Finding the area between a graph and a line

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x13dr.ck
x13dr.ck 2015-12-2
编辑： Walter Roberson 2015-12-4
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

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采纳的回答

dpb
dpb 2015-12-2
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...
  1 个评论
dpb
dpb 2015-12-2
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.

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更多回答（1 个）

Kirby Fears
Kirby Fears 2015-12-2
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.

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