Performing an IDW on specific area of map

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2 weeks ago
NeilOgle
New Contributor

While not a complete novice with GIS, it has been many years so please bare in mind if replying. 

What Ive done is import a table with the headings: Sample Number, Eastings, Northings and 2 columns of attributed data for each point data that Id like to plot (d13c and d15N).  The sampling points can plot fine using the IRENET95 coordinate system.  See screenshot below.

NeilOgle_0-1623071498491.png

Its at this point that problems begin.  Id like to perform an Inverse Data Weighted Analysis on the data with a power factor of 2.  However, I only want to perform this analysis a) within the land mass and only certain parts of it and  b) excluding unsampled Northern Ireland

Now Im aware a mask could be applied to the map to only perform IDW on a specific area but its at this point where my expertise runs out.  Without some form of mask the resulting plot is not what is required:

NeilOgle_1-1623071498496.jpeg

I half-expected to see such because as I say there are no defined boundaries.  I imported a map from Esri Ireland Country Boundary 2019 - Overview (arcgis.com) but when I tried to plot onto this I saw no points at all.  So its at this point Im going to need some help. Could someone start me off

Thanks

Neil

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2 Replies
JayantaPoddar
MVP Esteemed Contributor

By default, the tool takes the rectangular extent of the input layer (Xmin,Ymin,Xmax,Ymax).

Here is what you could do (add the steps in a model builder, if this is a repetitive process).

1. Create a bounding polygon from the points layer using either Aggregate Points tool or Minimum Bounding Box, whichever suits your processing extent.

For Aggregate Points, keep changing the aggregation distance until you receive a desired polygon extent.

2. While executing the IDW tool, define the Mask parameter (above polygon) in Tool Environments.

JayantaPoddar_0-1623077936524.png

 

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SteveLynch
Esri Regular Contributor

Natural Neighbor interpolation will not extrapolate outside the convex hull defined by the input points.