MATLAB: How to color and plot arrows in directions specified by an array of angles

2darrowscolorMATLABplotquiverscale

I have 3 vectors of data specifying x, y, and theta. I would like a 2-D plot of arrows at each (x,y) coordinate, with each arrow pointing in a direction indicated by theta, where theta is between 0 and 360 degrees.
In addition, I would like the color of arrows to be customized. For example, arrows pointing in the NorthEast direction (0 to 90 degrees) should be red, arrows pointing in the NorthWest direction (91 to 180) should be green, and so on.

Best Answer

You can draw arrows on a plot in MATLAB using the QUIVER command and modify the COLOR property.
Below is an example:
clear; clc; close all
% Arrows in the following
% red = 0 to 90 degrees (counterclockwise)
% green = 91 to 180 degrees
% blue = 181 to 270 degrees
% cyan = 271 to 360 degrees
% Data is organize as (x, y, theta in degrees)
data = [1 1 120
       2 2 190
       3 3 220
       4 5 60
       6 2 150
       7 4 280
       8 1 310];
% Plot to figure 1
figure(1);
% Identify data from 1st quadrant and store in q1, identify data from 2nd quadrant
% goes to q2, and so on. A value of 1 indicates it is in the quadrant, 0 otherwise
q1 = (data(:,3) <= 90);
q2 = (data(:,3) > 90)  .* (data(:,3) <= 180);
q3 = (data(:,3) > 180) .* (data(:,3) <= 270);
q4 = (data(:,3) > 271) .* (data(:,3) <= 360);
hold on; 
% See DOC QUIVER
% Use QUIVER to specify the start point (tail of the arrow) and direction based on angle
% q1, q2, q3, and q4 are used to generate four different QUIVER handles (h1, h2, h3, and h4)
% This is necessary for varying colors based on direction
% Based on equations: x = x0 + r*cos(theta), y = y0 + r*sin(theta)
% In the usage below, x0 = data(:,1), y0 = data(:,2), theta = data(:,3) * pi / 180
% Can also specify a scale factor as the last argument to quiver (not specified below)
h1 = quiver(data(q1 == 1,1), data(q1 == 1,2), cos(data(q1 == 1,3) * pi/180), sin(data(q1 == 1,3) * pi/180));
h2 = quiver(data(q2 == 1,1), data(q2 == 1,2), cos(data(q2 == 1,3) * pi/180), sin(data(q2 == 1,3) * pi/180)); % sin is negative in 2nd quadrant
h3 = quiver(data(q3 == 1,1), data(q3 == 1,2), cos(data(q3 == 1,3) * pi/180), sin(data(q3 == 1,3) * pi/180));
h4 = quiver(data(q4 == 1,1), data(q4 == 1,2), cos(data(q4 == 1,3) * pi/180), sin(data(q4 == 1,3) * pi/180)); % cos is negative in 4th quadrant
% Set colors to red for 1st quadrant, blue for 2nd, green for 3rd, cyan for 4th
% Also, turn scaling off. get(h1) will return additional property-value pairs
set(h1, 'Color', [1 0 0], 'AutoScale', 'off')
set(h2, 'Color', [0 1 0], 'AutoScale', 'off')
set(h3, 'Color', [0 0 1], 'AutoScale', 'off')
set(h4, 'Color', [0 1 1], 'AutoScale', 'off')
% Done plotting
hold off;
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