message "point rejected" when draw low cord

Hi, Chris,
The following is the actual message I received when draw low cord:

Point rejected: Y coordinate is too low.

It seems that it only allow me to draw low cord at or above the ground
elevation. Your help is greatly appreciated.

Wei

For HEC-2 we do not allow ypu to go below the ground. But in HEC-RAS we do allow that.

Chris