Problem: Values of -9999 not recognized as NoData by the Density function
In the Density function of Spatial Analyst, values of –9999 are not being recognized as NoData.
The Density function does not recognize the value –9999 as NoD. Only the raster interpolators recognize such value as NoData.
Solution or Workaround
Reclassify your data and make values of –9999 into NoData.