# Machine Learning-Based Calibration of Low-Cost Air Temperature Sensors Using Environmental Data
## 2 Materials and Methods  
### Data Acquisition  
The low-cost air temperature sensor used in this study is shown in Figure 2. The sensor probe \"SHT-71\" (Sensirion AG, Staefa, Switzerland) for measuring air temperature and relative humidity is covered by the plastic box. The specification of the sensor probe is given in Table 1. The sensor was connected to a sensor node of the e-kakashi prototype version (PS Solutions Corp., Tokyo, Japan), which consists of ports of air temperature, relative humidity and solar radiation sensors, the GPS unit and the 3G networking module. Measured data were uploaded to our database on Amazon Web Services via the 3G network.  
Before the outdoor experiment was conducted, an indoor experiment was carried out to assess the accuracy of the sensor. Low-cost and high-accuracy (reference) sensors were installed inside a building for 15 days to allow the accuracy of the low-cost sensor to be evaluated without the influencing factors of solar radiation, rain or wind. The high-accuracy sensor had a forced aspiration system and radiation shield (CPR-AS-12-AC, Climatec Inc., Tokyo, Japan), and the accuracy  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value (\({}^{\circ}\)C)** \\ \hline Measuring range & \(-40\) to 123.8 \\ Resolution & 0.01 \\ Accuracy & \(\pm 0.5\) (25 \({}^{\circ}\)C) \\ \(\pm 0.9\) (0 to 40 \({}^{\circ}\)C) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specification of the sensor probe of the low-cost air temperature sensor.  
Figure 2: Appearance of the low-cost air temperature sensor.  
of its sensor probe (C-HPT-5-JM, Climatec Inc., Tokyo, Japan) had been certified by the Japan Meteorological Business Support Center. The specification of the sensor probe is given in Table 2.  
The experimental sites and sensor setups are shown in Figure 3. We installed one sensor node in the Tea Experiment Station of Saga Prefecture (\(33^{\circ}7^{\prime}3^{\prime\prime}\) N and \(129^{\circ}59^{\prime}41^{\prime\prime}\) E) and two sensor nodes in the Institute for Sustainable Agro-ecosystem Services of The University of Tokyo (\(35^{\circ}44^{\prime}23^{\prime\prime}\) N and \(139^{\circ}32^{\prime}29^{\prime\prime}\) E; \(35^{\circ}44^{\prime}20^{\prime\prime}\) N and \(139^{\circ}32^{\prime}33^{\prime\prime}\) E). Hereinafter, these locations are denoted as sagatea-111, tanashi-117, and tanashi-118, respectively. Since AMeDAS is also installed at the Tea Experiment Station (\(33^{\circ}7^{\prime}1^{\prime\prime}\) N and \(129^{\circ}59^{\prime}40^{\prime\prime}\) E), we used the air temperature measurements from AMeDAS as reference data. The AMeDAS data were supplied by MetBroker [8]. At the experimental site in Tokyo, air temperatures measured by sensors installed at \(35^{\circ}44^{\prime}9^{\prime\prime}\) N and \(139^{\circ}32^{\prime}26^{\prime\prime}\) E were used as reference data. The sensors at the experimental site in Tokyo do not have a forced aspiration system, but are installed in Stevenson screens to shield them from direct sunlight while allowing natural air circulation by shelters made of wood in a double-louvered design (Figure 4).  
We used air temperature data taken every hour from November 2014 to September 2015 in Saga and from April 2015 to September 2015 in Tokyo. All of the data collected in this study are available online [29].  
Figure 4: Appearance of Stevenson screens (photographed by [PERSON], distributed under a CC BY-SA3.0 license).  
Figure 3: Location map of experimental sites and sensor setups (Pin1: tanashi-117; Pin2: tanashi-118; Pin3: sensor measuring reference air temperature for Tokyo site; Pin4: sagatea-111; Pin5: AMeDAS in Saga site). (**a**) Experimental sites in Japan; (**b**) sensor setup in Saga; (**c**) sensor setup in Tokyo.