Comparison of the precision of three commonly used GPS models

Development of science in various fields has caused change in the methods to determine geographical location. Precision farming involves new technology that provides the opportunity for farmers to change in factors such as nutrients, soil moisture available to plants, soil physical and chemical characteristics and other factors with the spatial resolution of less than a centimeter to several meters to monitor and evaluate. GPS receivers based on precision farming operations specified accuracies are used in the following areas: 1) monitoring of crop and soil sampling (less than one meter accuracy) 2) use of fertilizer, pesticide and seed work (less than half a meter accuracy) 3) Transplantation and row cultivation (precision of less than 4 cm) (Perez et al., 2011). In one application of GPS in agriculture, route guidance precision farming tractors in the fields was designed to reduce the transmission error that deviate from the path specified in the range of 50 to 300 mm driver informed and improved way to display (Perez et al., 2011). In another study, the system automatically guidance, based on RTK-GPS technology, precision tillage operations was used between and within the rows very close to the drip irrigation pipe and without damage to their crops at a distance of 50 mm (Abidine et al., 2004). In another study, to compare the accuracy and precision of the receivers, 5 different models of Trimble Mark GPS devices from 15 stations were mapped, the results indicated that minimum error was related to Geo XT model with an accuracy of 91 cm and maximum error was related to Pharos model with an accuracy of 5.62 m (Kindra et al., 2006).
Due to the increasing use of GPS receivers in agriculture as well as the lack of trust on the real accuracy and precision of receivers, this study aimed to compare the positioning accuracy and precision of three commonly used GPS receivers models used to specify receivers with the lowest error for precision farming operations as well as the efficiency of the work done in different situations.
In this study, three commonly used GPS models belong to GARMIN CO. were selected for comparison. This company is the world biggest manufacturer of GPS device. Three models include eTrex VISTA, MAP 60 csx and MAP 78s that in recent years have been the most widely used receivers in precision agriculture (Figure 1, Table 1). To assess the accuracy and precision of the receivers, 9 recording stations were selected in a field (20×20 m2) and detailed mapping by the odolite camera under high precision compass networks and regular conditions (figure 2) was identified. To reduce the error of multi-path, a relatively open and unobstructed place in the Abbas Abad field of Bu-Ali Sina University were considered. This study was conducted in a Completely Randomized Design (CRD) with factorial analysis to examine three factors, at three levels, each in three replication including weather conditions (clear, partially cloudy and full cloudy sky), time of day (9 am, 12 am and 4 pm) and three different models of receiver (MAP 60 csx, eTrex VISTA and MAP 78s), in 9 local stations. Difference of deviation value at each station with the mean value of latitude and longitude recorded at same station was used to precision calculate on (equation 1) and the difference of deviation value at each station with a deviation of the actual position latitude and longitude of the same station was used to calculate the accuracy (equation 2). The base station position (No.1) was determined with an accurately large-scale map. Then, the positions of other stations were defined with camera and compass in exact rectangular grid by underlying base station. Mean error for each station using equation (3) and the precision and accuracy and the definitions of each receiver was calculated.
To display the geographical distribution stations and the registered location data for GPS devices ArcView software (v3.3) was used (Fig.3). The real location of stations and registered by each receiver position has been determined. Information recorded in Map Source software, including all longitude and latitudes registered for each station and receiver were transferred to Excel Software (2007). Table 2 shows the mean precision values recorded in each weather conditions. The results obtained by equation 1 (the mean error at each station) showed that the GPS MAP 78s model has the lowest error of 91 cm, VISTA eTrex model has a maximum error of 4.7 meters and MAP 60 csx model has mean error of about 2.64 meters. The analysis of variance of models and weather conditions and the time of day with the interactions between factors have been shown in Table 3. Results showed that there is significant difference (0.01