Snapper

Features whose geometry is changed by the transformer.

Features that leave the transformer without being changed.

• If a line or area feature snaps down to a single location, it will be output as a Point geometry, through the Collapsed port.
• If an Aggregate, MultiArea, or MultiCurve geometry has any parts snap down to a single location, a Point geometry for each part will be bundled up into a MultiPoint geometry and output through the Collapsed port. The remaining parts which did not snap down to a single location (if any) will be output as a group through the Snapped port.

If Group By attributes are selected, features are only snapped to other features with the same values in the group by attributes.

Process At End (Blocking): This is the default behavior. Processing will only occur in this transformer once all input is present.

Process When Group Changes (Advanced): This transformer will process input groups in order. Changes of the value of the Group By parameter on the input stream will trigger processing on the currently accumulating group. This may improve overall speed (particularly with multiple, equally-sized groups), but could cause undesired behavior if input groups are not truly ordered.

Considerations for Using Group By

There are two typical reasons for using Process When Group Changes (Advanced) . The first is incoming data that is intended to be processed in groups (and is already so ordered). In this case, the structure dictates Group By usage - not performance considerations.

The second possible reason is potential performance gains.

Performance gains are most likely when the data is already sorted (or read using a SQL ORDER BY statement) since less work is required of FME. If the data needs ordering, it can be sorted in the workspace (though the added processing overhead may negate any gains).

Sorting becomes more difficult according to the number of data streams. Multiple streams of data could be almost impossible to sort into the correct order, since all features matching a Group By value need to arrive before any features (of any feature type or dataset) belonging to the next group. In this case, using Group By with Process At End (Blocking) may be the equivalent and simpler approach.

Note: Multiple feature types and features from multiple datasets will not generally naturally occur in the correct order.

As with many scenarios, testing different approaches in your workspace with your data is the only definitive way to identify performance gains.

When this parameter is set to End Point Snapping, the transformer:

• Snaps lines together if the distance between their end points are within the specified tolerance.
• Snaps point features to points or lines if their distances are within the specified tolerance.
• Area features will not be altered by the transformer when run in this mode.
• When two features are snapped together, the feature that entered the factory most recently is the one that is modified

When this parameter is set to Vertex Snapping, the transformer does the following:

• Snaps vertices of lines together if their distances are within the specified tolerance.
• Snaps point features to points or lines if their distances are within the specified tolerance.
• Area features are altered by this operation as its vertices are snapped.
• When two features are snapped together, the feature that entered the factory most recently is the one that is modified.

When this parameter is set to Segment Snapping, the transformer does the following:

• Snaps vertices of lines together if their distances are within the specified tolerance.
• Snaps vertices of segments to other segments if their distances are within the specified tolerance at any point along the segment. New vertices will be introduced to the segment where the new snapped vertex has been moved to.
• Segment snapping may cause duplicate points, where segments have degenerated to a point.
• Segments which cross other segments will have new vertices introduced at the point of intersection, but the segments will remain unbroken.
• Area features are altered by this operation as its vertices and segments are snapped.
• If a segment and a point are both within tolerance, the transformer will snap to a vertex in preference to a segment, even when the segment is closer.

Snapping Distance specifies the distance, in ground units, that the snapping occurs between features.

When Snapping Type is Segment Snapping, this transformer performs data cleaning that is meant to improve the robustness of the results when they are used in other algorithms by ensuring that no two vertices in the snapped output are within tolerance of each other.

If Tolerance is set to Automatic, a tolerance will automatically be calculated for the cleaning operation, based on the size of the inputs. Otherwise, a custom tolerance may be used.

This parameter applies only when the end point of a feature is being snapped. It controls how lines are modified when they are snapped.

• NEVER: the endpoint of a line is moved when it is snapped and no additional vertex is added.
• ALWAYS: the original end point (start point) of the line becomes the second from the end (start) and a new vertex is added to complete the snap.
• FORWARD_ONLY: a new vertex is added only when doing so creates an angle greater than 90 degrees with the original line segment. In this case, if adding the vertex would cause a less than 90-degree angle, the old end point is still moved.

Any features entering the transformer whose length is less than or equal to the tolerance will be treated specially: they will be output as Untouched, and other features (but not other short features) will not be able to snap to them. If the option is not selected, features like this will collapse to a single point and will be dropped.