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I'm rather new to using GIS (QGIS, specifically). I've mapped out a cohort of around 550 patients across Long Island, New York based on their full address by using MMQGIS plugin. This data is represented as "Single Symbol" "simple marker"s across the map. In the same layer, their 'PCL Score' (integer ranging from 17-85) is also assigned to each patient.

How can I either edit the current layer or add a new layer so that I have one continuous gradient of color showing how high or low the PCL trend is based on location? Basically, this is much like an elevation map where higher elevations are darker color and lower elevations are lighter color.

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    What you're trying to do is called interpolation and the result is sometimes called a heatmap. gis.stackexchange.com/questions/256 is a good place to start, though we have many questions on the topic. There are different methods of interpolation and some are more suited to particular kinds of data than others. The two quick ways in QGIS are IDW and a Heatmap plugin. Note that with the current version of QGIS (2.8.1) there's a bug that produces results with coordinate system errors. – Chris W Apr 6 '15 at 21:25
  • This has been very useful so far. I'm trying to use the interpolation method. However, whenever I try to use the plugin, the Interpolation attribute dropdown is blank. – Chris Ray Apr 7 '15 at 14:38
  • It might be because of the space in the field name, if "PCL Score" is actually what it currently is; it may not be able to see that field. You said it was integer data type, so it should work. Field names have some restrictions/rules. It's best if they start with a letter, contain no special characters other than underscore, have no spaces, and in the case of shapefiles be less than 13 characters long. – Chris W Apr 7 '15 at 20:09
  • Does it matter if the variable is joined from another layer (CSV file)? – Chris Ray Apr 7 '15 at 20:44
  • I can't say definitively, however that is often an issue and it's usually best to export a join result or add a new field and field calculate the desired join field's values into it prior to running any geoprocessing tools or analysis based on that attribute. – Chris W Apr 7 '15 at 20:47

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