Finding a specific value in Y-axis value with known X-axis value

Hi, I am trying to find a specific value in Y-axis value with known X-axis value as follow;

Alt = [11000,12000,13000,14000,15000,16000,17000,18000]; % Altitude [m]

Relative_density_ISA = [0.2971,0.2537,0.2167,0.1851,0.1581,0.1350,0.1153,0.0985]; % Relative density under ISA

I am trying to find the altitude which relative density 0.1291 is at.

I applied the -> R_D_wanted =interp1(Alt,Relative_density_ISA,0.1291);

but it gives me NaN result.

Please help, very much appreciated.

Answer 1

[Matt J]

Alt = [11000,12000,13000,14000,15000,16000,17000,18000]; % Altitude [m]
Relative_density_ISA = [0.2971,0.2537,0.2167,0.1851,0.1581,0.1350,0.1153,0.0985]; % Relative density under ISA

Alt_wanted =interp1(Relative_density_ISA,Alt,0.1291,'linear')

Alt_wanted = 1.6299e+04

[Ke Yeun Yong]

Thank you very much 😃

[Matt J]

You're welcome, but please Accept-click the answer to indicate that it worked.

Answer 2

[Dark_Knight]

Hello Ke Yeun Yong , hope this helps.

% Given Input Data
Alt = [11000, 12000, 13000, 14000, 15000, 16000, 17000, 18000];

Relative_density_ISA = [0.2971, 0.2537, 0.2167, 0.1851, 0.1581, 0.1350, 0.1153, 0.0985];

% Relative Density required
R_D_wanted = interp1(Relative_density_ISA, Alt, 0.1291); % 1.6299e+04

% Relative Density required
R_D_wanted = num2str(interp1(Relative_density_ISA, Alt, 0.1291)); % '16299.4924'

[Ke Yeun Yong]

Thank you very much 😃