# Interpolation

Delve into the fascinating topic of interpolation, a key concept in engineering mathematics. This comprehensive guide will unpack the meaning of interpolation and bring it to life through real-life examples. You'll gain insights into different types of interpolation such as spline and linear, learn about applying interpolation formulas and explore its practical applications across various engineering fields. Whether you're a student, a professional engineer or just an avid learner, this article is designed to broaden your understanding and extend your professional knowledge.

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## Understanding Interpolation in Engineering Mathematics

Interpolation is a key concept in engineering mathematics, used extensively in digital image and signal processing, computer graphics, and solving mathematical and physical problems. Fundamentally, it is a method of constructing new data points within the range of a discrete set of known data points.

Interpolation is the process of estimating unknown values that fall between known values. In engineering mathematics, it is used to predict missing or corrupted data in a sequence.

### Discovering the Interpolation Meaning

The term 'interpolation' originates from the Latin word 'interpolare', meaning 'to polish, or to alter'. In the context of engineering, interpolation takes changed or altered information and polishes it to provide an anticipated or estimated value.

Suppose you are dealing with a set of specific discrete data points. Between these known data, there might be an area where the data is unknown or absent. Interpolation comes into play here as it estimates the unknown by taking into account the characteristics of the known data.

Interpolation finds its origins in astronomical tables, helping to predict the position of celestial bodies. This laid the groundwork for its extensive use in modern day engineering.

This can be better understood with a simple mathematical formula. For a function $$f(x)$$, if it's known for certain values of $$x$$, say $$x_0, x_1, x_2, \ldots$$, the problem of interpolation consists of finding $$f(x)$$ for values of $$x$$ lying between the given points.

### Exploring Different Interpolation Examples

There are many types of interpolation methods used in engineering mathematics, each having its unique set of applications. Here, let's discuss three prime examples:

• Linear Interpolation: This is the simplest form of interpolation. In linear interpolation, it is assumed that the function follows a straight line between two points. The 'y' coordinate for a value 'x' is calculated by assuming a straight line between the previous and the next point.
• Lagrange Interpolation: This formula is used to find the equation of the polynomial which fits the given data points. Lagrange Interpolation proposes that any function can be represented as a combination of its values at different points.
• Spline Interpolation: Spline interpolation, or a 'spline', is a piecewise-polynomial interpolation because it interpolates on different intervals with different polynomials.
 Interpolation Method Roughness Measure Computation Linear Interpolation Medium Low Lagrange Interpolation High High Spline Interpolation Low Medium

Among these methods, selection highly depends on the nature and requirement of the task at hand. Some would require a balance between computation efficiency and smoothness, while some tasks would prioritally need a more accurate representation of data.

For instance, in image processing, bilinear interpolation is often used for resizing. Bilinear interpolation uses the closest 2x2 neighborhood of known pixel values around a point. For prediction, these pixel values are taken into account for a weighted average. This notion simply comes from linear interpolation applied in both directions.

## The Different Types of Interpolation in Engineering Mathematics

In engineering mathematics, interpolation methods play an essential role in the estimation of values that lie within a certain range of known data points. These methods come in several forms, each designed to handle specific types of problems. The different types of interpolation used frequently include Linear Interpolation, Polynomial Interpolation, Spline Interpolation and others. Each type comes with its own strengths, weaknesses and specific applications within the vast field of engineering mathematics.

### An Overview of Spline Interpolation

Spline interpolation is a form of interpolation where the interpolant, instead of being a single polynomial that covers the entire data set, is a series of lower-degree polynomials. Each of these smaller polynomials interpolates a subset of the data points. In essence, spline interpolation creates a 'smooth' function which avoids the problem of oscillation that can occur in polynomial interpolation.

In the numerical world, splines are incredibly handy as they provide a way of carrying out interpolation while maintaining a balance between computational efficiency and precision. Particularly, in applications such as image processing and computer graphics, spline interpolation is often the preferred choice.

The cubic spline is one of the most commonly used types of spline, defined as a piecewise function where the function between each pair of nodes is a cubic polynomial, the function is continuous at the nodes, and the first and second derivatives are also continuous at the nodes.

The cubic spline $$S(x)$$ on a given interval $$[a, b]$$, with partitions $$a = x_0 < x_1 < ... < x_n = b$$ is defined through $$S(x) = S_i(x)$$, on $$[x_{i-1}, x_i]$$, where each $$S_i$$ is a cubic function. The goal is to determine the coefficients of the individual polynomials.

Consider 5 data points:

x 0 1 2 3 4
y 1 3 7 13 21

When applying cubic spline interpolation to these data points, four cubic polynomials would be created, one for each interval $$[0,1], [1,2], [2,3], [3,4]$$. Different formulae, depending on the type of spline (for example, natural, clamped, or not-a-knot), are used to generate the 4 sets of cubic coefficients.

### Understanding the Basics of Linear Interpolation

Linear interpolation is the simplest form of interpolation. It is a method of curve fitting used to estimate the value of a variable from its surrounding data points. In clear terms, this method assumes that change between two data points is linear and while this may not always be the case in reality, it serves as a simple approximation that is exceptionally valuable in many scenarios.

The key idea behind linear interpolation is that it draws a straight line between two adjacent points and assumes that the function hardly deviates from this straight line in the interval.

Mathematically, the formula for linear interpolation between two points $$(x_0, y_0)$$ and $$(x_1, y_1)$$ is given by:

$y = y_0 + (x - x_0) \frac{(y_1 - y_0)}{(x_1 - x_0)}$

This formula essentially locates the point $$x$$ on the line between $$x_0$$ and $$x_1$$ and computes the corresponding $$y$$ value on the line.

The formula for linear interpolation can be visualised as creating a ratio of the distances between $$x$$ and the two $$x$$ endpoints, multiplying by the corresponding $$y$$ values, and adding the results.

Let’s say we have the points $A = (x_0, y_0) = (1, 1)$ and $B = (x_1, y_1) = (3, 3)$. Now we would like to estimate the value of $$y$$ at $$x = 1.5$$ using linear interpolation. Plugging into the formula, we would find that $$y$$ is equal to 1.5.

## Applying Interpolation Formulas in Engineering Mathematics

In any field of engineering ranging from robotics to aerospace, you might often encounter situations where a set of discrete data points is given, and the goal is to create a continuous domain. In these circumstances, you will have to apply interpolation techniques. The formula to use will meticulously rely on the problem requirements and the nature of data to be fitted.

### Simplifying the Interpolation Formula

Interpolation formulas may seem complex at first glance. However, a closer understanding can help to demystify and simplify these formulas. Generally, irrespective of the form, interpolation formulas project the desired data point onto the function that has been fitted to the known data points.

• Linear Interpolation Formula: Linear interpolation uses a linear function for each interval of $$x$$. The mathematical formula for Linear Interpolation $$L(x)$$ between an interval $$(x_0, f(x_0))$$ and $$(x_1, f(x_1))$$ is given by:
$L(x) = f(x_0) + \frac{f(x_1) - f(x_0)}{x_1 - x_0}(x - x_0)$
• Lagrange Interpolation Formula: The Lagrange Interpolation formula uses the basis polynomials $$L_k(x)$$ which are the product of the terms $$x - x_j$$ for $$x_j \neq x_k$$. The Lagrangian form for a function $$L(x)$$, interpolating at the points \{(x_0, f(x_0)), \ldots, (x_k, f(x_k))\}, is given by:
$L(x) = \sum_{k=0}^{n} f(x_k)L_k(x) = \sum_{k=0}^{n} f(x_k)\left( \prod_{\begin{subarray}{c} j=0 \\ j \neq k \end{subarray}}^{n} \frac{x - x_j}{x_k - x_j} \right)$

Linear interpolation considers a line between two points, whereas Lagrange interpolation seeks a polynomial of a given degree that runs through each point in the dataset.

### Practical Examples of Interpolation Formulas

Implementing interpolation formulas in real-world situations allows engineers to approximate values that aren't explicitly available within the given data.

For example, in the field of Computer Graphics, applications often need to calculate pixels' colours that weren't included in the original image. Bilinear Interpolation is one such method used here. This solution allows for the manufacturing of smooth and reasonably natural gradients of colour. This interpolation involves performing linear interpolation first in one direction, and then again in the direction perpendicular to the first. Practically, in Python, it can be implemented as follows:

def bilinear_interpolation(x, y, points):
points = sorted(points)               # order points by x, then by y
(x1, y1, q11), (_x1, y2, q12), (x2, _y1, q21), (_x2, _y2, q22) = points

if x1 != _x1 or x2 != _x2 or y1 != _y1 or y2 != _y2:
raise ValueError('points do not form a rectangle')
if not x1 <= x <= x2 or not y1 <= y <= y2:
raise ValueError('(x, y) not within the rectangle')

return (q11 * (x2 - x) * (y2 - y) +
q21 * (x - x1) * (y2 - y) +
q12 * (x2 - x) * (y - y1) +
q22 * (x - x1) * (y - y1)
) / ((x2 - x1) * (y2 - y1) + 0.0)


Similarly, in the field of Robotics, the joint space of a robot may be defined at certain discrete timestamps, but to generate a smooth movement, the control signal needs to be provided continuously. Cubic spline interpolation is commonly used for this purpose.

Interpolation formulas also play a considerable role in the field of Environmental Engineering, where measurements may only be available at discrete locations but predictions or models need to be constructed over a larger spatial domain. Techniques such as kriging are used for such spatial interpolation.

## Practical Applications of Interpolation in Engineering

Interpolation methods pervade many spheres of engineering. These tools of mathematics help engineers in predicting values, creating efficient algorithms, designing systems and many other tasks. These concepts influence how engineers approach problem-solving and data use, providing smoother, higher resolution outcomes that enhance the safety, efficiency, and reliability of engineering solutions.

### Interpolation Applications in Different Engineering Fields

In nearly every engineering field, you will find interpolation techniques in use. This is because engineering problems often require an estimation or prediction of data between two known points. Below are a few highlights of its utilization:

• Robotics Engineering: Robotics involves planning smooth paths for moving parts and ensuring synchronized operation of complex systems. Cubic spline interpolation is useful in these scenarios for generating trajectories that robots will follow. With the data points corresponding to positions at certain times, the interpolation creates a continuous function for the robot to move smoothly.
• Aerospace Engineering: In flight navigation and control systems, the control variables often take the form of discrete points. Interpolation becomes crucial to predict values in between these points. It is also used in tasks like generating high-resolution topographical maps from limited sensor data.
• Civil Engineering: In designing buildings and infrastructure, engineers use data from various surveys and measurements. To fill in the gaps between these data points, they utilize interpolation techniques.
• Electrical Engineering: Signal processing frequently involves estimates of data points not measured or known, making interpolation indispensable. It is also commonly used for audio signal enhancement, where it helps recover or heighten specific frequency ranges of interest.

One fundamental mathematical principle in the above applications is the Lagrange interpolation formula:

$L(x) = \sum_{i=0}^{n} y_i l_i(x)$

Where $$l_i(x)$$ is the Lagrange basis polynomial, given as $$l_i(x) = \prod_{j = 0, j \neq i}^{n} \frac{x - x_j}{x_i - x_j}$$.

### Career Opportunities Related to Interpolation Applications

As engineers across multiple disciplines apply interpolation techniques, several career opportunities exist for individuals with expertise in these methods. These can range from entry-level positions to research and leadership roles. A few examples include:

• Data Analyst: Data analysts often use interpolation techniques to fill missing data and prepare datasets for further analysis.
• Software Engineer: In software engineering, interpolation is used in graphics and image processing, user interface design, and even game development.
• Environmental Engineer: In environmental engineering, interpolation techniques are applied to model and predict environmental parameters over a large area from measurements taken at discrete locations.
• Structural Engineer: Structural engineers use interpolation methods in their modeling techniques to create accurate representations of structures from sparse measurement or simulation data.
• Control System Engineer: These engineers design and optimise systems for controlling processes in various industries. They use interpolation to improve the accuracy of control systems, especially in cases where system models are based on discrete data points.
• Research Scientist: Interpolation techniques are valuable in scientific research, especially when experimental data are limited or sparse.

Being well versed in practical and computational applications of interpolation methods can open doors to many exciting career opportunities. A deep understanding of these concepts will serve you well in problem-solving and decision-making in the real world, irrespective of the engineering field you choose to plunge into.

## Interpolation - Key takeaways

• ### Interpolation Meaning

- It originates from the Latin term ‘interpolare’, which means ‘to polish, or to alter’. In engineering, it takes altered information and polishes it to provide an anticipated value. It fills the gap by estimating the unknown data based on the characteristics of known data.
• ### Interpolation Examples

- Leading types are Linear Interpolation, Lagrange Interpolation, and Spline Interpolation. Linear Interpolation assumes a straight line between two points; Lagrange Interpolation proposes any function as a combination of its values at different points; and Spline Interpolation includes different polynomials for different intervals.
• ### Spline Interpolation

- This form utilizes a series of lower-degree polynomials each representing a subset of data; it avoids oscillations that occur in polynomial interpolation. Cubic spline is a common type where a cubic polynomial represents the function between each pair of nodes.
• ### Linear Interpolation

- In this simplest form of interpolation, it assumes a linear change between two points, drawing a straight line between them. The formula: y = y0 + (x - x0) *((y1 - y0)/(x1 - x0)), computes the corresponding ‘y’ on the line.
• ### Interpolation Applications

- Interpolation is used extensively in robotics, aerospace, civil, and electrical engineering for estimating or predicting values, designing systems, and leading high-resolution outcomes in diverse scenarios.
What is interpolation?
Interpolation in engineering is a method to estimate values between two known values. It involves constructing new data points within a set of known data points. Different types of interpolation include linear, polynomial, and spline.
What is linear interpolation? Please write in UK English.
Linear interpolation is a method in engineering where an unknown value within two known values is estimated by assuming a linear relationship between the known values. It is a basic technique in computer graphics for generating intermediate values.
How does one perform interpolation? Please write in UK English.
Interpolation involves finding an estimated value within two known values in a sequence. First, identify two points with known values that bracket your unknown value. Then, calculate the slope between these points and use this to estimate the unknown value using linear proportions.
How can I interpolate between two values?
To interpolate between two values, determine the proportion of the difference between the two values that Your Target is, then apply the same percentage difference to the value. The formula for linear interpolation is value = y0 + (x-x0) * ((y1-y0) / (x1-x0)).
What are some applications of interpolation in engineering?
Interpolation in engineering is used in image processing for resizing and remapping, in civil engineering for terrain modelling, in control systems for approximating values between discrete points, and in computer-aided engineering for solving differential equations.

## Test your knowledge with multiple choice flashcards

How does the application of interpolation techniques benefit different engineering disciplines?

What is the role of the Lagrange interpolation formula in engineering applications?

What is the origin and meaning of the term 'interpolation'?

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