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I have a KML file created in Google Earth that has points where we sampled 11 monitoring wells (my work uses Google Earth Pro and I'm trying to introduce QGIS). I have a CSV that has the contamination levels for constituents (arsenic, DDD, benzo, etc). Currently I can import both files to QGIS and join them on the monitoring well names no problem but...

Objective

  1. I want to use either kriging and IDW interpolation to create contour lines to see where the contour levels reach a certain level.

Problems

1 After I've joined the KML, exported to a shapefile (or just kept it as a KML, tried both) when I go to do an IDW interpolation, none of the contamination values are showing up in the drop down menu.

  • My running theory is that the joined data is being interpreted as a string an not a double (I noticed after looking at the attribute table). Which leads me to problem 2.
  1. How do I make sure the joined values from the CSV are being interpreted as doubles?

  • I've made sure to format cells to make them "numbers" or "General" in Excel

  • I've done a work around by just creating a new column directly on to the KML file, specified it as a double and then tried to do the IDW interpolation. Which brings me to problem 3.

  1. Using the workaround method the IDW allows me to pick the parameter to interpolate but the exported raster doesn't show up and when I look at the upper and lower color value bounds, they are the same.

Side Note: I also tried to use the SmartMap plugin for kriging interpolation and it exported a table that looked correct but no heat map.

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  • Upper and lower bounds are the same: I see this happen with IDW, when I set the Distance coefficient P too small.
    – peter
    Commented May 1 at 6:35
  • If you still don't get what you hoped for: try Grid(IDW with nearest neighbor) and Heatmap from the toolbox.
    – peter
    Commented May 1 at 6:36
  • I set my coefficient P as 1, 2, and 3. With similar results. The core issue is problem one. We have projects with a lot of data and I can't always use the work around method.
    – BiosphereEngineering
    Commented May 2 at 12:47
  • I tried this with a few points that were spaced about 50m apart, and set P to 50 too. That worked for me.
    – peter
    Commented May 2 at 13:34
  • I'll give it a go. Any idea on how why when I join the CSV the contamination values are being set as strings and not doubles?
    – BiosphereEngineering
    Commented May 2 at 13:37

1 Answer 1

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When adding the Delimited Text layer to your project (Layer | Add layer | Add delimited text layer) you will see the following dialog:

Delimited text import dialog

Under the Sample data heading, you see the data types that QGIS has inferred. Change the column headers there to the right data type and proceed with the import.

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  • That worked! Thank you, I guess there is some difference on how it interprets data types when the file is drag and dropped versus imported like that.
    – BiosphereEngineering
    Commented May 2 at 16:41

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