Define a Warp File Semi-interactively

Use the semi-interactive method of defining a warp file for scanned maps or images that need to be georeferenced. When adding projection information to scanned images from paper maps, make sure you use the same projection used in the map. For example, if the map is in UTM, the projection for the image should also be defined as UTM.

The instructions below describe the procedure for defining a warp file for an image of a map that has a pixel coordinate system.

  1. Display the image you want to warp in a map by selecting Grid and Image > Display on Map > Grid from the menu bar.

  2. Select the Coordinates > Georeferencing > Define a Warp menu item. The Define a Warp dialog is displayed.

  3. From the Definition mode drop-down list select "Semi-interactive" and click OK.

  4. The Map Projection question box is displayed with the message: "Define the output coordinate projection?". If you know the projection, click the Yes button, otherwise click the No button. See the Coordinate Systems topic for more information on defining map projections.

  5. The system displays a message informing you to choose (up to) four locations to use as control points for warping. To ensure that the locations are precise, choose locations that use intersecting lines or meridians.

  6. Take note of the coordinates from the paper map for each of the control points you define on the image. You will also need the projection and datum from the map. Click OK to continue.

  7. A crosshair cursor appears on the map. Using your mouse, click on the location of the first control point.

  8. If you click the right-mouse button, a popup menu will appear that enables you to zoom, pan or redraw the map screen. You can use this function to zoom in to the location of each control point.

  9. The system displays a dialog box showing the current coordinates (i.e. pixels) and the new coordinates (i.e. UTM). Enter the projection coordinates in the field and click OK.

  10. Specify the location and coordinate values for the remaining control points.

    An alternative to clicking the last control point with the left-mouse button is to click the right-mouse button and select Done.

  11. Now that you have defined a warp file, on the Project menu, select the Edit a File option to view the warp file coordinates in a text editor.