Errors in establishing the Reference Plan

The positioning of reference plane is crucial for accurate location of a building. It needs to be exterior to the building, critically located so that determinant points along the section plane can be drafted from it. In this case, the closest possible permanent points from which a reference plane external to the building that could be generated was located 18.0 meters away. As a result, 4 points inside the building were chosen as the corners of the quadrilateral (reference plan) and measurements for interconnected spaces were taken with respect to the same. Over the length of 25.0 meters, it was observed that the cumulative error as a result has been 150 mm. 

After a discussion of the approach with Ar P. Latorre, it was realized that the process theoretically was correct, but in practice, the changes of errors, including human errors, area higher.  Also it was realised that drafting smaller areas from a larger reference plane negates errors than the other way round. The Diagram below shows the reference grid suggested by Ar P Latorre (Figure-1) and that executed at Site (Figure-2). 

Ar. Latorres recommendations were based on the possibility of external reference plane, which will be at this juncture a time consuming exercise being subjected to permission form the client. 

The consultants have now identified critical points (that include perfect corners, openings, significant niches or larger moulding) from which dimensions can be transferred to connected spaces. These points will be triangulated first and will serve as a reference grid for smaller interior details. The consultants have also in the meanwhile requested the clients to permit construction of bearing points within 3 meters from the built surface, which will serve as an external reference plane. 

On redrafting with the aforementioned approach, it has been found that the difference between the location of old and new points has been almost 40mm. 

Lesson learnt

• Incase an external reference grid is not feasible or cannot be availed - draft only  critical points to determine the overall profile of the section plane. The other details, like the niches, mouldings etc, in this case, should be drafted thereon. 
• The larger the grid the more correct it will be (perhaps).
• Avoid making moulding or other details as the critical points except when unavoidable. In this case, as there were higher number of chipped and or broken corners, one had to take edges of mouldings as reference corners.