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ADB study compares agricultural land measurements using GPS and satellite data

The Asian Development Bank (ADB) recently released a working paper comparing land measurement through Global Positioning System, Self-Reports, and Satellite Data.

The authors explain that timely, cost-effective, and high-quality land measurement data through national statistical reporting playds an important role in the formulation of policies targeting poverty reduction, agricultural growth, and the welfare of agricultural households.

The compass-and-rope method is an option for land measurement where two or three people measure the area of a plot using tools such as calculator, compass, measuring tape, and ranging poles. It is accurate but labour-intensive.

The most common land area measurement technique is farmer self-reporting because it enables inexpensive collection of information from a single question. However, the data from self-reporting has been shown to vary significantly from more accurate estimates derived from Global Positioning System (GPS).

However, GPS still requires walking along the boundary of a plot with a GPS device to obtain an area estimate, introducing significant time and financial costs. The paper proposes using Google Earth instead for land area measurement. In the case of Google Earth, the physical map can be scanned and overlaid with the actual Google Earth image, and the plot boundaries can then be retraced to create a new digitized plot boundary to compute plot area.

The study was conducted in four pilot provinces in four countries: Savannakhet Province (Lao PDR), Nueva Ecija Province (Philippines), Ang Thong Province (Thailand), and Thai Binh Province (Viet Nam). The four were selected as part of a technical assistance (TA) project of the ADB to promote the use of satellite-based technology in estimating rice area and production.

Both improvements in data quality and implementation costs were taken into consideration for evaluating the survey methods.

The study compared Google estimates to GPS estimates, finding few statistically significant differences between these two measures in all countries, with the exception of Vietnam, where a a difference of 16.4 percentage points was observed. The researchers say that the deviations may occur either because of GPS measurement or Google measurement and be of larger magnitude because plot sizes are small in Vietnam.

The fixed cost associated with procuring GIS software needed to calculate area from GPS instruments or Google Earth images was zero in the study as the researchers used a freely available and open source platform called QGIS (previously known as Quantum GIS). However, variable costs per plot would likely be different between GPS and Google methods.

So, the researchers considered four components of variable costs: plot boundary mapping, printing of paper maps versus procuring GPS instruments, farmer compensation, and consultancy fees.

They arrived at an estimated cost of $16.46 per plot for GPS and $10.27 in the case of Google Earth, a 37.61% reduction. The paper notes that for a survey with 4,000 plots, which is typical of multi-topic agricultural surveys such as the Living Standards Measurement Study of the World Bank, the cost savings from using Google Earth with the study’s cost structure would be $6.19 x 4,000 = $24,760.

Since the differences between Google and GPS measures are not statistically significant, while cost differences are, Google Earth images could provide a viable alternative land measurement technique. The paper also notes that as remotely sensed data becomes publicly available, it may become a less expensive alternative to link to survey data than rely on GPS measurement.

Read the paper here.

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