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Gradient or Slope Representation in Various Infra Projects


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?


Gradient or Slope Representation in Various Infra Projects

Image represents 3 Series of Levels

  1. Estimated Formation Level (Straight Line)
  2. Blue Line – Actual Readings
  3. Red Line – Interpolated Values at 10 M interval

You may note that because of interpolating the values there is difference in Area Calculation.

When We Measure Area with Actual Readings


Gradient or Slope Representation in Various Infra Projects

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


Gradient or Slope Representation in Various Infra Projects

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.

  1. Grid Interval – If grid interval is too close, although the calculation may be accurate, will result in additional computation time and also the result (report) generated will run into multiple pages and number of supporting cross section drawings will be more.
  2. Surface Characteristic – If surface has more undulations, setting closer grid interval is more meaningful. For Flatter surfaces, by keeping grid interval closer, it will not make much difference.
  3. Input Data – If initial survey is done at closer interval, setting closer grids will have better result. If the survey points are scattered and surveyed points fall apart then keeping closer grid level will not make much difference in result.

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.