Page 71 - Nutrient Management Planning Guide

66

Field Assessment

(rise) 2 + (run) 2 Slope length =

(6 m) 2 + (47 m) 2 =

36 m 2 + 2209 m 2 =

2245 m 2 = 47.4 m =

= rise ÷ run x 100 Slope grade

6 m ÷ 47 m x 100 = 12.8% =

Calculating Grade and Length of Slope

Using a GPS unit, it is determined that a slope in one particular field has a run of 47 m and a rise of 6 m. The grade on this slope would be:

The approximate grade of this slope is 13% and the length is 47 m.

There are several practical methods to determine slope grade including visual approximation, using a clinometer, and using a GPS. Choosing a method will depend on availability of technology and the level of precision required.

Visual Approximation »

The least precise, but simplest method for approximating slope is visual evaluation. Visual evaluation is useful for determining whether slope exceeds certain critical threshold grades (e.g., slope greater or less than 4%). This method is used in the Alberta Environmental Farm Plan field assessment process.

Clinometers »

A relatively precise, yet simple method for estimating slope is using a clinometer (Figure 3.1.9). This tool measures the angle or percent (or both) grade of a slope.

Photo courtesy National Resource Conservation Service

Figure 3.1.9 Slope Grade Measurement Using a Clinometer

Clinometers range in price from $20 to more than $300, depending on the sophistication of the instrument. Designs for simple, inexpensive homemade clinometers can be found online.

GPS »

A GPS unit can be used in the field to determine slope (Figure 3.1.10). Most GPS units provide relatively accurate coordinates that can be used to calculate the grade and length of slope (see equations in Figure 3.1.8).

tip

To get an accurate slope reading, pick a reference target that is clearly visible at eye height. The line of sight should run as close to parallel with the slope grade as possible. The reference target can be anything from a post, a vehicle or another person.

tip

Several post secondary training institutions offer short courses on the use of Global Positioning System (GPS) technology for agricultural and land management applications. Contact your local institution to find out about upcoming learning opportunities.