How do I calculate the area between a curve and a horizontal line?

I want to calculate the area of a region below a specific y-value for a curve. This is the code I'm working with now, but it is only highlighting the area I want and not returning a specific value.

%% variables

p1d = Profile18(:,1);

p1e = Profile18(:,2);

hwMark = 4181.0164;

%% graph and x-sectional area highlighted ~2.5m above lowest channel elev.

plot(p1d,p1e);

title('Profile #1');

ylabel('Elevation (m)');

xlabel('Distance (m)');

hold on

area(p1d, min(p1e,hwMark), hwMark);

Answer 1

[Roger J]

Capture the return value from "area()", and then find the x values for the two points where p1e crosses hwMark.

Now you can calculate the area of the shaded region, if you realize that it is equal to the area of the rectangle, minus the area under the curve p1e.

Something like the following:

a = area(p1d, min(p1e,hwMark), hwMark);
range=find(a.YData ~= hwMark)
areaUnderP1e = trapz(p1e(range(1):range(end)))
areaOfRectangle = hwMark*(range(end)-range(1))
area = areaOfRectangle-areaUnderP1e