I have a shp file with the location (lat, lon) of lightning spatial distribution. I want to produce a density map of lightning occurrence per square KM. I used density point function and I don't know how to transform the output in square KM. My map's unit is kilometer I should use 1 Km radius? I'm using ArcGis 9,3 I appreciate any answer.

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    Can you provide some more details about your data, your requirements and what you have tried so far? The more detail you provide, the better an answer you can expect. – sgrieve Nov 28 '12 at 10:33
  • I want to produce a density map of the events/square Km. I have tried so far the density point function. On data frame properties I checked Units=Kilometers and Display=Kilometers, Coordinates= geographical->World->WGS 1984. In point density in the field "Area Units" the option is Square map Units (I can not change it). I proceed with density point and the output is a raster classified by 10 classes. How I will interpret the raster. Are these pixels or events/square KM. Thank you – John Matsa Nov 28 '12 at 11:45
  • You will not get a correct density map when using geographic coordinates, due to the large metric distortion they introduce. Instead, use a projection suitable for your area of interest that is as accurate as possible and either equal-area or close to it. When working at a global scale, divide the globe into overlapping portions, project them and compute their densities separately, unproject the densities, and mosaic those into a global map. – whuber Nov 28 '12 at 16:53

Choose square kilometres for the area units parameter in the density tool. Then your result raster is lightnig per square kilometer. You do not have to transform anything.

The search radius is independent of the area units parameter. Larger values of the radius parameter produce a smoother, more generalized density raster. Smaller values produce a raster that shows more detail.

  • Is there any procedure to follow in order to select the correct value of radius? – John Matsa Nov 28 '12 at 11:51
  • The choice of the radius depends on the distribution of sample points. I usually start with the default value for the radius which ArcGIS suggests. Then I look at the result (and sometimes I change the colorramp). Are there too many details visible in the result? Then I try a larger radius. Do I see no local variation? Then I try a smaller radius. – Jens Nov 28 '12 at 13:40
  • Jes, thank you so much for your help. In my case in Point Density function I can not choose the square kilometres for the area units parameter in the density tool. By default is Square Map units. Any idea? – John Matsa Nov 28 '12 at 15:44
  • @JohnMatsa - This ("Square Map units") is related to your coordinate system. Consider the response of whuber. – Jens Nov 29 '12 at 7:12

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