Aligned dimension

Hi 

 

when I put an aligned dimension in for example a - b it will give the distance (for example 1500). If I want that distance to be exactly 1500 i type 1500 in the command line and nothing happens i.e. no dim line. If I do the same thing with a linear dimension i can type the distance in and it does it. If I try to measure a line that is aligned (i.e not horizontal or vertical) and use Smart dimension it will only give me a horizontal or vertical dimension not the length of the angled line. I've gone through the options but cant seem to rectify it. Am I missing something or am i trying to do something Draftsight cant (I can do this in AutoCad so hopefully its a setting). Hope I've explained my dilemma in an understandable way.