Meaning of uncertainty value returned by Soilgrid API

Geographic Information Systems Asked by Tomas on August 30, 2020

I am trying to wrap my head around the uncertainty value and quantiles returned by the Soilgrid 2.0 API. I went through this document, but cannot find a clear answer.

As an example, I issue the following request to find pH of a certain (lat,lon) value:

and get as results:

  'values': {'Q0.05': 52,
   'Q0.5': 77,
   'Q0.95': 84,
   'mean': 73,
   'uncertainty': 4}

(deleted the rest of the response since I don’t see anything else relevant).
According to the FAQ, the interval [52, 84] contains the true pH value of the soil with 90% probability.
I’m interested in cutoffs at 5.5, 6.5 and 7.5 of the pH values, so this interval is not very informative. I sampled over 2000 points and see that this is the typical case, not an outlier, i.e. these intervals are always very wide.

Am I interpreting this correctly?

On the other hand I see the uncertainty = 4 value included in the response, and had hoped this would somehow offer an alternative, but I can’t find what the exact definition of it is.

How is it calculated?

How can I use it?

One Answer

The linked document does not refer directly to SoilGrids v2.0 product. For information about the units please refers to the information provided in the FAQs

The values for Q0.05 and Q0.95 (respectively 52 and 84 in your example) refers to the predictions percentiles (5% and 95% respectively). This is explained in the FAQs see here and here for more information.

Uncertainty is an additional layer produced mostly for display purposes in the portal. More details are in the FAQ

If you need cutoff values you can use the values for mean or median (depending on the needs for your application) for an if/else condition. Finally, please be aware that the REST API for SoilGrids v2.0 is still under active development.

Answered by Jorge Mendes on August 30, 2020

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