When it comes to measuring plans accurately, one of the fundamentals is measuring the internal wall thickness.
It only takes one of the walls to be thicker than the rest and you can easily have 20 – 30cm unaccounted for. This may not sound like a lot but it can have implications on the area and on adjacent rooms.
I have used many different tools over the years to create floor plans, from pen & paper/CAD to iPhone apps that claim to do the job without a laser. In my opinion PlanEdge is the best I have come across by quite some distance. By allowing the accurate measuring of internal walls, the plan comes together without the inevitable problems when an arbitrary wall thickness is assumed.
As you can see from the plan above, buildings rarely have a consistent wall thickness and accounting for this variation not only creates a true representation of the plan, it also provides an accurate assessment of the area.