Thank you both for the response. With some investigation, I found that 'Edit line properties' gives an error 'The selected polylines are of unequal category' if one open line and one closed line are selected. This was what was stopping me using the method suggested to reassign. Is this by design? On your first point, when importing a shape file, the 'Polyline-attribute header specification' window allows the Elevation setting to be (disabled), so I would suggest that might allow you to automatically identify elevations to ignore. Typically I create elevation and roughness shape files separately. Thanks again for the help, Daniel