New possibilities of using permanent gps stations in modern geodynamic research

By the time the satellite technology was introduced geodynamic research had been based mainly on the analysis of stability of special points of geodetic triangulation and trilateration networks measured by means of classical measurement techniques, including, above all, linear (distancemeter) measurements, precise angular measurements, precise levelling measurements and gravimetric surveys. The frequency of the measurements of such networks depended mainly on the expected displacement values and the accuracy of the measurement techniques. In many cases deermination of slight displacements was difficult due to the application of insufficiently accurate obsevation techniques. Analysis of stability in the position of points of a geodynamic network was often conducted separately for horizontal (triangulaton, trilateration) networks and for vertical ones: “horizontal” and “vertical” displacements were determined.

Another serious difficulty which had to be faced consisted in determining a rational frequency of the network measurements: on the one hand, it was posible to detect secular variations using the most accurate observation techniques available at that time but only after a relatively long period of time. On the other hand, the need to detect unexpected sudden changes called for more frequent measurements.

It is evident that GPS observations conducted permanently at many intrnational stations and processed on the day-to-day basis supply us with currently up-dated material which is extremely valuable for geodynamic analyses. Compared to classical techniques analyses of mutual position of points located hundreds and thousands of kilimetres from each other have become relatively easy. The geodynamic network established in Japan which consists of 900 permanent GPS stations located at a distance of 20-30 km from each other can serve as an example.

The growing accuracy of the satellite observation techniques, especially of the GPS ones, lays down particularly high requirements as regards stabilization of geodynamic points. It appears to be necessary that we should not only consider unambiguous submillimetre centering of antennas of satellite receivers and take account of many other factors that effect the accuracy of observations (e.g. examination of groundwater level, periodical variations in gravity values, atmospheric effects, etc.) but where is also a growing need to use deep stabilization of points that are of prime importance. This stabilization is not a stabilization just “below the ground freezing level”, it is a stabilization based on pile foundations that are approximately 25-30 metres deep. It has been found out that the 30m deep subsurface ground layer is subject to periodical displacements depending on the time of the day, the season of the year, the humidity level, etc.

Students’ field practices with DGPS to measure the beach erosion

In the frame of the course “Special Services for the Environment and the Territory” for civil engineers at the Udine University (Italy) part of the field practice activity was devoted to the use of the DGPS in order to measure the coast line of a beach and to compare it with previous measurements.

The dynamic behaviour of a beach is very important for the economy of the tourist places and its measurement is fundamental for engineering works aimed at avoiding or minimizing the loss of sand. With the cooperation of local authorities the students have measured the 2.5 km long beach of Grado (northern Adriatic Sea). The fixed station was located on the point GRADO of the Alpine Traverse where both the WGS 84 and the national Gauss-Boagra grid coordinates are known. The rover station was used in kinematic mode: a student followed on foot the shore line carrying the antenna on a pole. With a moderate walking speed and a sample rate of 5 sec the shore line is sampled every 6 m. The measured data was converted from WGS84 to grid coordinates.

The comparison was made with the numeric technical map in scale 1:5,000 of the region “Friuli-Venezia Giulia”. The flight was carried out six years ago and the precision of good recognizable points is about 15 cm, not so far from the precision of the kinematic DGPS. The comparison must consider also the actual tide levels during the flight and GPS measurements.

The result of the comparison shows an erosion of tens of metres especially where the groins perpendicular to the coast are absent or destroyed. The computation of the area of change is complicated by the above mentioned groins that interrupt the continuity of the shore line, on the other hand, the groins are good reference points to check the goodness of the transformation from one coordinate system to another.

The sand coast lines can vary their position up to several hundreds of metres due to natural and artificial causes: the rivers carry a great amount of sand that is distributed by the streams along the coasts. This movement is not constant, it varies with the seasonal meteorological conditions, but the most important changes

cover periods of several years and depend on a great amont of causes, not all well known. At Grado the most part of the sand coast is used as beach and therefore has a remarkable economic value; if the beach is decreasing with the time (erosion) the administration must carry the sand from another part, so the beach is artificially expanded.

Real-time kinematic surveying

RTK GPS surveying is a kinematic technique where a stationary “base station” through the use of a radio modem broadcast carrier phase GPS data to one or more roving receivers. The rover must track satellites in common with the base. The GPS data from both the base and rover are processed immediately in the roving receiver. The result is an accurate 3D vector between the base and the rover. The rover’s position is displayed and logged in “Real-Time”. The main advantage of the RTK surveying is that the quality control of an established position can be carried out in the field. A position is approved if there was:

- static occupation time minimum 5 seconds,

- GDOP < 7,

- 5 satellites,

- position accuracy entered specified before the start of the survey.

What are benefits of RTK GPS surveying? GPS antenna can be mounted on vehicles to provide contour and topographic surveying data continuously. A surveyor can walk a boundary line and survey while walking. Surveyors can find points that are buried or obscured by tall grass, overgrowth and snow because real-time kinematic accuracies are similar to GPS control accuracies. Positions are always overdetermined, feature-coding is done by the surveyor at the detail point and independent checks are easy to make. Equipment can be used by less skilled operators. Real-time kinematic GPS provides the functionality for many of the surveyor’s routine tasks such as: collection of topographic, and cadastral survey data and establishing project control, different stakeout operations etc. One of the main assets of RTK surveying is reducton of the surveying crew: complete work with the equipment can be done by one person.

There are two main drawbacks of the method: the site should be suitable for GPS surveying and survey area is limited by the line-of-sight coverage of the radio link. Radio repeaters could extend the system’s range and radio broadcast GPS data over and around major obstructions such as hills and very large buildings.