How to screen digitize

Screen digitizing is the process of creating and/or editing a segment or point map while an existing raster map is displayed as a background in a map window. The raster map can be for instance a band of a satellite image, a color composite, a scanned map, or a scanned photograph. By using the mouse, you can directly digitize elements of interest on the background map.

1. Display the background map:

Display the map which you want to use as background in a map window. When the map already has a correct georeference, i.e. when the status bar of the map window shows Row Col values and XY-coordinates, continue with step 4.

2. Creating a georeference:

When the map is using georef None, i.e. when the status bar of the map window only shows Row Col values, open the File menu in the map window, and choose the Create Georeference command.

The Create Georeference (in map window) dialog box appears:

For a satellite image, for a scanned map, or for a scanned photograph when a Digital Terrain Model (DTM) of the area is not available, create a georef tiepoints.

Specify the coordinate system you want to use.

For scanned photograph which was taken with a normal camera (i.e. no fiducial marks on the photo) and when a DTM of the area is available, create a georef direct linear.

Specify the coordinate system and the DTM you want to use.

Tip:

For a correct behaviour of a georef direct linear and subsequent screen digitizing, it is essential that you have marked the 'Interpolation' check box in the Properties dialog box of your DTM raster map. For more information, refer to the Raster Map Properties dialog box.

The Tiepoint editor will be opened.

For a scanned photograph which was taken with a photogrammetric camera (i.e. fiducial marks on the photograph and known principal distance of the camera) and a DTM of the area is available, create a georef orthophoto. Digitizing on such an aerial photograph is called monoplotting; continue with topic How to monoplot.

The remainder of the current topic deals with georefs tiepoints and georefs direct linear.

3. Using the Tiepoint editor:

In the Tiepoint editor, you have to insert tiepoints, also called ground control points, which establish relationships between the map pixels (row,col) and real world XY- or XYZ-coordinates.

To add tiepoints:

first, click at a recognizable point in the map without coordinates which is displayed in the map window,
the Add Tiepoint dialog box appears. In this dialog box, the row and column values at the position of the click are filled out.
When you already have some tiepoints, the dialog box will also come up with a suggestion for the XY-coordinates. This suggestion is the result of the calculation with the existing tiepoints and using the current transformation method. This suggestion is a measure of the quality of the current tiepoints. The suggestion is merely a suggestion, it is advised to enter your own XY-coordinates to prevent false accuracy.

Then:

click at the same position in a map which already has correct XY-coordinates and which is displayed in another map window (Master/Slave), or
digitize the same point in an analog paper map which is stuck on a digitizer, or
read the correct XY-coordinates for this point from a table or an analog paper map, and type these XY-coordinates in the dialog box.

When editing a georef direct linear or a georef orthophoto, you can optionally specify a Z-value for the inserted tiepoint; otherwise the height value as found in the DTM is used.

The inserted tiepoint appears in the map window and in the tiepoint table.

For a georef tiepoints, you need to select a transformation method:

For satellite images, an affine transformation will usually do (mathematical minimum of 3 tiepoints).
For a scanned photograph (without DTM), a projective transformation is recommended (mathematical minimum of 4 tiepoints).

A georef direct linear, which performs a direct linear transformation (DLT) has a mathematical minimum of 6 tiepoints.

Tips:

You should always insert more tiepoints than is mathematically required.
Tiepoints should be well spread over the map (XY).
For a georef direct linear, the tiepoints should also be well spread in Z-direction and they should not be co-planar, i.e. in Z-direction, the tiepoints should not be on a (tilted) plane. You may have a problem with coplanar tiepoints when for instance during screen digitizing you notice a shift between the location of the segments or points which you were digitizing and the location where these digitized segments and points are appearing in the photo on the screen. For more information, see ILWIS objects : georeference direct linear.

For more information on the Tiepoint editor, see Georeference Tiepoint editor : functionality.

When finished editing the georeference, leave the Tiepoint editor by choosing Exit from the File menu or by clicking the Exit Editor button in the toolbar. The raster map will now use the coordinates as stored in the georeference you just created. The raster map is still displayed in a map window; the status bar now shows Row Col values and XY-coordinates.

4. Creating and/or editing a segment or point map on the background map:

Have the raster map which now uses correct XY-coordinates and which you want to use as background displayed in a map window.

To create a new segment or point map, open the File menu in the map window, and choose Create Segment Map or Create Point Map. The Create Segment Map or the Create Point Map dialog box appears. Type a map name, specify a domain and optionally type a description. Accept the coordinate system and map boundaries which are already filled out.

To edit an existing segment or point map, choose the Edit Other Map command from the Edit menu of the map window, then select the desired editor and select a map.

The Segment editor or the Point editor is opened. You can now add, edit and delete segments or points by using the mouse as usual.

When your background map uses a georef direct linear, you may see that an edited segment or point does not exactly appear at the position of the pencil pointer: this is a measure of the quality of your georeference in the area. Errors will usually be larger along the edges of a photograph.

When the deviation is small, your georeference is good enough;
When the deviation is large, you can check the number, even distribution and quality of the tiepoints in your georeference, the quality of your DTM, etc.

5. Resampling the background image, yes or no:

The crux of this topic is that you do not need to use the resample operation in order to screen digitize on images which use a georef tiepoints or on scanned photographs which use a georef direct linear. An advantage of using the original image or photograph instead of a resampled one is that the original image is not disturbed by the resampling; in general, features of interest will be easier to recognize using the original data.

Performing a resample operation is only required before using MapCalc when you want to combine raster maps with for instance a georeference corners with raster maps using a georef tiepoints or a georef direct linear. In this case, before using MapCalc, resample for instance your image with the georef tiepoints to a georef corners.