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ŠUMARSKI LIST 7-8/2016 str. 48     <-- 48 -->        PDF

state that using higher masts brings an increase in operating costs as well as technical difficulties, e.g. the weight of the mast, problems with electrical power, the mast tilt, etc. and shows insignificant improvement in terms of accuracy, especially in the leaf-off season (Brach and Zasada, 2014).
Considering the graphics of 3D positioning differences for TP01 and TP02, it is seen that smaller errors have been estimated for TP01 point, where satellite visibility is better. Analyses for TP01 show that APPS service generally gives better solutions. For TP02, APPS and GAPS services give close solutions to each other.
CONCLUSION
ZAKLJUČAK
Traditionally, static GPS observations are collected and relative technique are implemented for high accurate GPS point positioning applications. To achieve this, high cost geodetic grade GPS receivers and post-processing software are used. Accuracies estimated from relative technique basically depend on observation duration and baseline length between points. Data collected are post-processed either scientific or commercial software with respect to aim of the study and baseline length between points. The accuracy of point positioning estimated is frequently found in mm to cm levels.
For high precise and Real-Time differential positioning, GPS-RTK techniques have been preferred. Today, either single-base RTK (classic-RTK) or network-RTK technique has been used for this purpose to achieve cm level positioning accuracies.
Recently, PPP technique, which is an absolute positioning technique, has been used widely as an alternative to the above mentioned techniques. This technique, by having the ability of using the GPS pseudorange and carrier phase observations together, provides high accurate solutions with respect to observation duration. However, in this technique, minimum 20 minutes or more observation duration is required for GPS carrier phase integer ambiguity (float) resolution. As well as increase of the use PPP technique, the web based online positioning services, being free of charge, implemented this technique are gradually increasing. These services also present a significant alternative to the high cost scientific and commercial software. Developments of PPP technique and web based online services provide several advantages to the users with respect to time, cost, accuracy and labor.
For the applications that conducted for protecting and assessment of the natural resources of forest environment, this services are important alternatives while considering these advantages. In this study that conducted for this purpose, 3-day repeated static GPS observations with 3-hours observation duration per day have been collected. The observations have been processed by commercial software and web based online services and results have been compared. In all web based services except APPS, IGS final ephemerides products have been used. In APPS service, final ephemerides products provided by NASA JPL have been used. The results obtained by services using PPP technique show that the most reliable solutions in terms of 3D positioning differences have been provided by APPS and GAPS services.
Considering 3D positioning differences, the positional accuracies for all services implemented both relative and