Soil Survey and GIS
Soil 206 – Soil Ecosystem Lab
Objectives:
After completing this laboratory the student should be able to:
1. Appreciate the complexity of a soil survey and the tremendous resources available through the
NRCS.
2. Have an understanding of the Rectangular Survey System and the Universal Transverse Mercator
coordinate system.
3. Locate a quarter section of land on a digitized county map and determine the most prominent
mapping unit and soil series present on that parcel of land.
4. Determine the acreage of the predominant soil series polygon in a quarter-section and describe the
land forms, vegetation and anthropogenic features of the same.
5. Describe the soil series and suggest a possible best use for the parcel.
6. Determine engineering limitations of the mapping unit.
Soil Survey
A soil survey report is a record of occurrence of soil mapping units on the earth’s surface. In
addition, a soil survey report contains information and data about the use and management of the soil. A
soil survey report is an inventory of soils in a given area, usually a county, in which information about the
uses, capabilities and limitations of the soils in the inventory are included.
Mapping units represent the boundaries between different soil series on a map of the surveyed
area. These mapping units are represented as polygons on the soil survey. These mapping units are
constructed from information gathered by soil scientists as they evaluate the landscape by digging or
boring holes to determine the profile characteristics. The profile characteristics and the landscape
location and position are evaluated to determine the best use of the soil and engineering properties. Soil
surveys are developed to allow the user to predict the behavior of a soil without having first hand
experience with the particular soil and it is a comprehensive source of information regarding the soils in a
particular area. It must be stressed that the information contained in the report is based on a survey and
is not a specific site survey. It is neither practical nor possible to evaluate every potential site included in
the survey and since soils and landforms are highly variable, even over short distances, an onsite
investigation must be conducted to verify the predicted soil properties.
Rectangular Survey System
To make it possible to match a parcel of land with the information about that parcel contained in a
Soil Survey, the parcel must have an identification symbol or name. The basis for location of a particular
tract of land is the Rectangular Survey System, sometimes called the township-range system. The
starting point of the rectangular survey is determined by the intersection of a survey line running east and
west called a base line and another line running north and south called a principle meridian. There are
34 surveyed principle meridians in the U.S. After the principle meridian and the base line were
established, parallel lines to each of those lines were surveyed at six mile intervals, six miles north, 12
miles north, etc. (towns, T); then six miles east, 12 miles east, etc. (ranges, R) and then the south (T)
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and west (R) intervals were done until the entire area had been divided into a 6-square mile patchwork
called townships. These townships are numbered in relationship to their distance from the starting point
so a designation of T2N, R3W would indicate the parcel covers an area 6 to 12 miles north of the base
line and 12 to 18 miles west of the principle meridian. See the following graph.
Principle Meridian
T4N
T3N
T2N
T1N
Base Line
T1S
T2S
T3S
T4S
R4W
R2W
R3W
R1E
R1W
R3E
R2E
R4E
The highlighted township would be described as T4S, R4W and would represent 36 square miles as it is
6 miles on each side. This township is divided into 6 one-mile square parcels called sections. Each
parcel with the section is numbered, beginning in the upper right hand corner, proceeding to the left and
then counting in an S pattern until the bottom right hand corner is reached and labeled as section 36.
See the following diagram. Each one-square mile section is then divided into quarters and then each
quarter is divided into quarters, if needed.
Township with Numbered Sections
6
5
4
3
2
7
8
9
10 11 12
Section Divided into Quarters
1
NW ¼
NE 1/4
SW ¼
SE 1/4
18 17 16 15 14 13
19 20 21 22 23 24
30 29 28 27 26 25
31 32 33 34 35 36
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The legal description of the highlighted township, section and quarter section, if this parcel was in Idaho
is:
“The Northwest quarter of Section 15, Township 4 South, Range 4 West of the Boise Meridian or
using the numerical notation the description would be NW ¼, Section 15, T2N, R3WBM”.
This rectangular survey system has a major drawback; it imposes a rectangular system on a
curved earth. Because of the curvature, sections are not always exactly one square mile. On the average
sections are about 50 feet shorter on the north edge than on the south. This makes the use of a
correction line every 24 miles necessary and a corresponding adjustment in the size of sections 1-6 on
the north side of the section and sections 6, 7, 18, 19, 30 and 31 on the west side of the abovementioned
correction line.
Coordinate System
As mentioned above, when the spherical surface of the earth is projected onto a planer surface it
is quickly recognized that the surface cannot be transformed without distortion. To overcome this
problem hundreds of map projections have been developed, each with their own strengths and
weaknesses. With the advent of the Geographical Information System (GIS) map projections with the
least distortion were selected. Unfortunately, the distortion of the map projection is dependent upon the
shape and orientation of the object, so each region chose a projection system with the least distortion for
that area. Idaho uses the Universal Transverse Mercator (UTM) system which is recognized worldwide.
This system divides the earth’s surface into 60 UTM zones and then establishes a false northing and
false easting in the SW corner of the meridian so each coordinate pair (x and y axis) within the zone is a
positive integer.
The rectangular survey system and the UTM system have been coordinated with each other to
allow us to locate the parcel using the township-range system and then allowing us to establish the exact
longitudinal and latitudinal coordinates with the UTM system. This interface is extremely important as the
data is linked to the parcel using the UTM system. For additional information on GIS, please refer to
“Introduction to Geographic Information System”, by Kang-tsung Chang, McGraw Hill, 2002.
Soil Survey Exercise
Access NRCS website at: http://soils.usda.gov/
We will be working on an exercise using the web soil survey. http://websoilsurvey.nrcs.usda.gov/app/
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