Ok this is going to be pretty hard to say in words but I l have a go.

Basically

I have a set of data points in a xyz co-ord system and I am using interpolation techniques to predict an unknown z at a given xy

What I think is that the distribution of the xy data will affect the accuracy of the z calculated but i have no idea how to quantify that statisically.

To try and explain it better if say the unknown z at the known xy is the centre of say a square. In one square (a) we have 10 xyz points all located in the top left corner, say then in square (b) the 10 points are evenly distributed around the square. I think the z from square b will be more accurate then the z from a but is there a way of quantifying the distribution.

Probably best to note i hope to use excel to do this