Griddata 'v4' interpolation

I'm using an equation node so that I can use the griddata('v4') method for interpolating an (x,y,z) Data set. For the most part, it seems like the interpolation is generating the right points. But in some cases I'm running into some unexplained results.
I'll plot a series of points (x,y,z), with the max z value equal to -60. Generally, when I plot (x,y) points with a z value of -60 (depending on the values of the neighboring points) I would expect to get a value of a little above -60 or a little below. Most of the time these results are produced. Other times, in the same data set, I have points that have z values of -110, -115, and -90, that end up creating a new, interpolated z value of -70. This jump in the z value makes me question whether or not I'm using the proper interpolation technique.
The data that I'm graphing should look like a hill, with a single peak, as the altitude decreases with distance. Smaller hills may appear as the distance from the center peak grows, but there should never be a peak near the value of the maximum peak.
I've attached a vi that shows 3 views of the data:
1. Plot - location of points with z values attached
2. 2d Mesh - 2d view of data after mesh/interpolation
3. 3d Graph - 3d view of data after mesh/interpolation
Some of the reasons for this sort of error may be due to the clustered nature of some of the lower z values. My only reasoning for these erroneous peaks is that these lower values are some how stacking together, which end up creating poor results. Does anyone have any ideas about how to get around this?
Attachments:
meshing, peaks.vi ‏4909 KB

Yeah I've looked online at matlab interpolation function here:
http://www.mathworks.com/access/helpdesk/help/techdoc/index.html?/access/helpdesk/help/techdoc/ref/griddata.html&http://www.google.com/search?hl=en&q=matlab+griddata&btnG=Search
This looked okay to me. I think I've determined that the number of samples and the spacing between the samples really does matter in the final output. I am plotting then interpolating between nearly 200 points. There are a few other posts online that I'm going to continue to look at to see if there are any differences between the interpolations styles.
I'm still having a hard time understanding the two resultant peaks that are generated after I interpolate over the entire data set (both of which are nearly equal in value). To try and isolate the problem, I interpolated over part of the data set, the portion which I believe to generate the 2nd erroneous peak. But when I did this, I saw a peak intensity which is 20 values lower than what was generated with all the data (this is what I expect). So, I really don't see anything which might indicate there is anything wrong with the interpolation. I'll post again, if I figure out what's going on. Thanks for the response.

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Thanks for the VI. I do understand that the X and Y data need to be monotonically increasing and formatted as a mesh as you showed me in your VI. However, the problem I run into is when generating the Z matrix. I have scattered points that have been measured in different X and Y locations, and do not represent a regular 2D grid. Sometimes the data is measured in a few points around a circle, for instance, but I want to interpolate over a rectangle containing the circle.
So, my Z data looks like this (see attached image):
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Attachments:
zdata.JPG ‏28 KB

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mapandscale.vi ‏89 KB

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