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How is Kernel Density converted to Count

ShawnGazzano

I'd like to convert the values in the range of a kernel density layer to represent case counts instead of density.

This blog post, http://blogs.esri.com/esri/arcgis/2013/05/28/how-should-i-interpret-the-output-of-density-tools/, makes reference to converting density to count.

Is there a tool in ArcMap that can do this? If not, is there a way to determine the number of points in a kernel density range?

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DanPatterson_Retired

You use the identify tool...don't forget for floating point rasters, the standard symbology will not be unique values and it will be unlikely that you can convert it to such given the possible range of values. The data are there...they are just classed into ranges, so if you want to see individual values, use the identify/info tool or use the Extract Values to points tool. Good luck

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DanPatterson_Retired

to quote the blog

Even when it makes sense to interpret the cell values, it is often much more meaningful to report the expected count within a cell than it is to report the cell density value. Density is count divided by area, so multiplying the density by area will give an expected count. Using the example of crimes in a city over one month, suppose we use an output cell size of 0.5 kilometers, and we see that one cell receives a value of 16 crimes per square kilometer. Since the area of the cell is 0.25 square kilometers (0.5 km * 0.5 km = 0.25 km²), we can multiply the density by the area to get an expected count of 4 crimes (16 crimes/km² * 0.25 km² = 4 crimes). This means that if crime conditions do not change month-to-month, we expect to see about 4 crimes in that cell the following month. If another cell has an expected count of 0.5 crimes, the interpretation would be that if the conditions for crime do not change, we expect about one crime in that cell every two months.

It is a simply a process a multiplication process which can be accomplished through the raster calculator given your cell size

ShawnGazzano

Thank, Dan. What I'm unclear about is "....we see that one cell receives a value of 16 crimes per square kilometer."

How is the value of the cell deciphered? Do I look at the list of ranges of the kernel density layer? If the first range has a value of 0 - 250, does that mean cells in that range have a value of 250 points?

DanPatterson_Retired

There will be a value for the cell. Don't confuse the symbology range with the values for a given cell. Also kernel density is observations per unit area. To get the count, follow the outline in the blog example

ShawnGazzano

Yes, there will be a value for the cell; but, where? Is it in the points layer or the kernel denisty layer? Is it in Properties, or do I use the Identify tool?

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