CONTROLS & ITS METHODS
The horizontal control consists of reference marks
of known plan position, from which salient points of designed structures may be
set out. For large structures primary and secondary control points are used.
The primary control points are triangulation stations. The secondary control
points are reference to the primary control stations.
Reference grids are used for accurate setting out
of works of large magnitude. The following types of reference grids are used:
2. Site Grid
Survey grid is one which is drawn on a survey plan, from the
original traverse. Original traverse stations form the control points of the
grid. The site grid used by the designer is the one with the help of which
actual setting out is done. As far as possible the site grid should be actually
the survey grid. All the design points are related in terms of site grid
coordinates. The structural grid is used when the structural components of the
building are large in numbers and are so positioned that these components
cannot be set out from the site grid with sufficient accuracy. The structural
grid is set out from the site grid points. The secondary grid is established
inside the structure, to establish internal details of the building, which are
otherwise not visible directly from the structural grid.