Error in distance check ?

Since the formula editor came about we are often using expressions to set elevation from slices above or below.

We typically use an expression like z(Idx,n - Noslice,n) - 0.01 for setting elevation for certain selected nodes equal to the elevation 0.01 meter below the nodes directly above.

This seems to mostly work well, but sometimes we get the message "Verify assignment, invalid data" indicating that the slices are crossing.

Since the formula specify a fixed distance, the necessary room must off course be available below the upper slice, but even then we get this message and since all changed nodes are set to have a minimum distance of e.g. 0.01 m to the nodes above, no crossing should be possible. If we apply the solution, move slice top/down, nothing seems to happen, but problem is resolved. My question is, how does Feflow check for invalid assignment of elevation and how precise is this check ??. Can it be that actually there is no problem even though we get the "verify assignment, invalid data" message ??.

Have you checked the setting for Minimal Slice Distance in the Editor Settings of the Problem Settings dialog? The distance defined there will determine when the verification dialog will appear. This is to make sure that the verification does not only check for intersection, but also for very small distances. If - for example - a distance of 0.01 m is set there, and you set a distance of 0.01 m, it may happen that the dialog comes up due to to internal rounding/variable conversion that makes 0.01 m into something that could be just a little bit larger than 0.01 m.