Statewide Soil Map Creation-Is there a maximum spacing for borings? 2

[Ron247]

In creating soil maps for a large area such as a state, is there some maximum spacing for soil borings or other observation methods? Knowing a common spacing would also be helpful.

I am not talking about soil borings we may request for a construction project or road, but I am talking about large soil maps.

I would also like to know if there is a resource that explains how these large scale maps are researched and created.

I have been trying to explain to a friend why the state soil map for his area does not show any expansive soils in his county but am positive his house is in an area that is proven to have subdivision size pockets of it.

Also, am I correct in telling them a soil map unit does not mean 100% of the soil is of that classification?

 

[GeoPaveTraffic]

I do not know of any standards for these maps. They are typically created by the State Geologic Survey or State Geologist. Most were created in the 60s and 70s.

These maps are general ideas of what kind of soils to expect, nothing more. They are created mostly from arieal photography with very limited ground truthing. Likely NO borings were used, just surficial soil observations.

Mike Lambert

 

[DBronson]

Ron247:

The state soil maps I'm familiar with are prepared by the US department of Agriculture. As I understand it, the soil surveys vary in sample density depending on the type of land - farm land in Iowa is has much denser sampling than forested mountain areas in, say, Idaho. The Soil Survey Manual has a extensive discussion of this. Link

Regards,

DB

 

[GeoEnvGuy]

The maps I am familiar with use a couple different sources of info and include the following.

Well records
River bank observations
Observations from the road
Aerial photos

The maps also have general areas of exposed bedrock mapped with a note saying upto 6 m of material unclassified between bedrock outcrops.

All in all they tell me whether I can expect alluvial, esker or glaciolacrustrine deposits and that's all.

 

[cvg]

state soil survey maps are very general in nature. (the first work maps were developed at 1" = 1 mile) you can always expect variability. they were primarily meant for agricultural use, not engineering. detailed investigation is always recommended using field investigation, sampling and testing.

https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_052289.ppt

 

[Ron247]

Thanks a lot for the responses. It appears I had some misconceptions about how they are created. I was told once it was mostly a combination of aerial photography and soil borings. That is why I wonder how far they spaced the borings.

I can't complain about whoever told me that since I have repeated it multiple times.

I did know there was room for variability in the maps.

Thanks for the links cvg and DBronson, they were helpful

Thanks again all.

 

[1503-44]

When designing and installing cross country pipelines I have noted considerable variations to official soil classification maps. There are no systematic methods used to establish standard distances for borings. They are based on many sources of data, most rather unscientifically determined. You have to be pretty lucky to have any actual soil boring supporting such maps. Most are based on exposed cuts, or simple color change. If we want real soil boring or test pit data, we get it done ourselves. Anything else is pretty much a shot in the dark.