Thank you for the reply.
So the DTM values are supposed to be NULL where there are obstacles present (building, trees etc ), but due to some noise the values are not NULL , but should also not be considered ground heights. This makes it confusing to even use the DTM data, as you are never sure if the DTM value should refer to a NULL value, or noise or the actual height.
Is there a way to obtain clutter heights from DTM and DSM in any other way? I am new to this and some information about this would be helpful.