Correlate images with AutoCAD Raster Design

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Once a raster image has been inserted into a drawing, such as a satellite image,

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its location and orientation can be controlled by correlating it.

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Correlating an image positions it within the AutoCAD coordinate system with the correct scale, rotation, and coordinates.

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The two commands used to correlate images in the Raster Tools are Match and Rubber Sheet.

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The Match command performs a simple, two-point rotation and translation of the image.

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The Rubber Sheet command performs a multi-point rubbersheeting across the entire image.

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Rubbersheeting is the process by which a layer is distorted

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to allow it to be seamlessly joined to an adjacent geographic layer of matching imagery.

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Using the Raster Tools, this is done by selecting control points.

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On the ribbon, Raster Tools tab, Correlate panel, click Rubber Sheet.

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The Rubbersheet – Set Control Points dialog appears.

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Here, you add the control points to the drawing, which are arranged in pairs—a source point and a destination point.

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Click Add Points.

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The dialog box disappears, and in the drawing, you are prompted to pick a source point.

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Pan and zoom the drawing into the northwest corner.

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Click the center of the gray plus (+) in the image as a source point.

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You are then prompted to specify a destination point.

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Click the center of the purple plus (+) next to the 10.

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These two points are now matched, as shown with the highlighted line.

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Continue to correlate the remaining gray and purple points in the image, ensuring to match at least four.

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When you are finished, type “D” for Done, and the Rubbersheet dialog opens again.

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The next step is to select the method of rubbersheeting, either Triangular or Polynomial.

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The Triangular method uses all the control points to triangulate the image,

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then performs a series of small transformations on those triangular areas.

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The area to be transformed is called the convex hull, and is defined by the outermost destination points.

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Image data outside the convex hull is discarded.

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The Triangular method provides the most accuracy.

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The Polynomial method uses the specified control points to perform a single transformation based on the entire image.

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However, the polynomial method does not always result in perfectly matched control points.

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This method is less accurate than the Triangular method.

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Errors appear only with the Polynomial method, and are shown graphically on the image after control points are entered.

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You may also be able to reduce errors by adding, deleting, or moving control points.

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You can also increase the polynomial degree—the more control points, the more it can be

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increased—but this also creates more warping in the image.

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The key is to use the lowest possible polynomial degree that still gives acceptable results.

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Click Cancel to close the Rubbersheet dialog.

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Now look at an example in a different drawing.

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Once you have inserted an image into a drawing,

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you can use the Match and Rubber Sheet commands to change its size, location, or orientation.

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From the ribbon, Raster Tools tab, Correlate panel, click Match.

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Zoom in to the raster image and select the upper-left corner of the room as the first source point.

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For the correlating destination point, zoom back out and select the upper-left corner of the rectangle.

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For the second pair, click the lower-left corner of the room in the raster image and then pick the lower-left corner of the rectangle.

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The Match command scales and orients the image to match the existing sketch.

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At this point, you can either save the image or bind the base raster image into your drawing file using the Embed command.

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Now look at a different example on a different drawing.

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This time, on the Correlate pane; click Rubber Sheet to open the Rubbersheet – Set Control Points dialog.

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Click Add Points to start adding control points.

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Zoom into the drawing and pick source and destination points for each control point, correlating at least four.

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Be sure to pick points from various locations around the image.

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Remember, the more points you use, and the more you scatter them, the greater the accuracy.

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Once all of the points are selected, type “D”, and then on your keyboard, press Enter to complete the command.

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Back in the Rubbersheet dialog, notice that selecting Polynomial adds the degree of error

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for each point and makes the Degree setting available.

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Also, you can click Export to export this point list to an external data file, which can then be imported into another drawing.

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This can be useful if you plan to repeat the process.

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Double-check the selected Method, Triangular or Polynomial, and then click OK.

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You can save the image correlation without saving the current AutoCAD drawing.

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To simply save the image data, in the Insert & Write panel, select Save.

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To save it in a different location or with a different file name, click Save As.

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Click Cancel to close the dialog.

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The Insert & Write Panel also includes options to export, embed, or capture an image,

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and to create a new blank image.

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Click Image Export, and in the Export dialog, expand the Files of type drop-down.

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Here, you can select from several available file formats, including TIF, TGA, PNG and JPG.

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Click Cancel to close the Save As dialog.

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Finally, if you plan to use the same image with different correlations on different projects,

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you can export only the image correlation by creating a world file.

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On the Insert & Write panel, click World File.

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By default, the file is saved with the same name and location as the associated image, but with a “w” at the end of the extension.