I am trying to Krige in ArcGIS from point samples of a chemical concentration. Towards the center, most of the points are larger, and on the periphery, all of the points are 0. The generated raster looks good in the center and towards the edges, but beyond the outside points of zero, the raster values trend back up. I set the major range of the semivariogram to be 50, and the trending upwards is beyond this range of influence for any of the points that are greater than 0. So it seems that Arc is setting an expectation somewhere around the average of all the points. Is there are way to set the expectation to 0 in ArcGIS. If not, what is a simple way to Krige in R or QGIS where you can set the expectation and the parameters of the semivariogram?
Here's a screenshot:
See how near the periphery, especially upper left and lower right corners, the value trends back up, when it should be zero.