Finding X Y coordinates from surface plot

Hi All, n00b to MatLab and I have a question that is giving me fits!

peaks is a built-in example function of two variables in MatLab. You can see what peaks looks like with surf(peaks).

Using MatLab, what is the maximum value of peaks (i.e., the maximum z value)?

This one I understand. I use Z = peaks; then max (max(Z)) to get the answer. No problem!

Using MatLab, what is the x, y coordinate of the maximum value?

This is my frustrating issue!

My prof assigned this but hasn't returned a response to my email and I'd like to get this done ASAP. Any advice would be appreciated!

Answer 1

[Paulo Silva]

[X,Y,Z] = peaks;
I=find(Z==max(max(Z)));
X(I); %x values corresponding to the points where Z is maximum
Y(I); %y values corresponding to the points where Z is maximum

To test if values are correct do

[X,Y,Z] = peaks;
surf(X,Y,Z)
hold on
I=find(Z==max(max(Z)));
plot3(X(I),Y(I),Z(I),'x')

[Darin]

Thank you SOOOOOOOOOOOOOO much! My prof is...well she's an idiot.

[Paulo Silva]

She's just being a good teacher, by searching and trying to do the homework you always learn something.

[Ilias Iord]

This is very helpful indeed, but I am looking for one step further:

I have two vectors of co-ordinates (1-n) for X and Y and a surface (S) with n-n corresponding values.

The question is, how can I get the X, Y values for the single minimum value of S. Because S is n-n, the above solution does not work.

%calculation of drawdown in confined aquifer

clear
clc
% Parameter and variable values
xabs=3000;
yabs=xabs;
xmin=-xabs;
ymin=-yabs;
x=xmin:50:xabs;
y=ymin:50:yabs;

x1=1500;
y1=600;

x2=700;
y2=1800;

x3=1200;
y3=900;

x4=500;
y4=800;

rw=0.3; %well radius
rinf1=2000; %m   radius of influence
rinf2=3000;
rinf3=5000;
rinf4=500;

k=2.58; %m/day

q1=1200; %m3/day
q2=1500;
q3=2000;
q4=500;
a=45; % m (confined aquifer width)
hw=30; %m ( well drawdown depth)

s1=drawdown(xabs,x1,y1,rinf1,k,q1,a);
s2=drawdown(xabs,x2,y2,rinf2,k,q2,a);
s3=drawdown(xabs,x3,y3,rinf3,k,q3,a);
s4=drawdown(xabs,x4,y4,rinf4,k,q4,a);

s=s1+s2+s3+s4;

"Drawdown" is the function that calculates S for the well data I need the coordinates for the minimum peak.

Thanks a lot guys!