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i added the following points to ArcMAP using "Add XY Data".

x,y,z
7,39,117
27,17,122
27,61,123
46,36,136
52,14,138
55,46,141
56,56,137
65,24,144
65,36,145

Changed raster output size in "options" through "spatial analyst" as follows

Left 0
Bottom 0
Top 70
Right 70

And cell size as follows

Cell size: 10
Number of rows: 7
Number of columns: 7

Then got two rasters with IDW interpolation. First with the following options:

Z value field: z
Power: 2
Search radius type: Variable
 Number of points: 3
 Maximum distance: Empty
Output cell size: 10

Second raster with the following options:

Z value field: z
Power: 2
Search radius type: Fixed
 Distance: 1
 Minimum number of points: 3
Output cell size: 10

Compared two rasters outputted using above two options for IDW interpolation by converting the rasters to ASCII files. Results are different. Can anyone explain why? thanks in advance!!!

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1  
They use different neighborhood specifications! (One finds the 3 nearest points, the other finds all points within 1 unit of the interpolation location.) When the neighborhoods differ, the points used for interpolation differ, the z-values will differ, and the weights will differ, so of course the results are different. –  whuber Feb 9 '11 at 18:54
    
@whuber: "One finds the 3 nearest points"-yes. "the other finds all points within 1 unit of the interpolation location."-yes. but, in "1 unit (in map units)" there should be no points, so i have put min. number of points for this case 3. So that it finds exactly 3 points as well. Am i missing smth? thanks in advance!!! –  user1918 Feb 9 '11 at 19:57
1  
If ArcGIS thought this, then the output in the second raster would be completely null, because no radius-1 neighborhood would contain the minimum 3 points. I suspect there is a miscommunication concerning how distances are computed. Have you defined a coordinate system for your point layer? If not, all bets are off (and you will find a lot of ArcGIS bugs ;-). –  whuber Feb 9 '11 at 22:00
    
Perhaps mention which cells are different, with which values. Calculations can easily be repeated by hand. But I also think the second grid should consist mostly of NULL values (may be -99999 in ascii grid) –  johanvdw Feb 14 '11 at 23:26

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