contour plots in MATLAB - Mini Project: Build & Apply

Creating Contour Plots in MATLAB

📖 Scenario: You are working as a data analyst and need to visualize the height variations of a terrain using contour plots. Contour plots help show lines of equal height, making it easier to understand the shape of the land.

🎯 Goal: Build a MATLAB program that creates a contour plot from a grid of height data. You will first set up the data, then configure the grid, apply the contour plotting function, and finally display the plot.

📋 What You'll Learn

Create a matrix called Z representing height values.

Create vectors X and Y representing the grid coordinates.

Use the contour function with X, Y, and Z to generate the contour plot.

Display the contour plot with labeled contour lines.

💡 Why This Matters

🌍 Real World

Contour plots are used in geography to show elevation, in meteorology to show pressure or temperature, and in engineering to visualize data like stress or heat distribution.

💼 Career

Knowing how to create and interpret contour plots is useful for data analysts, engineers, and scientists who need to visualize complex data in a clear way.

1

DATA SETUP: Create height data matrix

Create a matrix called Z with these exact values: [1 2 3; 4 5 6; 7 8 9].

MATLAB

% Create the matrix Z with the specified values
% Your code here

Hint

Use square brackets and semicolons to create a 3x3 matrix.

2

CONFIGURATION: Create grid coordinate vectors

Create vectors X and Y as [1 2 3] to represent the grid coordinates.

MATLAB

Z = [1 2 3; 4 5 6; 7 8 9];
% Create vectors X and Y with values [1 2 3]
% Your code here

Hint

Use square brackets to create the vectors.

3

CORE LOGIC: Generate the contour plot

Use the contour function with X, Y, and Z to create the contour plot and assign the output to C and h.

MATLAB

Z = [1 2 3; 4 5 6; 7 8 9];
X = [1 2 3];
Y = [1 2 3];
% Create contour plot using contour(X, Y, Z)
% Your code here

Hint

Use square brackets to capture multiple outputs from the contour function.

4

OUTPUT: Label and display the contour plot

Use clabel with C and h to label the contour lines and display the plot.

MATLAB

Z = [1 2 3; 4 5 6; 7 8 9];
X = [1 2 3];
Y = [1 2 3];
[C, h] = contour(X, Y, Z);
% Label the contour lines
% Your code here

Hint

Use clabel(C, h) to add labels to the contour lines.

Practice

1. What does a contour plot in MATLAB primarily represent?

easy

A. A 3D surface plot

B. Lines connecting points with the same value

C. A bar chart of values

D. A scatter plot of points

Solution

1. Step 1: Understand contour plot purpose

   A contour plot shows curves that connect points with equal values on a grid.

2. Step 2: Compare with other plot types

   3D surface plots show height as a surface, bar charts show bars, scatter plots show points, so they differ.

3. Final Answer:

   Lines connecting points with the same value -> Option B

4. Quick Check:

   Contour plot = lines of equal value [OK]

Hint: Contour plots connect equal values with lines [OK]

Common Mistakes:

• Confusing contour plots with 3D surface plots
• Thinking contour plots show individual points
• Mixing contour plots with bar charts

2. Which of the following is the correct syntax to create a basic contour plot in MATLAB using matrices X, Y, and Z?

easy

A. contour(X, Y, Z)

B. plot(X, Y, Z)

C. contourplot(X, Y, Z)

D. surface(X, Y, Z)

Solution

1. Step 1: Recall MATLAB contour syntax

   The function to create contour plots is contour(X, Y, Z).

2. Step 2: Eliminate incorrect functions

   plot() is for 2D plots, contourplot() is not a MATLAB function, surface() creates 3D surfaces.

3. Final Answer:

   contour(X, Y, Z) -> Option A

4. Quick Check:

   Use contour() for contour plots [OK]

Hint: Use contour(X,Y,Z) to plot contours [OK]

Common Mistakes:

• Using plot() instead of contour()
• Typing contourplot() which is invalid
• Using surface() which makes 3D plots

3. What will be the output of the following MATLAB code?

X = 1:3; Y = 1:3; [Xg,Yg] = meshgrid(X,Y); Z = Xg.^2 + Yg.^2; contour(Xg, Yg, Z, 3);

medium

A. An error because contour needs vectors, not matrices

B. A 3D surface plot of Z values

C. A scatter plot of points (Xg, Yg, Z)

D. A contour plot with 3 contour lines representing levels of Z

Solution

1. Step 1: Understand meshgrid and Z calculation

   meshgrid creates grids Xg and Yg; Z is sum of squares, a smooth surface.

2. Step 2: Interpret contour call with 3 levels

   contour with 3 as last argument draws 3 contour lines at different Z levels.

3. Final Answer:

   A contour plot with 3 contour lines representing levels of Z -> Option D

4. Quick Check:

   contour with level count = contour lines count [OK]

Hint: Last argument in contour sets number of lines [OK]

Common Mistakes:

• Thinking contour creates 3D surface plots
• Believing contour requires vectors, not matrices
• Confusing contour with scatter plots

4. Identify the error in this MATLAB code snippet for contour plotting:

X = 1:5; Y = 1:5; Z = X + Y; contour(X, Y, Z);

medium

A. contour requires only two arguments

B. X and Y must be matrices, not vectors

C. Z must be a matrix matching grid size, not a vector

D. No error, code runs fine

Solution

1. Step 1: Check dimensions of X, Y, Z

   X and Y are vectors; Z is vector sum, not a matrix grid.

2. Step 2: Understand contour input requirements

   contour needs Z as matrix matching X, Y grid sizes; here Z is vector, causing error.

3. Final Answer:

   Z must be a matrix matching grid size, not a vector -> Option C

4. Quick Check:

   Z size mismatch causes error [OK]

Hint: Z must be matrix matching X,Y grid for contour [OK]

Common Mistakes:

• Passing vector Z instead of matrix
• Assuming contour accepts vector inputs
• Ignoring dimension mismatch errors

5. You want to plot contour lines only at specific Z values: 5, 10, and 15. Which MATLAB command correctly achieves this given matrices X, Y, and Z?

hard

A. contour(X, Y, Z, [5 10 15])

B. contour(X, Y, Z, 3)

C. contour(X, Y, Z, 'Levels', [5 10 15])

D. contour(X, Y, Z, 'LineLevels', [5 10 15])

Solution

1. Step 1: Recall contour level specification syntax

   To specify exact contour levels, pass a vector of levels as the fourth argument.

2. Step 2: Evaluate options for specifying levels

   contour(X, Y, Z, [5 10 15]) uses vector [5 10 15] correctly; contour(X, Y, Z, 3) sets number of lines, not levels; options C and D use invalid syntax.

3. Final Answer:

   contour(X, Y, Z, [5 10 15]) -> Option A

4. Quick Check:

   Use vector of levels as 4th argument [OK]

Hint: Pass vector of levels as 4th argument to contour [OK]

Common Mistakes:

• Using number instead of vector for levels
• Trying unsupported name-value pairs
• Confusing level count with specific levels