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Geolocation (light analysis)

Barancle Geese. Foto by Götz Eichhorn

Barnacle Goose. Foto by Götz Eichhorn

MT-Geolocation processes light data in order to calculate the route of an animal. The program identifies the time of dawn and dusk, time of noon and midnigt and by these information it calculates the position (the co-ordinate) twice for each 24 hours.

The recorded data should be equidistant concerning the time (e.g. each 30 secs one sample). In case they do not, we provide a utility tool which is able to 'fill the lacks'.

MT-Geolaction works in this way: In a first step the data will be imported and presented graphically. If necessary, the data can be pre-processed. Most important for the light analysis is the ability of MT to replace data sections (see Manual, chapter Process data, Sections ). By this feature it is possible to replace a bad data section by a copy of the preceding or succeeding day, and so achieve good results of in fact bad data sections. In case the data had been sampled by a quite large sampling rate (e.g. 15 min) then there is the possibility to store intermediate points. MT can fill in additional points thus decreasing artificially the sampling rate. The points can be filled in by a nonlinear splining process.

Some devices records e.g. each 1 minute a light values but stores only the max. value of e.g. 10 minutes in order to spare logger’s memory resources. This method leads to a timing-error. MT can deal also with such data, eliminating this error.

The analysis works threshold oriented. MT searches the times of the begin and of the end of day at the dimlight-phases. If the light intensity values exceeds the 'sunrise-threshold' then MT sets at this time the 'begin-of-day-marker'. In the same way MT identifies the end of each day. By means of these markers (the exact time of sunrise and sunset) MT calculates the current co-ordinates of the animal. If the animal moves fast east- or west-directioned then errors are induced by the deviation of the 24-h cycle. (i.e. a full day may appear longer or shorter than 24 hours). MT is able to reduce this error by various sophisticated methods.

The threshold oriented method requires the knowledge of the exact thresholds for sunrise and sunset. MT is able to determine these thresholds (before performing the actual light analysis) provided that the logger had recorded at least three days at the same location light values before the animal started his trip.

Ideal: Start the logger at least three days at the start point before deploying it at the animal!All the analysis - the determination of the thresholds as well as the calculation of the route - is performed under full visual control of the user. Each found time of sunrise or sunset can be corrected by the user.

The calculated route will be exported into a text file and may be visualized onto a globe. The globe can be zoomed, rotated and tilted freely. However the globe is mainly intended to check the results and to give an overview of the route; it is not a real geographic system.

The accuracy of the results depends on many factors: The quality of the data, the animal and it's speed, the sampling rate,... With very good data experienced users have achieved a mean accuracy of less than 20 km. Restriction: Geolaction cannot work properly near the poles and about 10..12 days around equinox.

Barnacle goose

Fig. 1 In a first step the user may repair corrupted data sections and smooth the data. The lower graph represents the original data, in the upper graph there are the same data after some pre-processing. (The lower graph is visualized here only for demonstration purposes. It is not necessary for the analysis.)

Fig. 2 MT calculates the exact time of dawn and dusk for the first 5 days. The dim-times are indicated by red and blue cursors. By means of these calculations it is possible to find the appropriate values for the thresholds - which are necessary for the analysis.

Fig. 3 The analysis. MT searches the times of dawn and dusk according to the thresholds (the dashed lines). It calculates by these magnitudes the daylength, the time of noon,... and at last by this values the longitude and latitude.

The possibility of MT to draw splines may be helpful to find the corrcet time of dawn and dusk. If neccessary the data can be replaced by the spline.

Colours indicate monthly decades starting from February onwards until May.
Left Wadden Sea in 1st decade of May, spent 18 days in the Baltic and left the Baltic 21 May.

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