DEMDistanceCalculator

A reference DEM raster. Only the first raster feature received for each Group By grouping will be used.

3D line features.

Raster DEMs representing the 3D distance between a line and the corresponding point on the reference DEM.

Any input features entering the Line port that are not 3D lines will be rejected with the fme_rejection_code attribute set to INVALID_VECTOR_GEOMETRY_TYPE.

Any input features entering the RefDem port that are not valid DEM rasters will be rejected with the fme_rejection_code attribute set to INVALID_REF_RASTER_GEOMETRY.

If more than one DEM raster is received for a Group By grouping, the extra rasters will be rejected with the fme_rejection_code attribute set to EXTRA_REF_RASTER_FEATURE.

Use this parameter to organize lines into groups, with each group of lines having its own reference DEM.

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.