IDW: Coincident points and Maximum Distance

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04-11-2013 07:02 AM
XimenaOsorio
New Contributor
The ArcGIS Help file says the Default Maximum Distance, when using the Variable Search Radius in IDW, is the extent's diagonal, however, no value is automatically filled in the corresponding box after I define the extent, so is ArcMap really using this default value if the box is empty?

Regarding coincident points (different measurements at the same location) when using IDW, it is possible to set the Environment "Geostatistical Analysis" to Min, Max, Mean, etc., but is this selection really honored when running IDW using Spatial Analyst, or does it only apply to tools from the Geostatistical Analyst extension? I am confused because the ArcGIS help says: "Tools that honor the Coincident Points environment define how coincident data is treated in Geostatistical Analyst". If the Coicident Point selection is not honored when using Spatial Analyst, how do I know what IDW is doing about them? Thanks a lot for your help!
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3 Replies
SteveLynch
Esri Regular Contributor
If you think that the Default max distance is not honored then you could do a simple test.

Spatial Analyst does not honor the Geostatistical Analyst's Coincident points option as you and the documentation state.

Accordinding to the Spatial Analyst IDW help
Some input datasets may have several points with the same x,y coordinates. If the values of the points at the common location are the same, they are considered duplicates and have no affect on the output. If the values are different, they are considered coincident points.

The various interpolation tools may handle this data condition differently. For example, in some cases the first coincident point encountered is used for the calculation; in other cases the last point encountered is used. This may cause some locations in the output raster to have different values than what you might expect. The solution is to prepare your data by removing these coincident points. The Collect Events tool in the Spatial Statistics toolbox is useful for identifying any coincident points in your data.


-Steve
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XimenaOsorio
New Contributor
Thanks Steve. However, for a research I am conducting, I need to understand and provide a citation/reference of how Spatial Analyst�??s IDW handles coincident points. So far, I haven�??t found a description in any ESRI document.
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SteveLynch
Esri Regular Contributor
Spatial Analyst's IDW ignores all the coincident data. For example, if you have 5 points with the exact same location, then SA's IDW will ignore points (2,3, 4 and 5) or it'll ignore points (1, 2, 3 and 4). This is stated in the help, see the 4th usage tip in http://resources.arcgis.com/en/help/main/10.1/index.html#//009z0000006m000000

Regards
Steve
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