Trying to interpolate beyond sample points to polygon extent

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10-03-2013 05:39 PM
MattGray
New Contributor
I have a polygon shapefile of an oil basin, and a point file of sample points that fall within the polygon.  I am using Natural Neighbor to try to interpolate a z-value from the points, but every time i run it, it only produces a raster to the edges of the points. I am trying to get it to interpolate to fill up the entire polygon of the basin.  I have heard you can interpolate beyond the extent of sample points, but I cannot get it to work.  I have tried everything including setting the raster processing extent to that of the polygon extent, and setting the polygon as a mask in the environment settings of the Nearest Neighbor tool. This video describes what Ive tried starting around the 6:25 mark. This is for a big map project I am struggling with at work. Any help greatly appreciated.
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6 Replies
DanPatterson_Retired
MVP Esteemed Contributor
extrapolation beyond the data might make for a good map but potentially erroneous data, I would advise against it.  Interpolation within a point cloud is often a stretch in itself...beyond it could be hoping for a miracle
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MattGray
New Contributor
This is understood, the boundary I am trying to extrapolate to is not far beyond the point extent.  I realize that data beyond the points will be filled will error.  I have tried setting the processing extent to all sorts of things and cannot get a different result.
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SteveLynch
Esri Regular Contributor
Have you read the first Usage tip in Natural Neighbor's help?
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MattGray
New Contributor
Are you talking about :

"If the cell center of the perimeter cells of the output raster fall outside the convex hull (defined by the input points), then those cells will be assigned NoData values. If an input point falls within one of these perimeter cells and the cell center falls outside the convex hull, the cell will still be assigned a value of NoData." ???

How would I use this to enlarge the processing extent of my points?
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SteveLynch
Esri Regular Contributor
Natural Neighbor can't extrapolate outside the convex hull. You'll have to use another interpolation tool.
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MattGray
New Contributor
Ah, Makes sense now.  It has been the interpolation method that has so far made contours  that best matched the data.  I will have to fiddle with IDW more then.
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