# 8 Difference Between Simpson’s Rule And Trapezoidal Rule In Surveying

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Accurate measurement and calculation of areas are fundamental tasks in surveying, as they form the foundation for designing structures and understanding land features.

The Trapezoidal Rule and Simpson’s Rule are two well-established techniques commonly used for calculating areas based on discrete data points. In other words, both methods are based on numerical integration techniques and serve as effective tools for approximating area.

## Trapezoidal rule

The Trapezoidal Rule is particularly useful in surveying when dealing with irregularly shaped regions, where it becomes necessary to calculate the area accurately. The method derives its name from the trapezoidal shapes formed by connecting adjacent data points with straight lines.

Trapezoidal Rule approximates the curve or data points with a series of trapezoids and then calculate the sum of their individual areas to estimate the total area. The curve or data points are divided into small segments, and each segment is treated as a trapezoid with its top side formed by connecting the two adjacent data points with a straight line.

The area of each trapezoid is calculated using the formula for the area of a trapezoid:

Area = (1/2) * (sum of the lengths of parallel sides) * (height)

Once the areas of all the trapezoids are calculated, they are summed up to obtain an approximation of the total area under the curve or between the data points.

Formula:

The mathematical representation of the Trapezoidal Rule can be expressed as follows:

∫[a, b] f(x) dx ≈ (h/2) * [f(a) + 2f(x₁) + 2f(x₂) + … + 2*f(xₙ₋₁) + f(b)]

where:

• a and b are the lower and upper limits of the integration, respectively.
• f(x) is the function representing the curve or data points.
• x₁, x₂, …, xₙ₋₁ are the intermediate points between a and b.
• h represents the interval between adjacent points and is given by h = (b – a) / n, where n is the number of sub-intervals.

• The method is straightforward and easy to implement, requiring minimal computational complexity.
• It can handle both uniformly spaced and non-uniformly spaced data points.
• For relatively small intervals and a limited number of data points, the Trapezoidal Rule can provide reasonably accurate results.

Limitations

• While it can yield acceptable accuracy for curves that are relatively smooth, it may introduce significant errors when dealing with curves that have rapid variations or sharp bends.
• The accuracy of the approximation is dependent on the distribution of data points. Unevenly spaced data points can lead to less accurate results.
• The Trapezoidal Rule is specifically designed for approximating areas under curves or between data points and may not be suitable for other types of integration problems.

## Simpson’s Rule

Simpson’s rule is a numerical approach to finding definite integrals where no other method is possible. It provides a more accurate estimation of the area compared to simpler methods like the Trapezoidal Rule. The method is particularly useful in surveying when dealing with irregularly shaped regions, where higher precision in area calculations is required.

The core principle behind Simpson’s Rule is to approximate the curve or data points with a series of quadratic polynomials (parabolas) instead of straight lines used in the Trapezoidal Rule.

The value of a definite integral is approximated using quadratic function or a series of quadratic polynomials (parabolas) instead of straight lines used in the Trapezoidal Rule.

In this rule, the boundaries between the ends of ordinates are assumed to form an arc of parabola. Hence Simpson’s rule is sometimes referred to as parabolic rule. This approach allows for a better fit to the curve and more accurate area calculations. In many cases the method is only used when great accuracy is required.

To use Simpson’s Rule, the curve or data points are divided into an even number of intervals (n), each having three points (x₀, x₁, x₂) that lie on the curve. These three points are then used to construct a quadratic polynomial that passes through them. The area under each parabolic segment is calculated using the formula for the area of a parabola:

Area = (h/3) * [f(x₀) + 4*f(x₁) + f(x₂)]

where:

• h is the interval width, given by h = (b – a) / n, where a and b are the lower and upper limits of the integration, respectively.
• f(x) is the function representing the curve or data points.

The areas of all the parabolic segments are then summed up to obtain the final approximation of the total area under the curve or between the data points.

Formula:

The mathematical representation of Simpson’s Rule can be expressed as follows:

∫[a, b] f(x) dx ≈ (h/3) * [f(x₀) + 4f(x₁) + 2f(x₂) + 4f(x₃) + 2f(x₄) + … + 2f(xₙ₋₂) + 4f(xₙ₋₁) + f(xₙ)]

where:

• a and b are the lower and upper limits of the integration, respectively.
• f(x) is the function representing the curve or data points.
• x₀, x₁, x₂, x₃, …, xₙ are the points on the curve, spaced at regular intervals.