APPLICATION OF GPS RTK METHOD IN CONSTRUCTION OF SETTLEMENTS

and rover receivers (Fig. 1.) operating simultaneously. Real-time kinematic method is based on the radio connection between the reference and rover receiver, as well as on the application of OTF (on-the-fly) procedures. The application of an OTF procedure provides the initialization even in the case when the signals from a sufficient number of satellites are lost. It means practically, that after passing through a tunnel, forest or some narrow street, it is possible to initialize the measurement in a very short period of time. The distance between the reference and rover receiver should not be longer than 5 km; the ideal distance is 2 km (URL 1). The antenna of the reference receiver with the radio device is set upon the point with 3D known coordinates. Using the radio connection, the corrections from the reference receiver are sent to the rover with its antenna set upon an unknown point (Fig.

At the rover, the GPS measurements from both receivers are processed in real time and stored in its memory. Figure 1. Basic components and operation principle of the GPS RTK measuring system The measuring system measures actually 3D vector between the known and unknown point in WGS 84 geocentric Cartesian coordinate system. The calculated 3D vector is added to the 3D coordinates of the known point in order to determine the position of the unknown point. Hence, the determined position of the unknown point, i.e. its coordinates depends on the quality of the coordinates of a known point, and the quality of the computed vector (URL 1). Reference receiver with radio module Observer with rover receiver and radio module SGEM 200 6 - Section III 333 During the measurement, the coordinates of the unknown point are transformed into the local-state coordinate system using Helmert’s 7-parameter transformations. These parameters are known in advance; they are obtained on the basis of identical points (minimally three points) in both coordinate systems. Helmert’s transformation consists of 3 translations along the coordinate axes (tx, ty, tz), 3 rotations around the coordinate axes ( x,  y,  z) and scale change factor (M). The parame ters have to be entered into the measuring system before the measuring procedure itself, so that the transformation of coordinates in local-state coordinate systems can be performed in the real time, directly in the field. GPS RTK measuring systems functio n at one or two frequencies. The advantage of single -frequency equipment is that they are significantly cheaper than the dual - frequency systems, but their basic deficiency is lower accuracy and no OTF initialization possibility , so they have to rely on static initialisation procedures. According to Landau (Landau et al. 1995), the time needed for initialization of single - frequency and dual -frequency RTK systems, coming from the same producer, is approximately 4 – 8 minutes for the first system, and approxi mately 1 minute for the second system.

1 Application of GPS RTK method Except above -stated advantages, there is one more advantage of GPS RTK. It refers to the fact that there is no need for densification of an existing network since the distance betwee n the reference and rover receivers can be up to 5 km. Regard to all mentioned advantages of this method, its application is the most convenient for the following purposes: cadastral surveying (boundary regulation, various subdivisions, etc.), engineering projects (survey of large areas, staking out of large number of points, road and settlements construction, deformation monitoring etc.), preparation of geodetic documentation (digital plans and maps).

ASSESSMENT OF GPS RTK DATA QUALITY For quality (accuracy) assessment, it is necessary to determine precision of measurement – the repeatability of observations e.g. the maximum difference or standard deviation of repeated observations on each line (random deviations), and it is necessary to determine reliability of measuring according to independent checks (checks on biases and blunders).

1 Precision of GPS RTK data Precision of results (coordinates) obtained by GPS depends on: precision of satellite location determination (ephemerides error), precisio n of distance measuring between receiver and satellite, satellite geometry (satellite constellation above the horizon of the point during observation), other factors that have influence on satellite signal (e.g. multipath). Ephemerides are set of orbital parameters which GPS receiver uses for determination of GPS satellite position and its velocity. The data about satellite orbit is called broadcast ephemeride, and it is received through satellite signal during 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 334 measuring. For high precision measuring, it is possible to obtain the other data about satellite orbits, so called precise ephemerides, for post processing of GPS data. The precision of measuring the distance from receiver to satellite depends on: clock errors in satellite and receiver, errors in satellite signals (ionosphere and troposphere refraction), and environment effect. Geometry i.e., satellite constellation during observation directly effects measuring quality, while quality indicator of achieved results (point coordinates) is expressed with DOP (dilution of precision) factor. The precision is higher if DOP factor is lower. DOP is a scalar value that is expressed as a ratio of position precision  p and measuring precision  R: Rp σDOPσ   . Various factors can influence on the GPS signal and disturb it. Therefore, it is not possible to apply GPS RTK measuring technique in forests, urban areas with high objects blocking the signal near of various electrical and radio sources, and it is very important that the elevation of satellite abo ve the horizon of the point is minimum 10?- 15?. The most problematic source of errors due to environment is multipath; the reflection of signals from the near objects. These objects are large, plane surfaces, e.g. large water bodies, buildings and vehicles . This problem is especially present within the city area. In these cases, classical terrestrial methods, such as total station systems, can be used instead of RTK.

2 Reliability of GPS RTK data The determination of GPS RTK measuring results reliability is performed by using additional, independent observations. There are several procedures that ensure independent controls: Using one reference station - position of certain points is determined twice. Reference receiver is placed on a known point of geode tic network, while a rover receiver is placed on an unknown point and their coordinates are determined. After one period of time rover receiver, once again occupy the same unknown point to determine their coordinates. If the differences between two sets of coordinates are within allowed tolerances, the measuring is reliable. Using two reference stations - the second measuring of the same point is achieved in relation to another reference station, and the same as before the two sets of coordinates are compar ed. This method is more reliable than the first one. Using terrestrial measuring methods - measuring the distances between certain points whose coordinates are obtained by GPS RTK method. If the distance determined from point's coordinates is equal to a measured distance, or within allowed tolerances, the measuring is reliable. Using different receiver or another observer to obtain the new set of coordinates. Changing the antenna height.

GPS RTK METHOD IN CONSTRUCTION OF SETTLEMENTS Based on the stated advantages, GPS RTK method has been chosen and used for the designing and staking out a future settlement at the territory of the town Knin. For the defined project, it was necessary to survey the whole territory of the future settlement in order to make project documentation. First, it was necessary to make digital base (plan) for settlement design. Since it was a larger (about 30 hectare) and open area, it had been decided that GPS RTK method would be used because of already SGEM 200 6 - Section III 335 mentioned advantages. Classic al geodetic methods could not be efficient, especially because there was no adequate, i.e. sufficiently dense existing geodetic control (Fig. 2.). The application of classical methods would not be economically efficient and practical for the defined tasks. However, the points of the existing geodetic control were used as reference stations because they were sufficiently dense to meet the condition of optimum distance between the reference and rover receivers (<2 km). The existing points were also used for the application of classical geodetic surveying for the area within the forest, i.e. for the part that GPS RTK method was not applicable. After completing the project documentation, i.e. establishing future land parcels where family houses will be placed, all characteristic points (break points) were to be set out. For this purpose, the GPS RTK method was used too. Further, these two phases in the construction of the settlement are described – survey and stake out in order to indicate all specific character istics of the GPS RTK method.

1 Survey of the settlement construction area The survey of the whole area (about 2500 points) was made in two days by means of GPS RTK method. The orthophoto plan was used as the sketch document (Fig. 2.). After the reconn aissance of the field, those known points were select as reference stations, which meet the basic conditions of GPS survey. Figure 2. Orthophoto plan of the area Golubići with the overview of the existing geodetic network After the receiver with antenna has been set up over the known point, the following should be done (URL 2): enter the name of the point into the receiver, enter 3D coordinates of the (local -state) point, measure the height of the antenna and enter it, enter the datum and projection parameters, enter the transformation parameters. The reference receiver must be compatible with the rover receiver (sample rate-observation of GPS epochs must be as quick/short as possible –1 sec - URL 1). 504 503 502 513 508 509 507 516 517 506 514 515 512 511 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 336 After completing the above -mentioned actions, the reference receiver is activated and the configuration of the rover is started. All necessary parameters should be entered into the rover receiver (datum / projection / transformation / sample rate). It is necessary to enter the height of the antenna into the receiver, and all changes. After that, the rover receiver starts operating. In order to determine the position of an unknown point, it is necessary to track the minimum of 5 (recommendable 6) GPS satellites in 5 continuous GPS epochs. So, to determine the coordinates of one point, minimally 5 seconds are needed. Based on the surveyed points, a digital vector plan was made (Fig. 3.). Figure 3. Vector plan of the area Golubići Overlapping the vector plan with the orthophoto plan one can notice that the details coincide very well (Fig. 4.). Based on the documentation, the project for the future settlement could be made according to the development plan. SGEM 200 6 - Section III 337 Figure 4. Vector plan overlapped on the orthophoto plan of the territory Golubići

2 Staking out the parcels of future settlement After completing the project, the future land parcels were formed (Fig. 5.) and the coordinates of characteristic (break) points were determined. For staking out these points GPS RTK was also used. Within two days, the points were set out and marked in the field. 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 338 Figure 5. Subdivision of parcels on the vector plan of the territory Golubići In the procedure of setting out it is necessary to consider all factors, as well as the surveying procedure. All necessary parameters are to be entered into the receivers so that the points could be set out. Unlike the survey used for determining the coordinates, the procedure of setting out has known coordinates of points, so they have to be entered into the receiver and then transferred into the filed. In the procedure of staki ng out the receiver leads us to the point, indicating on display in which direction we have to move with regard to the north (Fig. 6.). In this way, we come gradually closer to requested position of the point. When the receiver indicates we are not to move any more, the antenna is put onto the ground, and the point is marked at that place. Due to the deficiencies of GPS RTK method, a certain part of the territory was measured with the classical surveying method. There were also classical surveying methods used for staking out. However, it was a very small area (about 2 ha) and using classical methods did not slow down the whole procedure. Surveying and staking out with combined methods was performed as showed on Fig. 6. (Seeber 1993). GPS RTK method was used to determine the coordinates of new geodetic control, from which classical surveying and staking out was performed in the field where using GPS RTK method was not possible. SGEM 200 6 - Section III 339 Figure 6. The combination of GPS RTK and classical measuring methods

3 Ana lysis of results For precision and reliability analysis of achieved results, the certain number of points (approx. 50) was observed and staked out using GPS RTK method but from two different reference stations. With analysis of achieved coordinates, it was determined that the differences between two sets of coordinates are within 2,5 cm, which was beyond allowed tolerances. In addition, the certain number of distances (approx. 25) between characteristic points was measured using classical methods i.e., using total station. Measured distances were compared with distances determined from point coordinates obtained by GPS RTK. The analysis showed that differences between distances were within 3 cm, which was beyond allowed tolerances. According to accomplish analysis it was obvious that our measuring were sufficient precise and reliable i.e., accurate.

CONCLUSION The example presented in this paper shows the possibilities of applying GPS technology in construction of settlements. GPS RTK method is being appl ied more and more, especially when the task is to be done very quickly, because the coordinates of the points can be obtained immediately in the field. Today’s instruments provide high accuracy and quick and economically efficient measurement. Great advant age of GPS RTK method is that there are only a few geodetic points with known coordinates required in the circle of about 5 km. Nevertheless, this method has also its deficiencies. One of the basic and largest deficiencies is that there should be no large objects and sources of electrical and radio signals near the receiver (block of signals, mulipath, interference). The deficiencies are overcome by combining this method with the classical surveying and staking out methods. The application of this satellite s based method, which provides simultaneous determination of coordinate in the state coordinate system, is the most convenient for cadastral surveying and engineering projects, especially when the geodetic control is destroyed or missing. REFERENCES Landa u, H., C. Pagels and U. Vollath (1995): Development and Implementation of a Centimetr e-Accurate Real -Time -Kinematic Positioning System. 8th Int. Tech. 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 340 Meeting of the Sat. Div. of the U.S. Inst. of Navigation, Palm Springs, California, 12-15 September, 1485 -1491.

Seeber, G. (1993): Satellite Geodesy: foundation, methods, and applications, Walter de Gruyer, Berlin –New York. URL 1: Main Roads Western Australia - Recommended field procedure for real - time kinematics (RTK) global positioning system (GPS) surveys. http://www .mainroads.wa.gov.au/NR/mrwa/run/start.asp ,03. 05. 2006. URL 2: USDI - Standards and guidelines for cadastral surveys using global positioning system methods. http://www.blm.gov/nhp/efo ia/wo/fy00/im2000 -181a.pdf , 04. 05. 2006. 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org