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I am fairly new to GIS.

I am currently evaluating wildlife sightings across transect lines and want to calculate the occurrence of encounters, per area surveyed. So essentially look at encounter rates across square mile grid cells to create a thematic map.

I am using Arcgis 10.4 for desktop and so far I have created a fishnet grid within a polygon of the surveying area and I have joined this with my sightings point data. From this I can see counts of sightings in each cell, however, what I want to depict are sightings per square kilometer. I am totally stuck from this point on. I have tried a few things but nothing seems to be quite right!

  • Can't you create your initial grid in kilometres? – Hornbydd Jul 9 '16 at 12:14
  • I have already set the grid cells as I want them to be in kilometers, it is more how do I calculate encounter rate/distance covered x by square kilometer and produce an output to show these encounter rates? Rather than what it shows at the moment which is counts of points in each square kilometer cell. I hope that makes sense! As I said I am very much a novice! – Becca Crow Jul 9 '16 at 12:20
  • Can you refine what you are asking? If you want to transform counts to density per square KM; you are required to divide each cell's count by the area. In your case the area is constant and equals to square KM, thus it won't affect the visual result. Anyway you can use field calculator to divide counts / area. – dof1985 Jul 9 '16 at 12:41
  • My apologies! It doesn't help that I am so new to this so don't really know the correct terminology. You are right, I want to transform counts to density per square KM. The entire survey area is overlayed with a grid made up of of square KM cells, so I want to take my count values, divide them by the total area surveyed to produce a density value. I have tried to use the field calculator but it doesn't seem to work? I have also tried to turn my join layer (grid+sightings points) to raster and then use the raster calculator but this also doesn't work?? – Becca Crow Jul 9 '16 at 12:51
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    @BeccaCrow a key to a good answer is to give details on your efforts to solve the problem; it allows you to focus your question on the problem you are unable to solve. Can you provide details on why field calculator failed? You should use the edit option below your question to give full details in the question's body. – dof1985 Jul 9 '16 at 13:09
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I have since figured this out.

Firstly I created a new field in the attribute table of my grid/sightings join layer and using the field calculator, calculated cell size (1 square KM). By multiplying this with the amount of cells there were in my grid I produced a whole area coverage value.

I then converted my counts to density values. I created another field in the same table and using field calculator divided my counts by the whole area value and then multiplied this by 1 square KM to produce density per square km.

Then in symbology properties for the same layer, changed my value field to density so it now displays these values on my map instead of counts!

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