Does FME carry out any internal geometry operations that I should know about

Question

Does FME carry out any internal geometry operations that I should know about?

Answer

Yes. We describe FME as a semantic product; this means FME has a rich data model that can handle any geometry and also that we try to automate translations as much as possible to prevent the need for user intervention.

Because some source and destination formats don't have the same rich data model, some source features would often contain geometries classed as "invalid" by the destination format, so that FME has to process them in some way to make them valid. Because we do this automatically you probably won't even realise it is happening.

Some faults with geometry are fixed when the data is read (to give a good generic representation for FME use) and some when the data is written (to give a representation that conforms to the destination format).

Some faults are just too much for FME to automatically fix, in which case the user will need to apply some transformers (eg a SelfIntersector transformer) to search for and fix a specific issue.


It should be emphasized that FME's processes will not change the overall shape, area, length or size of a feature. The sort of faults fixed are not ones that would have this effect.


Some example faults that FME might fix automatically are...

• Duplicate points on a line feature
• Duplicate line segments on a polygon feature
• Polygon orientation (clockwise/counter-clockwise)
• Twisted or figure-of-eight polygons
• Donut polygons where the inner boundary touches the outer boundary at more than a single point

...where usually the only noticable difference in the data is a change in the number of vertices.


The type of data requiring most cleanup is vector data converted from raster; in such data it's common to find strange interactions where cell outlines meet or overlap and cause figure-of-eight problems.