Finding the area between a graph and a line

basically i need to find the distance (area) of the graph above the 1000mph mark on the graph (y axis).

i have tried using the trapz function however i can only seem to get the total area of the graph.

how would i go about this?

the x axis should read as hours, not seconds, so just ignore that. a quick response would be greatly appreciated.

so far the only code i have is:

for c=1:9
        int=(trapz(Time_h,Velocity_milesph(:,c)));
        disp(int)
end

what would i subtract from the trapz function to get just the area above 1000mph?

thanks

[x13dr.ck]

Answer 1

[dpb]

for c=1:9
  ix=Velocity_milesph(:,c)>=1000;
  int=(trapz(Time_h(ix),Velocity_milesph(:,c(ix))));
  disp(int)
end

You'll still need to normalize by dt of course...

[dpb]

You may want to check on the granularity of the result of the check and improve the endpoints by interpolation to find the crossover if there's a fair disparity between the end values returned and the setpoint value.

Answer 2

[Kirby Fears]

The area below your curve and above Y=1000 is obtained by integrating (Velocity_milesph - 1000) over X values were Velocity_milesph is over 1000. The below code adjustments should be what you're looking for.

for c=1:9
    interval =  (Velocity_milesph(:,c)-1000) > 0;
    fastArea = trapz(Time_h(interval),Velocity_milesph(interval,c)-1000);
    disp(fastArea)
end

I'm not sure if trapz() is the right method for you since it uses unit spacing. Be sure that 1 unit of Time_h is considered very small relative to the amount of time that velocity is over 1000. Otherwise you will have large errors.