Problem: True curves are converted to densified line segments
If the Arc Tool or the Tangent Curve command is used to create curves in a coverage or shapefile, the arc is converted to a densified line when the sketch is complete.
Shapefiles and ArcInfo coverage data formats do not support circular, tangent, or any other types of parametric curves.
Solution or Workaround
In the geodatabase, curves are not composed of smaller arc segments as they are in a coverage, but instead are stored as single segments.
When working with true curves in the geodatabase there are some things to be aware of. When importing data some things to know are:
Curves in CAD files are copied over into true curves in the geodatabase with Copy/Paste and the LoadObjects commands.
Curves stored in coverages (with or without COGO attributes) are not translated into curves in the geodatabase, but instead come over as a set of line segments. There is a developer sample (Curve Conversion) that can be used to translate these features to true curves.
Once the true curves are in the geodatabase, they are displayed and act like true curves as long as an ArcGIS client is used. Note that true curves are not seen if an ArcGIS clients is not used. For instance, if ArcIMS is used (without the ArcMap Server), then true curves are not displayed in the data. This is because the geodatabase stores two representations of the curve segments. One representation is the true curve that can be seen by ArcGIS clients, and the other is the multiple segment view (what is seen with coverages in workstation ArcInfo), which is displayed with non-ArcGIS clients.
Convert data to the geodatabase format, and edits using the Arc or Tangent Curve tools work as expected.