In ArcGIS 10.1, I have two layers: 1) raster of Topographic Wetness Index 2) vector layer of polygons representing soil types

I am wondering how to accomplish what I am after, that is to get information for polygons from the underling raster. In other words, I would like to extract the mean TWI for each polygon in order to calculate what is (on average) the TWI for each soil type.

1 Answer 1


With the Spatial Analyst Extension, using Zonal Statistics as Table should give you what you need.

  • thanks for the prompt reply. May be my question was not clear. It is my understanding that extract by polygon will 'cut' the raster according to the polygons. What I am after is extract the mean value per each polygon from the underlying raster. Something similar to what 'add surface information' does in relation, e.g., to the polygons mean z (i.e., elevation). Instead of z, I would like to have the mean value of the TWI raster. I hope my poor English will not lead you astray.
    – NewAtGis
    Jun 20, 2014 at 13:51
  • @Maksim you are right with zonal stat, but not with extract by polygon. You should edit your answer with your comment to make things more readable.
    – radouxju
    Jun 20, 2014 at 14:04
  • @radouxju edited
    – Maksim
    Jun 20, 2014 at 15:00

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