In grading projects while calculating the quantities, we can do the quantity calculation at different grid interval. While doing earthwork calculations with different grid interval, we get different quantities (Volume).
a) What is the Reason for difference in calculation by changing the grid interval?
Image represents 3 Series of Levels
You may note that because of interpolating the values there is difference in Area Calculation.
When We Measure Area with Actual Readings
Sl. 
Description 
Cutting 
Filling 
1 
Actual Reading 
116.3011

241.955 
2 
Interpolated Value at 10 M 
128.569 
217.510 
In the following picture, you can see the drawing with grid Interval at 5 M
When grid is interpolated with smaller grid interval, then values are much closer to actual reading. This results in calculation which is closer to actual calculation.
For Same data when we do the computation with different grid – results are as follows
Sl. No. 
Grid Interval 
Filling (Sq. M) 
Cutting (Sq. M) 
1 
Actual 
116.359 
243.076 
2 
5 M 
116.303 
241.761 
3 
10 M 
128.569 
217.313 
4 
15 M 
67.116 
234.869 
5 
20 M 
131.375 
160.510 
If we note that difference between actual calculation and calculation when 5 M interval is set is negligible. Whereas when the grid intervals are higher the difference in area is considerable.
With these results we can conclude that if the grid intervals are closer, accuracy of area calculation is higher.
For volume calculation, input will be area calculation for each grid line. Area is multiplied with grid interval to derive the volume. As we have observed, if the grid interval is more, then there is possibility of higher difference considered to actual calculation. So, higher the grid interval, volume calculations are also expected to deviate considerably from actual volume.
b) What is best suggested grid interval?
There are many factors which influence while deciding grid interval.
Considering all these factors, normal practice is to Keep 3 M interval where more accurate results are expected. 5 M in most of the projects and 10 M when the project area is very big.