18 February 2011

Soil Survey and GIS

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) 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.

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.

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.

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