Before construction, a survey grid set-out plan should be agreed in order to identify the exact survey points for property IV. Property IV requires the floor to be surveyed on a 3 m grid and elevations of adjacent 3 m grid points compared. The 3 m grid survey should be carried out using a precise level and staff as in Section 7.12 of TR34. Property II controls flatness and requires the difference in slope over 600 mm to be surveyed. Property II should be measured along lines that are uniformly distributed on the 3 m grid across the floor and which are used to assess property IV. The uniformity is required to ensure that the survey gives a reasonable sample assessment of the free movement area. This is achieved by using lines of the same length and ensuring that no readings are taken within 1500 mm of walls or columns. The minimum number of 3 m long survey lines should be calculated as floor area/30. Figure 4 illustrates a typical survey grid set-out plan although other configurations will be acceptable providing that the above requirements of line distribution are met.