I have a dataset of equally spaced, by 3 arcseconds, points. Each point represents the radio signal strength from a near by transmitter. I wanted to convert the point data set into a raster for better visual analysis. I used the IDW (spatial analyst) tool, expecting that a cell between points would interpolate a value somewhere between the two values, but not greater or less both.
In the picture below I'm using IDW on the point set that's visible. I set it to consider 4 data points and use variable radius. If you look at the picture, where the pixel inspector is, the cell that i'm pointing to is close to 4 other points for which I pasted the value in the picture. I expected that to get the value of that cell that the tool would consider the value of the 4 closest points and take the weighted average of the 4 which would be somewhere between -157.1 and -148.1, but the value the tool is giving is -106.03778. So where is that value coming from? I using the wrong tool for what I want?

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This is the identify all layers output from the same project. It has the IDW set up to evaluate based on 4 of the closest points, also I created an INT layer out of the idw layer. I used the identify tool and sampled a "square" of 4 data points then the top output is taken in the center of the "square.

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  • Can you run INT on the output raster for me and check the value again? Then just use identify and not the pixel inspector. I do not trust the pixel inspector. – If you do not know- just GIS Mar 11 at 23:15
  • OK. I ran INT, and I edited my original question with the output of the Identify from all layers. I took samples from 4 points as the corners of a square and then from the center of the square. I'm seeing the same issue. – Frank Mar 12 at 0:05
  • Also, I just realized that my input point feature has values from -54 to -287, but my IDW output only has values from -54 to about -112. Looking back at the "results" tab, I used the correct point feature as my input. But, I remember that previously I did a select on that layer for values > -112 and exported that as a layer, but with a different name, so I'm not using that by mistake. Would the IDW tool possibly be only using selected values if I didn't clear out the selected values explicitly? I restarted arcmap and ran IWD several times with same results though. – Frank Mar 12 at 1:26
  • post your point shapefile I think is the only solution. – If you do not know- just GIS Mar 12 at 1:42
  • change the search radius to fixed as well – If you do not know- just GIS Mar 12 at 1:50

What appears to be happening is that I had only values above -112 selected from an earlier operation a few weeks ago, so when I ran IDW only those were considered. I'd restarted my computer and Arcmap several times, so I didn't think that the layer would retain selection. I also rebooted several times. In the future I will make sure that selections are explicitly cleared. I should also have used fixed radius as well. That's a lesson learned

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