Currently, when defining a map area, one can draw a box or rectangle but always is horizontal / Vertical. this means if the area one maps at a angle let say 45 or 30 degrees, one cannot define the map area to be at the same angle. It would be great if one could define a map area that is as per the survey area. that way, when calculating volumes it could be a lot more accurate.

Volume/area calcs with the volumes module only ever take into account actual mapped areas, so the bounding box that is defined when creating the map shouldn't really be an issue. It is true though that the area of the rectangle as shown in the "mapped area" section of the map project will show the area of the bounding box.

True. but to be able to generate a accurate area and boundary box. it is very helpfull to be more specific in defining the boundry box.