How should I aggregate qualitative data from horizons to mapunits?

As you know, each component represents a column that has up to six layers, called horizons.

Components are each separate individual soils with individual properties and are grouped together for simplicity's sake when characterizing the map unit.

We often want to map data gathered and stored at the horizon level, but given that we're making a plan-view map, we need to aggregate the data from the multiple horizons to a single component value and without knowing exactly where the component boundaries are, we must aggregate the data from components to mapunits. When the data we need to aggregate are qualitative such as shrink-swell characteristics or texture of the soil horizons, we can't average the data or sum it up.

Aggregating horizon data to mapunits requires two steps. First is aggregating data from the horizons to the components and then aggregating from the components to the entire mapunit. There are a couple of options for aggregating qualitative horizon data:

  • Selecting the top horizon and using that for the entire component
  • This is an easy solution and also makes sense when talking about the wind erosion class, for instance, where the value for the top horizon is obviously the one that you care about. Simply take the data value for the top horizon and assign that value to the entire map unit.

  • Selecting the horizon that occurs at a set depth and using that for the entire component
  • This is obviously similar to the first option above. Sometimes however, the soil characteristics at a certain depth are more important than the characteristics of the surface. Simply take the data value for the horizon that is at the assigned depth and assign that value to the entire map unit.

  • Finding the horizon with a limiting property and using it for the component value
  • Like the other methods discussed, we are still going to assign the data value from one horizon to the component. However, now we are not going to necessarily use the value from a certain depth. Instead we will use the value that most limits us based on the purpose of the map and assign that value to the component. We can also use the limiting property method for quantitative data.

Once you have aggregated from horizon to component, you must aggregate again from the components to the entire mapunit (remember, we don't know the exact location of components). There are several ways to do this, including selecting the value for the largest component or selecting a limiting value.

Always remember that for different variables and different mapping purposes, different techniques are appropriate. Care is needed to make sure that the best method is being used.