(48G) Field Area Measurement (polylines)

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An extract from Statistical Annex and Copies of Questionnaire, MSU International Development Working Papers, Green Revolution Technology Takes Root in Africa, Department of Agricultural Economics, MICHIGAN STATE UNIVERSITY, Working Paper No. 76, 1999 (11pgs.)

PART 2: TECHNIQUES FOR FIELD AREA MEASUREMENT
Field Area Measurement Using the Poly Program for the HP 48G CALCULATOR
1. Basics. The basic idea behind field area measurement is to identify the corners of the field, use a compass to take the bearing (from North) between consecutive (moving clockwise) points, and measure the side lengths between each of the points. The bearing and side length data will be entered into the HP 48 programmable calculators and area and the percent error will automatically be calculated (see Part B for instructions on using the calculators). The program works by dividing the field up into triangles, calculating the area of each triangle and summing them. The beauty of using the programmable calculators is that you can have the enumerators take the bearings and side measurements, then immediately check the area while everyone is still in the field. If the closing error is greater than 5%, the enumerators should redo the bearings and side measurements.
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The enumerators should be very careful about how they record the bearings and corresponding side lengths (see Appendix 2). They also need to roughly sketch the shape of the field, marking the corners (A,B,C,D etc.). This is important because in some cases we will have concave rather than convex polygons (see Figure 2). For a convex polygon, it doesn’t matter what point you start with when entering data into the HP48 program. If the polygon is concave, though, the starting point (that is, the first point that is entered into the HP48 program) MUST be the first point beyond the concavity, moving in a clockwise direction. Otherwise the program will overestimate the field area.
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Part B: Instructions for using HP 48G to determine farm plot area
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Part C: Notes on concave and convex polygons and formulas used in the HP48G POLY program
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Part D: POLY Program
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Well documented (equations, examples, etc. …)

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