How should I aggregate qualitative data from components to mapunits?

As you know, each map unit represents an area that has up to three different kinds of soil, called components.

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 component level, but 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 agricultural limitations, suitability for various flora and fauna, or management characteristics, we can't average the data or sum it up.

There are a couple of options for aggregating qualitative data:

  • Selecting the dominant component and using it for the entire map unit
  • This is the easiest solution and can work well, especially when the dominant component makes up a large percentage of the map unit (thus this is more often useful with SSURGO data rather than STATSGO data). Simply take the data value for the largest component and assign that value to the entire map unit.

  • Finding the component with a limiting property and using it for the entire map unit
  • Like the dominant component method, we are still going to assign the data value from one component to the entire map unit. However, now we are not going to necessarily use the value from the largest component. Instead we will use the value that most limits us based on the purpose of the map and assign that value to the entire map unit. For instance, we might be worried about using heavy equipment in a woodland environment [variable wdequip in table woodmgt]. If one of the components has a value that restricts the use of equipment more than the others (due to concerns about erosion, perhaps) then we use that value for the entire map unit, even if the component makes up a small percentage of the entire area. Why? Because we don't know where that component is! Thus we can't restrict equipment use in only that portion of the map unit.

Other answers discuss a related issue, where we take quantitative data like water holding capacity and then classify that data into qualitative classes like suitable / not suitable. 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.