To place one node interactively on the edge of a raster object or line

Select Place Nodes.
Turn on Raster Snap.
Turn off Vector Snap.
Select the appropriate color mask.
Select None as the Boundary option.
Choose Outline in the Entity option menu.
Identify the approximate position of the node by entering a data point.
The raster snap adjusts the actual position of the node and places it precisely on the edge of the raster object, or on either or both sides of a raster line.

Note: If both Raster Snap and Vector Snap are on, the raster snap is applied first, and then the vector snap.