points manually by double clicking into the X / Longitude and Y / Latitude fields the control points being edited will jump about in the drawing as their specified position moves about. Use Zoom to Fit to see the control points that have been added once manual editing has been completed.

 

Manifold maintains all coordinates to full precision. The default format for how many positions after the decimal point are shown in dialogs such as the Control Points pane is set in Tools - Options.

 

This topic is written showing the registration of an image as the target component. We can apply exactly the same procedure when registering a drawing (such as a CAD drawing of a factory plan) to known coordinates.

 

We can use tables to make lists of control points for use in georegistering images. When editing fields in tables, Manifold will automatically make sense of many different ways of entering latitude and longitude data into latitude and longitude type fields. This is extremely useful when entering coordinates for control points read off from paper maps that are marked using degrees, minutes and seconds notation. See the Create a Table and Add Records example for a step by step example of how one could add control points to a table using degrees, minutes and seconds format.

 

Suppose we have the GPS device in hand that was used to measure the known locations. We could have saved these in the GPS device as waypoints and then used the Manifold GPS Console to download the waypoints as points into a drawing. We could then use the Load Points command in the Control Points pane to load these points as control points. This would eliminate the need to enter coordinates by hand and thus also eliminate any possibility of typographic error when entering the coordinates.

 

For brevity, this topic and other georegistration topics use images as examples. However, the same procedures apply when georegistering drawings or surfaces .

 

See the Georegistration topic for additional details on how to use control points to georegister images and drawings.

 

Keep in mind that scanned images can be immense. An RGB image that is 12000 x 14000 pixels is over 800 megabytes in size. If you work with such large images you should have Windows 2000 or XP to avoid Windows bugs in 16 bit Windows versions such as Windows '95 or Windows '98. You should also have a fast processor and at least a gigabyte of RAM. Do not work with higher resolution images than is necessary.

 

See Also

 

Georegister a Scanned Paper Map