A vertical datum with high-precision leveling was created to determine the elevation of the datum points for the reference grids and the vertices of the densification polygons. The datum selected for calculating the compensation of the leveling network was the Igm (Cso) benchmark known as 170/12, located near Piazzale Roma.
Several benchmarks (UIMA and others), closest to the datum points of the Gps network, were selected as vertical references. The contour lines are primarily loops but in some cases, particularly in the dead-end streets and where there are no potential outlets, they are open polygonals. The high-precision contour lines consist of a total of 4015 benchmarks (65 Gps datum points, 65 elevation control points, 757 traditional datum points, 3128 densification polygonal vertices) for a total length of about 140 Km.
Measurements were made both ways, using digital levels with bar code staffs, automatically recorded in the internal memory and then appropriately stored in their original form.
The size of each station bar was built by standing with the level between the two staves, with uncertainty factor of no more than 2% of the distance between them. The distance between the instrument and the staff did not exceed 40m in the BF measurement mode (B = backsight, F = foresight).