**Land Area Measurement and Implications for Agricultural Analysis**

# **From Guesstimates to GPStimates:**

Land area measurement is a fundamental component of agricultural statistics and analysis. Land is a key measure of farmer wealth, a critical input in production, and a key variable for normalizing agricultural input use and output measures. Failure to adequately measure land limits the ability to analyze the agricultural economy. In an econometric analysis, mismeasuring land can potentially lead to questionable estimates of agricultural relationships.

In household surveys, land is commonly measured through farmer self-reported areas. Farmer reported land area is widely used since it can be easily incorporated into a standard household questionnaire and thus requires almost no additional time or money. The use of self-reported areas assumes that farmers are willing and capable of providing reasonably accurate estimates of land area. Evidence from previous studies suggests that land measures taken from farmers are measured with substantial error (Goldstein and Udry, 1999; Carletto, Savastano and Zezza, 2013). For data analysts, the question is one of how much error is associated with a self-reported measure and the implications of this error for conclusions drawn from the analysis of agricultural data.

As Global Positioning Systems (GPS) technology becomes more affordable, accurate and userfriendly, GPS-based area measurement provides a practical alternative to farmer self-reported areas that is increasingly being applied in surveys worldwide.<sup>3</sup> For example, in an assessment of agricultural data collection in Sub-Saharan Africa, Kelly and Donovan (2008) highlight GPS technology as having the potential to enable land area measurement to become a much less timeintensive and costly exercise. Using field experiments, Keita and Carfagna (2009) indicate that 80 percent of the GPS-measured sample plots were measured with negligible error when compared with compass-and-rope estimates, the accepted gold standard. Recent empirical evidence based on the 2005/2006 Uganda National Household Survey (UNHS) comparing GPS-based and selfreported measurement of parcel areas also suggests the existence of systematic errors in selfreported parcel areas (Carletto, Savastano and Zezza, 2013).

Our objectives in this paper are to systematically assess the suitability of GPS devices to replace farmers’ self-reporting of plot land area and to determine the implications of using a GPS-based system in agricultural analysis. Comparisons of household self-reported versus GPS-measured distances to market show that not only are self-reported distances characterized by measurement error, but also that the errors are non-classical and dependent on observable socioeconomic variables; this suggests that studies using self-reported measures may be estimating biased effects (Escobal and Laszlo, 2008). Given this is the case, Gibson and McKenzie (2007) argue that GPS can help overcome these problems of measurement in data collection. In the same vein, we demonstrate that using GPS for land area measurement may have extraordinary effects on the analysis of mundane agricultural questions.

> 3 The area of an agricultural parcel can also be measured by traversing or delineating parcel boundaries on orthocorrected remote sensing imagery. Traversing is completed using a tape and compass and is considered the gold standard of land measurement, but its implementation is time-consuming and costly. One Uganda study compared land measurement by traversing versus using GPS units, and found that the average time use per plot measured was over three hours for traversing, which was more than three times as much as the GPS technology (Schoning, Apuuli and Muwanga-Zake, 2005). Traversing is therefore infeasible in the context of national household surveys. Delineating parcel boundaries on satellite imagery is a potentially accurate alternative, but at present largely impractical, particularly in tree-dense areas and areas with regular cloud cover where the ability to make accurate and timely measures is limited. The spatial and temporal extent of national household surveys generally makes the acquisition and processing of such high resolution imagery largely cost prohibitive.

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