Entering Control Points - MapInfo_Pro - 2023

For MapInfo Pro to make a digitized copy of your paper map, it must be able to correctly interpret the location of the digitizing puck on the tablet as you trace the map. To do this, you must first set the parameters by specifying locational coordinates for several control points on the map. To set a control point, choose a point on the tablet and click a button on the puck. Type the coordinates that correspond to that point in the Digitizer Setup dialog box.

To add, edit, remove, or clear all control points, choose the appropriate button in the Digitizer Setup dialog box. A dialog box appears for each button to walk you through the steps.

Picking and Adding a Control Point

The buttons in the Control Points box (Digitizer Setup dialog box) allow you to add, edit, remove, or clear all control points.

To pick or add a control point:

1. Click Add to open the Pick Tablet Point dialog box.
2. Choose a point on the tablet. As you move the puck around the tablet, the Pick Tablet Point dialog box opens where that point is on the tablet. When you click a digitizer button, you select that tablet location as a control point.
3. Click a digitizer button or press Return to cancel.
4. You have selected a point on the digitizer tablet as a control point, and the Add Control Point dialog box opens. The Add Control Point dialog box lets you enter a name for the control point (optional) and X, Y coordinates.

For instance, to add a control point, choose Add. The Pick Tablet Point dialog box displays telling you to choose a point on the tablet and click a digitizer button. The Add Control Point dialog box opens prompting you for a label (optional) and the map coordinates for the point. When you are through, click OK. Repeat this process for each control point you enter.

Keep in mind that you must enter the coordinates in the units you specified in the Digitizer Setup dialog box. If your map unit is in degrees, be sure to type in the coordinates as decimal degrees. For a discussion on converting degrees, minutes, and seconds to decimal degrees see Converting Degree/Minute/Second Coordinates into Decimal Degrees.

Editing a Control Point using a Digitizer

If you make a mistake when entering a control point, you can edit it. High error values are often an indication that such a mistake was made.

To edit a control point, choose the point from the list of control points in the Digitizer Setup dialog box and click Edit. The Edit Control Point dialog box opens. Make the appropriate changes to the label or X and Y coordinates.

To edit a control point:

1. Choose the point from the list in the Digitizer Setup dialog box.
2. Click Edit. The Edit Control Point dialog box displays.
3. You can change the control point's label and its Map X and Map Y points by typing new values.
4. Press the Table X,Y button. The Pick Tablet Point dialog box displays.
5. Pick a point on the digitizer tablet and click any digitizer button.

The tablet's coordinates are changed accordingly.

You can also change the tablet position of the point through this dialog box. For instance, if the coordinates are correct, but you realized that you clicked on the wrong point on the tablet, you can choose the Tablet X-Y button in the Edit Control Point dialog box. This is easier than adding a new control point and removing the incorrect one.

You can also remove a control point by highlighting the point in the Setup dialog box and choosing the Remove button. To remove all points and reset the projection to that of the active Map window, choose Clear All.

Entering a Name for a Control Point using the Digitizer

To enter a name for a control point:

1. Type a name for the control point in the Label box.
2. Type the coordinate values for the point in Map X and Map Y.
3. Repeat the process at least three times.

   When four or more control points are properly added, an error value displays in the Control Points section of the Digitizer Setup dialog box.

   Most paper maps specify longitude and latitude in degrees, minutes, and seconds. Additionally, the MapBasic sample DMSCNVRT.MBX program displays a dialog box that converts coordinates.

4. Choose OK.

To begin digitizing:

1. Select a drawing command.
2. Press the D key. The cursor displays as a circle with a crosshair.

Removing Control Points

To remove a control point:

1. Choose the point from the list in the Digitizer Setup dialog box.
2. Choose Remove. The control point is removed.

To remove all control points, choose Clear All. All control points are removed and the projection is reset to the projection of the active Map window.

Saving Control Points

You can save your control points to a workspace. This is helpful if you must interrupt the session before you have finished entering your control points. When you open the workspace later, MapInfo Pro automatically turns digitizer mode on. There are some cases where MapInfo Pro will not be able to turn on digitizer mode. They are the following:

• If there are less than 3 control points entered in the workspace. You will need to enter at least 1 or 2 more control points in Digitizer Setup.
• If your control points run in a straight line. You must enter control points that are at different angles from each other.
• The digitizer size or resolution has changed since the workspace was saved. You will need to enter new control points in Digitizer Setup.

These problems do not, however, prevent the workspace from opening.

Control Point Error

You should use at least four control points when digitizing a paper map. When digitizing an aerial photograph or an image without a projection, you might want to use 30 or more control points.

MapInfo Pro uses the control points to perform an affine transformation, whereby the digitized objects are skewed, shifted and scaled to fit the control point locations.

MapInfo Pro determines the relative positions of the control points based on the coordinates you specify. It can then determine where those points should be on the tablet. MapInfo Pro then calculates an error estimate that is the difference between where the point should be on the tablet and where you actually clicked.

The error estimates give you an idea of how accurate the resulting computer map will be. Error values should be quite small, on the same scale as the resolution of your digitizer. In most cases errors should be in thousandths or hundredths of an inch. Those errors approaching a tenth of an inch are probably too large.

To minimize the likelihood of a high error estimate, increase the number of control points and be as precise as possible when specifying the coordinates for control points. Double-check that you specified the correct projection in Digitizer Setup.

Configuring Digitizer Buttons

To configure the buttons on your digitizing puck, click Buttons in the Digitizer Setup dialog box. The dialog box that displays will instruct you to click the digitizer button that you want to use as the main button and then to click the digitizer button that you want to use as the double-click button.

If you only have one button on the puck, you will use it for both drawing and completing the polyline/polygon. A single click enters a point when drawing an object; a double-click completes the object.