Map Reference

Digitizer Control Points

Second dialog box to establish a relation between digitizer coordinates (as received from your digitizer) and map coordinates.

The purpose of using Map Reference is to be able to use a digitizer:

to add points or segments to a new digital point or segment map, and/or
to edit an existing digital point, segment or polygon map.

This dialog box appears after you clicked OK in the Map Reference - Coordinate system dialog box.

It is assumed that:

your digitizer set up is correct:
- the Setup Digitizer command on the File menu of the Main window should be preceded by a check mark ;
you fixed an analog map onto your digitizer;
you know the real world coordinates of at least three points in your analog map; these points will be used as reference or control points. You can use for instance the coordinates of the corners of your analog map or the coordinates of crossings of grid lines on your analog map.

When the digitizer is on and properly set up, you should see, besides this Map Reference - Digitizer Control Points dialog box, a little floating window called Map Referencing which displays digitizer coordinates that change when you move the digitizer cursor on the digitizer.

The floating Map Referencing window contains the options:

0	Digitize Point
1	Next Point
2	Delete Point

If the digitizer coordinates in this little floating window do not change when you move the digitizer cursor on the digitizer, the digitizer setup is not correct and should be done again. For more information, see Setup Digitizer wizard or How to set up a digitizer.

Steps to take and dialog box options:

Position the digitizer cursor on the first reference or control point of your map on the digitizer tablet, and press button 0 on the digitizer cursor.
Under heading digitizer, the coordinates will appear as sent by the digitizer.
Depending on the setting 'metric (projected) coordinates' or 'geographic coordinates' as specified in the previous Map Referencing - Coordinate System dialog box, fill out the XY-coordinates or the LatLon-coordinate of the first reference or control point.

When using metric coordinates:

under heading coordinates, type the XY-coordinate of this point as read from your map: first the X-coordinate in meters, then the Y-coordinate in meters.

When using geographic coordinates:

under heading coordinates, type the LatLong-coordinate of this point as read from your map: first the Latitude in degrees, mintues, seconds, N or S, then the Longitude in degrees, mintues, seconds, E or W.

Continue with the next reference or control points.
Check the feedback in this dialog box on diff X and diff Y, Sigma, and Scale.

You can use the little floating window:

to go to a next reference point without inserting a new one (press button 1 on the digitizer cursor), and
to delete wrongly digitized reference points (press button 2 on the digitizer cursor).

Conformal transformation:	In a Conformal transformation, the scale in X and Y direction is the same. Mathematically a minimum of 2 reference points are needed but it is advised to specify at least 3.
Affine transformation:	In an Affine transformation, the scale in X and Y directory may be a bit different. Mathematically a minimum of 3 reference points are needed but it is advised to specify at least 4.

When finished, click the OK button.

In the Main window: the Map Reference command on the File menu of the Main window will now be preceded by a check mark ;
When you are already in the point, segment or polygon editor: the Digitizer, Map Reference command on the File menu of the point editor, on the File menu of the segment editor, and on the File menu of the polygon editor will now be preceded by a check mark .

When you are already in the point, segment or polygon editor and wish to start or to continue digitizing an analog map:

choose Digitizer, Active from the File menu of the point editor, from the File menu of the segment editor, or from the File menu of the polygon editor.

The Digitizer, Active command on the File menu of the editor will then be preceded by a check mark .

Now, you can start or continue to digitize your analog map. For more information, see How to setup a digitizer and start digitizing, sections Start digitizing a new map and/or Continue to digitize an existing map.

Tips:

For a good map reference in an affine transformation, the diff X and diff Y values should be in the range of -0.3 to 0.3 or less.
The Scale figure(s) should be close to the scale as specified on your analog map.
The OK button in this dialog box is grayed out when the Sigma is greater than 20.
A maximum of 6 reference or control points can be inserted.
When you take a break during digitizing, you can exit the editor and exit ILWIS and even switch the computer off. As long as your map is still at the same position on the digitizing tablet when you wish to resume digitizing later on, you do not need to reference your map again.
Map Reference settings are stored in the registry.