Snapping Nodes and Thinning Objects - MapInfo_Pro - 2023

The Snap/Thin command sets snap tolerances for nodes in different objects, and node and polygon thinning settings for your data. Snap/Thin also removes any self-intersections and overlaps in region data. The overlap removal is automatically done after any of the node snap and thin operations and will remove any overlaps, which survive these processes.

The Snap/Thin command has three main functions:

• The Inter-Object Node Snap - Snaps nodes from different objects together.
• The Node Thinning/Generalization - Removes nodes from objects while retaining their shape. When you thin region objects, shared boundaries are retained.
• Polygon Area Thinning - Removes small polygons from selection.

The Snap/Thin command is available whenever objects are selected in an editable table. These objects must either be all closed or all linear objects. You cannot use Snap/Thin a mix of closed and linear objects.

To Use Snap/Thin:

1. Select the objects you want to work with.
   Note: All of the objects selected must either be closed objects (for example, regions, rectangles, rounded rectangles or ellipses) or linear objects (for example, lines, polylines, or arcs). The selection cannot contain a mixture of closed and linear objects.
2. Make the table the objects are in editable.
3. On the SPATIAL tab, in the Edit group, click Snap/Thin to open the Set Values for Node Snap & Thinning dialog box.
4. Check Enable Node Snap, Enable Node Thinning/Generalization, or Enable Polygon Area Thinning.

   For more information, see Set Values For Node Snap & Thinning Dialog Box.

5. Click OK.

Why Use Snap/Thin?

Nodes from different objects that are very close to each other may have resulted from object creation or editing issues, and can result in small overlaps and gaps between objects. Using appropriate tolerance values will snap the nodes together and eliminate such overlaps and gaps.

Some objects may contain nodes that do not contribute to the object and/or application. The appearance of these nodes could be due to the resolution of the data. In some cases, the resolution may not be great enough to register subtle differences in node positions. In other cases, the objects may contain more resolution than is required for the application. This unneeded or unwanted resolution can add overhead in the display, manipulation, and storage of the objects. Thinning the data can remove nodes and reduce the complexity of the objects, and if done properly, can retain the shape and features of the object.

Inter-Object Node Snap

In addition to snapping nodes of different objects together, Inter-Object Node Snap eliminates small overlaps and gaps in region objects as well as connect polyline objects that are close, but do not touch each other.

The End Node Tolerance setting is the tolerance value for the end nodes of a polyline. If there is another node in the same object or a different object that is within the specified tolerance of an end node of a polyline, one or both of these nodes will be snapped together so that they occupy the same point. The Internal Node Tolerance is used for all other non-end point nodes of a polyline.

For polylines, the End Node Tolerance must be greater than or equal to the Internal Node tolerance. You will receive a warning message if they are not. In many instances, the End Node Tolerance and Internal Node Tolerance should be set to the same value, although they can be set separately.

For regions, the designation of end nodes and internal nodes is irrelevant, since the nodes of regions form closed loops. Therefore, when you specify tolerance settings for regions, the End Node Tolerance field is unavailable. The only tolerance value you specify is the Internal Node Tolerance. This value is used for all nodes in region objects.

Specify the unit of measurement for the snap tolerance in the Tolerance Units drop-down list.

Node Thinning and Generalization

In node thinning and generalization, nodes are removed using a 3-Node Collinear Deviation value. This value is determined by connecting three nodes to form a triangle and measuring the perpendicular distance between the second node and the line segment connecting the first and third nodes. If this distance is less than or equal to the 3-Node Collinear Deviation distance, the three points are considered co-linear and the middle (second) point is removed from the object.

The Node Separation tolerance is used to thin nodes within an object that are close together. If two successive nodes in an object are closer than the distance tolerance, then one of the nodes is removed from the object. Set the unit of measurement for the Thinning/Generalization values in the Distance Units drop-down list.

Polygon Area Thinning

In polygon area thinning, polygons that are smaller in area than the Minimum Area value will automatically be removed. The polygon can be part of a multi-polygon region, in which case only that polygon is removed from the region, and the rest of the polygons in the region are retained. Set the unit of measurement polygon thinning in the Area Units drop-down list.

You toggle Snap to Node by pressing the `S' key. If snap mode is active for a window, "Snap" displays in the Status bar.

• You can perform Snap to Node in more that one window at a time (only Map windows can use the snap feature). If you switch from a window that has snap on, to another window and then back, Snap to Node now stays on.
• Snap to Node tolerance is saved in the workspace.

To use Snap/Thin:

1. Open a Map window containing the layer whose objects you want to work with.
2. Make sure the layer is editable.
3. On the SPATIAL tab, in the Edit group, click Snap/Thin to open the Set Values for Node Snap & Thinning dialog box.
4. Specify the Inter-Object Node Snap settings and select the Enable Node Snap check box to activate the settings.
5. Specify the End Node Tolerance, Internal Node Tolerance, and the Tolerance Units.
6. Specify the Node Thinning/Generalization settings, and select the Enable Node Thinning/Generalization check box to activate the settings.
7. Specify the 3-Node Collinear Deviation, Node Separation, and Distance Units values.
8. Specify the Polygon Area Thinning settings and select the Polygon Area Thinning check box to activate the settings.
9. Specify the Minimum Area and Area Units for the polygons to be removed.

The Snap/Thin command will clean bad data (self-intersections and overlaps), even if no snap or thin values are used. It uses a related, but not identical, mechanism as is used in the Clean command, and thus may produce similar, but different, results than can be obtained using Clean. For example, overlapping polygons have the overlap removed, but which region retains the overlap, and which region it is removed from often are different from those that result from the Clean command. It is generally not easily determine this before the operation.

See Also:

Why Use Snap/Thin?