Possibly a strange one:

I have a dataset with loads of fields of environmental (explanatory) data, as a regular grid of points (black dots in image), and another dataset of sampled (response) values as non-gridded points (red dots).

I'd like to append all of the environmental data fields to the sample dataset, but am not sure how. Most of the environmental fields are simple numeric data e.g. depth, temperature, salinity, etc., and subsequently I'm hoping it's possible to have QGis calculate the values at each red dot based on an interpolation from the nearest 'n' black dot sites. Firstly, if someone knows if that's possible, and how I can do it, please let me know.

Secondly, some of the fields aren't so easy: one is substrate, so the values are character strings e.g. "sand", "gravel" etc, and thus I'd like the red dot field value to take the value of the nearest black dot site. Four of the fields are compass directions, i.e. numerical from 0 to 360, which won't pose a problem in most cases, but will do for bearings near 360/0, as a two point average of 359 & 0 will equal 179.5 rather than 359.5. Any thoughts on that much appreciated also. Again, using nearest black dot site will very probably suffice.

Thanks in advance!!


  • 1
    Just an idea for say 'substrate' you could create concentric polygons around each black point - with the % of the given substrate being reduced the further and further away you go. Then each red point could pick up these % numbers and add them together to get you a composite substrate. I.e. if a point is equally between 'Sand' and 'Gravel' it would pick up a 50% sand and 50% gravel. – dklassen Feb 21 '14 at 21:54

You could have a look at some interpolation techniques like natural nearest neighbor, Inverse distance weightening and Kriging.

  • natural neighbour works a treat for the substrate with some help from this guide [gracilis.carleton.ca/CUOSGwiki/index.php/…, using input as survey ID key and target as enviro substrate field. Substrate is the only eligible field for this, however, as it's the only text string. I might try to reclassify the direction fields as text in order to use the same process on them... Thanks for the help! – dez93_2000 Feb 24 '14 at 21:10
  • field reclassification process made possible using csvt process in this guide [qgis.spatialthoughts.com/2012/03/…, and some wrangling with the file in notepad. – dez93_2000 Feb 24 '14 at 23:20

Realised the best way to go about this was to use the same process I'd used to append the enviro data sources to the depth grid, to append them to the sampled points, namely by doing voronoi polygons from the individual enviro points sources (not required if enviro data sources are polygons), then adding polygon attributes to points as per @Underdark's tip here

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