For Vastu applications precise knowledge of directions is essential for which we use magnetic compass where the compass needle always shows towards North.
Place the compass in the middle of the building where the needle stops North direction is towards that side. 180 degrees from this side is South. 90 degrees to right side is East and 90 degrees to the left side is West.
If the needle stops some where within 22.5 degrees this may be called a regular (Disha) plot. If beyond 22.5 degree the same may be called skewed (Vidisha) plot
Locating directions by observing Sun