Registering the Coordinates of a Raster Image
Before you can overlay vector data on top of a raster image, you must first register the raster image so that MapInfo Professional can position it properly in a Map window. In the Image Registration dialog box, you can identify control point coordinates and specify the appropriate projection for the raster image.
Control points are the coordinates you identify on the raster image that MapInfo Professional can use later to match up to other layers. It is very important to provide accurate control point information when registering a raster image, so MapInfo Professional can display raster images without distorting or rotating them. Later, when you overlay vector data, MapInfo Professional distorts and rotates the vector data so both layers can line up properly. Identifying significant control points makes this match up process easier. We suggest you use highway/street intersections and prominent landmarks as control points, as they rarely move.
Specifying the correct projection of the raster image is also important for accurate display. Images that do not have known projections, such as unrectified aerial photographs, are less suitable for use with vector data.
There are two ways to register a raster image in MapInfo Professional. Each involves specifying the map coordinates of control points on a reference map and matching them with equivalent points on the raster image. To determine map coordinates, you can:
Note: If you scanned in the image from a paper map, the map most likely contains a graticule (latitude and longitude grid). You can choose those coordinates for prominent features and enter them in the Image Registration dialog box.
To register the coordinates of a raster image and create a .tab file from the raster image:
- On the File menu, click Open and specify the Raster Image file format in the Files of type drop-down list.
- Choose a raster image file and choose Open. The Display/Register dialog box displays.
- Click Register to display the Image Registration dialog box.
1 Control Points list box
2 Preview of raster image
- Choose the Projection button to specify the raster image's projection. It is easier to match up the maps if you select the same projection for the raster image as the vector images (layers) you want to use. Choosing the same projection minimizes image distortion when overlaying the vector map layers. You cannot change the projection from the Map window.
Note: If you do not set the projection, MapInfo Professional defaults to Longitude/Latitude or to the default table projection set by using the map window preferences you set.
- You can set the units for the control point entries by clicking the Units button.
- To select the actual coordinates to register in the raster image, click the Add button to add a new entry into the Control Points list box. The Add Control Point dialog box displays.
- Specify the coordinates from the paper map in the dialog box. The location of each point is marked on the preview image with a "+" symbol. You can change the Label to something more meaningful to you, such as US85/LA132, to describe an intersection in the raster map. You might also select a prominent building or other very specific address that is easily identifiable in both maps.
Note: If you have a MapInfo Professional map layer of the same area and projection, you can open it in a separate Map window by using Table > Raster > Select Control Point from Map.
- Repeat this process (step 6 and step 7) until you have registered at least three or four points on the raster image. Try to vary the position of these control points to better define the map. If too many points are in a straight line, an error displays.
If you are working with an image for which you do not know the projection (such as an aerial photograph), enter at least three control points to improve accuracy.
- When you have completed your control point entries, click OK. The raster image displays as a .tab file.
Use the + and - buttons to zoom in and out of the raster image. Zooming in on the image may help you obtain greater accuracy in control point location. If you are having difficulty finding a control point in the Image Registration image, click the control point entry in the Image Registration list and click the Find button to display the point in the image.
When you complete the Image Registration dialog box, MapInfo Professional saves the registration information in a table (.tab) file. In later MapInfo Professional sessions, you can re-open the table by choosing File > Open, without repeating the registration process, and without having to choose Raster Images as the file type in the Open dialog box.
Choosing Control Points from an Existing Map
To transfer the coordinates automatically from a vector map to a raster image, you need a vector map of the same image. Then, you display the vector map side by side with the Image Registration dialog box, which shows a preview of the raster image. Click a prominent feature in the vector map to determine its coordinates and transfer this control point to the Image Registration dialog box.
To register the vector map control points to the raster image:
- On the File menu, click Open and Raster Image file format.
- Choose the raster image file and choose Open. The Display/Register dialog box displays.
- Select Register. The Image Registration dialog box displays. Choose the Projection button to specify the image's projection, if necessary.
- To add control points, click the Add button to add a control point entry to the Control Points list.
- Click the Pick from Map button. The Edit Control Point dialog box displays with the new control points in the Map X and Map Y fields. Click OK to save these entries.
- Double click this entry in the Control Point list and click the matching control point location in the image pane. MapInfo Professional updates Image X and Image Y coordinates in the Edit Control Point dialog box with the selected coordinates.
- Click OK to save these entries.
Note: Remember to type a meaningful description of this location in the Label field.
- Repeat this process until you have identified three or four non-linear points in the preview pane of the Image Registration dialog box.
- After you have defined all of the control points, click OK in the Image Registration dialog box. The raster image displays in the Map window under the vector layer.
Note: You can modify these entries using the Table > Raster > Modify Image Registration options.
© 2007 MapInfo Corporation. All rights reserved.