R help - Apr 2007 - Get the: "standard deviation" and "mean value" of 3D-measurments

Hello,
I have got a numeric matrix with three colums of a few measurments. My x and
y coordinates are rising numbers. The z coordinate is the measurment. In
my matrix if have got ~6000 sorted values for one measurments multiplied 
with the number of measurments. An example in the end.

My aim is to get the "mean values" and "standard deviation"
for each value
between the measuements (of the z coordinate). But the problem is. I don't
have
always the same count of x and y values. See the example (29 4.8 xxx) is 
missing in measure two.

Has anybody an idea what I can do? I though I could split the measurments
an copy it horizontal in the matrix. But then I can't work with mean because
I don't have the same numbers in every measurment (~6000).

Thank's a lot.
Your Felix

PS: Thank you for the help in the past!



(x&Y polar coordinates) of two measurments
------------------------------------------
x	y	z
29	4.5	1.505713
29	4.6	1.580402
29	4.7	1.656875
29	4.8	1.735054
30	0	0
30	0.1	0.00096108
30	0.2	0.00323831
...
29	4.5	1.495148
29	4.6	1.568961
29	4.7	1.644467
30	0	0
30	0.1	0.00093699
30	0.2	0.00319411
30	0.3	0.00676619