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James Mathew

Linear regression on a semi-log scale

Asked by James Mathew
on 11 Jun 2013

Hi,

I'm trying to plot a linear regression line on a semi-log scale.

Y-axis Linear Received power in dB

X-axis Log distance in m

The program and data I'm using as as follows:

Inc=1;

D=10; %Max meaurement distance

d1=(1:Inc:D); %1m to max measurment distance (D) in Increments (Inc)

RXPdata1=[-45.0983; -53.3746; -54.4132; -56.8286; -59.2905; -60.2743; -60.6919; -59.4938; -64.0525; -62.6163]; %Measured data in dB

semilogx(d1,(RXPdata1(:,1)),'-or') %Plot measured data

hold on

%Graph Set up

title ('Plot showing measured data')

xlabel('Distance (m) [Log scale]')

ylabel('Recieved Power (dB)')

legend ('Measured Data','Location','EastOutside')

axis([0 10 -70 -35]);

hold on

grid on

Can anyone help?

James

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1 Answer

Answer by Miroslav Balda on 11 Jun 2013
 Accepted answer

The following code solves your problem:

% James.m
%  JAMES   Linear regression on a semilog scale
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%       2013-06-11
%
% Miroslav Balda
% miroslav AT balda DOT cz
%
Inc=1;
D=10;           %   Max meaurement distance
%
d1=(1:Inc:D);   %   1m to max measurment distance (D) in Increments (Inc)
RXPdata1=[-45.0983; -53.3746; -54.4132; -56.8286; -59.2905; -60.2743;... 
   -60.6919; -59.4938; -64.0525; -62.6163]; %    Measured data in dB
%
semilogx(d1,(RXPdata1(:,1)),'-or')          %    Plot measured data
hold on
%
%                %   Graph Set up
%
title ('Plot showing measured data','FontSize',14, 'FontWeight','bold')
xlabel('Distance (m) [Log scale]','FontSize',10, 'FontWeight','bold')
ylabel('Recieved Power (dB)','FontSize',10, 'FontWeight','bold')
%
axis([0 10 -70 -35]);
hold on
grid on
%
              %   Regression
%                
logd1 = log10(d1');
A     = [ones(size(RXPdata1)),logd1];
c = A\RXPdata1
y = A*c;
plot(d1,y,'o-', d1,y,'*-')
%
legend ('Measured Data','Lin. Regression','Location','EastOutside')

It is all. Good luck!

Mira

  2 Comments

James Mathew
on 11 Jun 2013

Mira,

Many many thanks, this has saved me hours of work!!!

James

James Mathew
on 18 Jun 2013

Mira,

Me again, any chance you could help me out again. If trying to find the following variables for the line:

- where the line intersects 0 (or it is 1?) - the gradient - the regression coefficient

Thanks in advance.

James


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