Raster Image Registration with MapInfo

What Is A Raster Image?
A raster image is a type of computerized image that consists of row after row of tiny dots (pixels). If you have a scanner and scanner software, you can create a raster image by scanning a paper map. After you scan a map image and store the image in a file, you can display the file using MapInfo.

There are many different raster image file formats. MapInfo can read the following types of raster image files: JPEG, GIF, TIFF, PCX, BMP, TGA (Targa), and BIL (SPOT satellite).

What Does It Mean To Register A Raster Image?
When you register a raster map image, you enter map coordinates (e.g. longitude/latitude degrees), and you indicate which locations on the raster image correspond to those coordinates. You must register each raster image before displaying the image in MapInfo, so that MapInfo can perform geographic calculations, such as distance and area calculations, when displaying the raster map.

The first time you open a raster image file within MapInfo, MapInfo displays the Register Raster Image dialog. By completing this dialog, you tell MapInfo how to register the raster image. MapInfo stores the raster image's registration information in a table file for future re-use. The next time you run MapInfo, you can re-open the raster table without repeating the registration process. Thus, you only need to register each raster image once.

Raster image files provided by MapInfo are already registered. You do not need to perform the registration process when you display the sample raster data included with MapInfo.

Registering A Raster Image

If you have not yet displayed your raster image in MapInfo, perform the following steps to register the image:

1. Choose File > Open Table. the Open Table dialog displays.
2. Choose Raster Images from the File Format drop-down list.  MapInfo displays a list of raster image files.
3. Choose the raster image file you want to open, and choose Open. MapInfo displays the Image Registration dialog. A preview of the raster image appears in the lower half of the dialog.
4. Specify the image's map projection by choosing the Projection button and completing the Choose Projection dialog.

If you created the raster image by scanning a paper map, the paper map may contain information about the map projection used. If you cannot determine the map's projection, use the default map projection (longitude/latitude). Choose OK.

5. Move the mouse cursor over the image preview in the lower half of the dialog, to a spot where you know the map coordinates (e.g. longitude/latitude), and click the mouse button. MapInfo displays the Add Control Point dialog.
6. Complete the Add Control Point dialog by entering the map coordinates that correspond to the location where you clicked on the map image. Choose OK.

Remember that any location west of the Prime Meridian has a negative longitude, and any location south of the Equator has a negative latitude. Thus, 73 degrees West corresponds to the X value -73.

If you are entering coordinates in degrees, you must enter decimal degrees as opposed to degrees/minutes/seconds.

7. Repeat steps 5 and 6 until you have entered at least three control points. To ensure accurate results, enter five or six control points. Each control point that you add helps MapInfo associate earth coordinates with locations on the raster image. Ideally, you should have at least one control point at or near each corner of the image.

The number of control points needed depends on the nature of your raster image. If you are not able to determine your map's projection, or if you are working with an image that does not have an actual map projection, such as an aerial photograph, you may want to enter twenty or more control points.

8. Choose OK when you are done adding control points. MapInfo displays the raster image in a Map window.

When you complete the Image Registration dialog, MapInfo saves the registration information in a table (.tab) file. In later MapInfo sessions, you can re-open the table by choosing File > Open Table, without repeating the registration process, and without having to choose Raster Images as the File Type in the Open Table dialog.
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