Matlab repeating x-axis for interpolation - matlab

Question

I have to interpolate wind directions. The data is given for every 1000 [ft]. for example:
%winddata input in feet en degrees
x=0:1000:10000;
grad=[340 350 360 1 10 20 30 35 34 36 38];
The interpolation works great, i use the function interp1. (please see code below.) However, the step from 360 degrees to 1 degrees is a problem. I want Matlab to interpolate from 360 to 1 degree directly (clockwise), instead of anticlockwise with the values degreasing from 360-359-358-...3-2-1. That doesnt make sense when you interpolate wind directions.
How can i command Matlab to repeate the x-axis and values every 360 degrees?
clear all;clc;
h=2000;
%winddata input in feet en degrees!
x=0:1000:10000;
degrees=[340 350 360 1 10 20 30 35 34 36 38];
%conversion to SI:
x=0.3048*x;
u=0:1:max(x);
yinterp1 = interp1(x,degrees,u,'linear');
figure(3)
plot(degrees,x,'bo',yinterp1,u,'-r')
xlabel('wind direction [degrees]')
ylabel('height [m]')
title 'windspeed lineair interpolated with function interp'

Answer 1

The problem is that Matlab, as smart as it is, doesn't realise that you're working with degrees, so therefore it sees no reason that 360 = 0. Therefore I believe your problem isn't with finding a way to repeat the plot every 360 degrees, but rather with the data you are feeding in to your interp1 function, as currently you are telling it there is a straight line between the points (0, 950) and (360, 750).
The easiest, but ugliest, method would be to just add 360 to your lower values, so your degrees vector read:
degrees = [340 350 360 361 370 380 390 395 394 396 398];
and then subtract 360 from your degrees and yinterp1 vectors:
clear all;clc;
h=2000;
%winddata input in feet en degrees!
x=0:1000:10000;
degrees=[340 350 360 361 370 380 390 395 394 396 398];
%conversion to SI:
x=0.3048*x;
u=0:1:max(x);
yinterp1 = interp1(x,degrees,u,'linear');
figure(3)
plot(degrees-360,x,'bo',yinterp1-360,u,'-r')
xlabel('wind direction [degrees]')
ylabel('height [m]')
title 'windspeed lineair interpolated with function interp'
xlim([-180 180]);
The obvious problem with this is it isn't able to be applied for all cases, but if you just need a one off, then it works well.
For a more generic solution you could have it so you manually enter a point below which values have 360 added to them:
clear all;clc;
h=2000;
% --------------------- Manual cutoff for rectification -------------------
limitDegrees = 180;
% -------------------------------------------------------------------------
%winddata input in feet en degrees!
x=0:1000:10000;
degrees=[340 350 360 1 10 20 30 35 34 36 38];
%conversion to SI:
x=0.3048*x;
u=0:1:max(x);
indecesTooSmall = find(degrees <= limitDegrees);
oneVec = zeros(size(degrees));
oneVec(indecesTooSmall) = 1;
vecToAdd = 360*ones(size(degrees));
vecToAdd = vecToAdd .* oneVec;
newDegrees = degrees + vecToAdd;
yinterp1 = interp1(x,newDegrees,u,'linear');
figure(3)
plot(newDegrees-360,x,'bo',yinterp1-360,u,'-r')
xlabel('wind direction [degrees]')
ylabel('height [m]')
title 'windspeed lineair interpolated with function interp'
xlim([-180 180]);
Both of the above solutions give the following:
EDIT: Substantially easier solution, just use rad2deg(unwrap(deg2rad(degrees))), or try to find an unwrap which works for degrees.