Input
This transformer accepts linear features with at least 3 points.
Replaces the geometry of the feature with a two-dimensional circular arc whose shape is estimated from the first, middle, and last point of the linear feature passed in. The result is an approximation only and unless the linear feature is previously known to be circular, the result could be completely different than the original feature.
This transformer is most useful when the feature was known to have been a circular arc originally, and was stroked into a line. This transformer can then be used to turn it back into an arc.
For example, a feature containing these points:
2,1
1,2
0,2.236067977 (sqrt(5))
will be turned into an arc feature with this geometry:
0,0
and these attributes:
fme_primary_axis 2.236067977
fme_secondary_axis 2.236067977
fme_start_angle 26
fme_sweep_angle 63
fme_rotation 0
Input
This transformer accepts linear features with at least 3 points.
Arc
The estimated arc features are output through this port.
Not applicable.
If the parameters for the arc are already available as attributes on the feature, then use the 2DArcReplacer or 3DArcReplacer transformer.
Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click 
beside the applicable parameter. For more information, see Transformer Parameter Menu Options.
Search for samples and information about this transformer on the FME Knowledge Center.
Associated FME function or factory: SnappingFactory, @ConvertToArc