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|ŠUMARSKI LIST 3-4/2016 str. 54 <-- 54 --> PDF|
strategies should be prepared beforehand. At this point, GNSS brings a key technology tool that helps the system operator to identify and monitor the exact location of the resource. Moreover, an integrated system, combined with a geographic information system (GIS) and GNSS, enables decision makers of the authorities to make appropriate decisions. Developments in the surveying technology provide the determination of information before harvesting operations are completed, unlike past years in which aerial photography was the only source of extracting the shape and location of wide blocks. The past technology was not so accurate. Today, GNSS makes it possible to acquire accurate information in real time. The required accuracy can be easily met with the use of the GNSS technology, which also makes it possible to determine the accurate locations where needed (Hoffmann-Wellenhof et al., 2008; El-Rabbany, 2006; Wolf, 2002; Pirti, 2005; Pirti, 2008; Pirti, 2010; Pirti, 2013; Bakula, 2013; Brach, 2014).
To provide sustainable management for forest life protection, an accurate forest boundary determination is of outstanding importance. In this concept, real time GNSS presents a good alternative which reduces the survey cost and duration by up to 75 percent of a project schedule and budget. However, there are some restrictions when surveying with GNSS, which are caused by the forest canopy. The heterogeneous pattern of the forest canopy, which differs from place to place, limits, disturbs or totally blocks the survey quality. For example, under heavy forested areas, GNSS receivers lose connection with GNSS satellites. Thus, the quality of GNSS survey under the forested area is determined by the type of forest canopy, which is directly related to a complete or partial signal blockage received from the satellites, where a significant role is placed by the leaves. The species and density of canopy leaves are in strong correlation between the signal quality and accuracy. As the canopy density increases, the received signal decreases. The same effect on signals occurs in terms of water percentage of canopy type or moisture. Species with higher water content have bigger obstruction reception. The pattern of the canopy behaves as a physical reflection surface for the signals. This reduces the number of visible satellite, which affects the accuracy of GNSS positions. The structure of the GNSS signal is line-of-sight, affected by solid objects around (Parkinson, 1996; El-Rabbany, 2006; Pirti, 2005; Pirti, 2008; Pirti, 2010; Pirti, 2013; Bakula, 2013; Brach, 2014). However, improvements on these obstructions can also provide users with a wide range of application facilities in forested areas, which helps making an agreement with authorities who avoid using GNSS under heavy forest canopy due to lack of accuracy. Moreover, Dilution of Precision (DOP) value is a significant indicator on the GNSS position accuracy. The Dilution of Precision indicates the accuracy that has a reverse effect between these two descriptions. Lower satellite visibility results in an increase in the DOP value and causes inaccurate position computation. Thus, the duration of observation should be prolonged in order to get more satellite signals and avoid worse satellite geometry. In certain conditions during a survey, the DOP does not allow a constant solution in the same day. Moreover, signal attenuation and multipath effect may also induce degradation of accuracy (Wolf, 2002; Parkinson, 1996; El-Rabbany, 2006; Pirti, 2005; Pirti, 2008; Pirti, 2010; Pirti, 2013; Bakula, 2013; Brach, 2014).
2. METHODS USED IN SURVEYING
Metode korištene u snimanju
2.1 Traditional Technique: Point Positioning with Total Station – Pozicioniranje točaka pomoću totalne stanice
Total stations (TS) are electro-optic surveying systems, which have been used for many years in geodetic precise positioning applications. Generally, TSs have been developed to survey short ranges (approx. 2-3 km). In geodetic metrology, there have been several total stations with different features for determination of precise coordinates in terms of applications based on surveying distances and angles. Some of this equipment, whose distance and angle surveying accuracies are different to each other, have the ability of operating without reflectors.
Although they allow surveying either in open-air areas or in urban, forested and indoor areas, these systems have some disadvantages. For instance, the requirement of direct line of sight, survey at daylight, bad weather conditions may be listed. In any case, total stations, which are currently still effectively used to determine precise point positioning for several engineering applications, are also often used for precise terrestrial surveying applications under forested areas (Pirti, 2005), (Wolf, 2002).
2.2 GPS/GNSS Technique – GPS/GNSS tehnologija
Global Positioning System (GPS), originally developed by the USA, is a system of positioning, navigation and timing, which has been commonly used by several disciplines for different applications over the last three decades. Today, GNSS consists of satellite technologies: GPS (USA), GLONASS (Russia), BeiDou (Chinia) and Galileo (EU) are used as a common abbreviation of space and satellite based positioning. In fact, these systems which enable a new manner to positioning and which are related to survey and navigation applications, have become effectively used for many services such as land management, environmental and urban planning, land use and agricultural policy, monitoring the global climate change, engineering and infrastructure services, evaluation and protection of forests and natural