A system using free floating devices to track and study litter in freshwater and coastal areas
The transmitters send their position, determined by the GPS/Galileo satellite network, several times a day to a server.
The transmission of the position data is carried out via the cellular telephone network, which whenever possible adds the data concerning the cell through which the connection is made.
The server, the system for organising the data, the software that selects the valid data, the progressive archive that is formed, the system for organising the data geographically and the system for representing the data and positions over the weeks are all part of the JunkTrack system and were developed as an original project by NAUTA scientific (www.nauta-rcs.it) together with NaturalGIS (www.naturalgis.pt).
The systems, consisting of sealed containers with PET casing, can be kept in a 'hibernation' condition using an external transducer.
As well as directly tracking waste floating in the current, if they are buried in the sediment of the embankments they can assess the erosion of the embankments themselves, waking up, and sending an alarm, only when they are freed from the sands.
The transponders, with dive profiles varying from a few centimetres upwards, follow the flow of the water and the winds and tend to run aground and accumulate in the shallowest areas and where the current is diminishing.
They tend to start up again when the water flow increases due to rainfall, the seasons and/or the regulation of artificial hydraulic weirs.
Monitoring these phenomena is important for assessing the floating load that is displaced by the waters and ends up in the sea, if not otherwise intercepted, at river mouths.
Considering that the transmitters are free to move in the water following currents and airflows on the surface (according to current observations the behaviour is very much affected by the direction and strength of surface winds), and that often the river bed runs by its nature embedded in the plain, sometimes far from cellular coverage, the data flow from individual transmitters can be intermittent, either due to lack of coverage or due to a temporary buoyancy in the water that prevents the GPS position from being received correctly.
Despite these two limiting factors, we are able to reconstruct the sequence of transmitter positions, identifying accumulation points and average speeds in the different sections.
The data representation system
The data are represented using an open-source map-server provided by NaturalGIS. The server is programmed to divide the river area into hexagons. The internal colour of the hexagon is determined by the total number of points that are surveyed within it. This number, i.e. the number of times an object has been detected within the boundaries, is also expressed in figures.
The size of the hexagon is scaled as a function of the zoom at which it is displayed, in order to improve the granularity of the data.
The choice of the hexagon as the cell shape derives from the more homogeneous distance between the centre and the periphery compared to the square.
The GIS allows at the moment to switch on/off the points of each launch, switch on/off the cells, switch on/off the tracks with the sequence of points of a single (or all) transmitters.
The map background can be chosen from several standards, including satellite photography.
The system loads and regenerates the complete list of points every night, and also automatically generates some auxiliary files including formats digestible by other GIS and Google Earth.
In the development project of the data analysis system we intend to represent in greater detail the components "time", "environmental variables", "speed".
In other words, we are interested in being able to represent the average speed of the trackers as a function, for example, of the flow rate during the period, seeing the evolution of the phenomenon over the weeks.
NaturalGIS LDA - Sistemas de Informação Geográfica, Portugal
The maps can display average speeds along the route, accumulation points, relationships with river regime, rainfall and all other relationships that can be identified on a geographical and temporal matrix.