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The first test was spread over 2 days, with moderate winds, rain overnight and during the first day, and temperatures below 8° C on the morning of the second. Some of the radio transmitters had been left out overnight and early bearings taken on these radios were difficult to fix and more variable than those taken the day before. Minor corrections to the aerials and position of the stations were made over the next 3-4 weeks and a second data collection run completed. The weather was fine and warm, though extremely windy, on that day. Figures 2.5 and 2.6 show the distribution of bearing errors for tracking stations J-1 and E-4 of the initial test, and the second, when the operators had more experience with the system. The overall mean error for bearings from J-1 and E-4 are shown in Table 2.1 (mean ±S.D).

Table 2.1 Mean errors from 2 tests, for each triangulation station

N J-1 tracking site E-4 tracking site Initial test: mean error (degrees) 215 -0.91 ± 2.45 0.73 ± 2.1 Second test: mean error (degrees) 215 1.35 ± 2.66 -0.49 ± 1.49

The effect of the weather was noted on several more occasions during actual possum triangulation as the temperatures changed overnight and the transmitters cooled down with exposure to the atmosphere and moisture after the possums left their dens. Some radios appeared to drift in frequency and/or vary in pulse characteristics with temperature changes. Both of these changes were distinct from signal strength variation due to movement through the area (some of the variation could also be due to the receivers).

Figure 2.5 Distribution of Bearing Errors - Initial test

Bearing Error

No. of Bearings

Figure 2.6 Distribution of Bearing Errors - Second Test

Bearing Error

No. of Bearings

Figure 2.7 shows the distribution of transmitter test locations and associated polygon areas. Test transmitter locations were divided into 3 groups - Group 1 in the lower part of the paddock (low arrow in Figure 2.7); Group 2 included transmitters west of and close to the rocky ridge (top arrow); and Group 3 the remainder. Groups 1 locations were difficult to receive from J-1 station and Group 2 from E-4. Most locations in these groups were out of sight of the respective receiving stations. Group 3 locations were spread throughout the site and almost all were line-of-sight to both stations. All bearings taken to transmitters in each group were included (5 for each point) to calculate the summary statistics in Table 2.2. Confidence or error arcs indicate the precision of bearings to a particular point and, except for signals received from the low part of the area by station J-1, arcs were <10° (95% confidence). These data are similar to that of Lee et al. (1985).

Table 2.2 Summary statistics of error test locations by group

Group and Group One Group Two Group Three

Station location ⇒ _{E-4 J-1 E-4 J-1 E-4 J-1}

Number of bearings 95 95 40 40 80 80

Mean Bearing (deg.) -0.65 1.95 -0.98 1.15 -0.08 0.69

Standard Deviation 1.49 3.74 2.01 1.27 1.64 1.44 Minimum Bearing -4 -6 -5 -2 -4 -2 Maximum Bearing 3 9 2 3 3 4 95% Confidence Arc (±) 2.92 7.33 3.94 2.49 3.21 2.82

Variation in mean error was inconsistent throughout the site. Table 2.2 indicates angular error (confidence arcs) from tracking stations for each group, but does not define the shape of the area around a point location in which the transmitter (animal) may be found. Many of the reference points had considerable angular error associated with them but the effect of this varied between tracking stations. Figure 2.7 shows the section of Backdrop paddock where possums were radio tracked. At lower left is the flat entrance to the paddock and top left is the northern ridgeline. The 2 squares along the right margin of the surface represent the tracking stations. Small black squares mark points where reference transmitters were placed and the larger irregular shapes the error polygons associated with them. Where transmitter points are not seen, the error polygon overlays them. The largest deviation of signals is seen in the lower part of the figure (arrow) and was due to the thick scrub and slight ridge between the transmitters and the lower tracking station (J-1). Conversely the upper station (E4) had difficulty in obtaining accurate locations on transmitters placed to the west of the rocky spur (top arrow). Although mean angular error was less than 1° overall, it is clear from this

surface and Table 2.2 that a few point estimates may be 20-40 metres from their true location (note the grid size).

Figure 2.7 Error Polygons from Radio Triangulation Error Measurements

Note: The scale of the figure is not linear as it is a draped surface. However, each of the grid cells covers a 30 metre by 30 metre area. See Appendix V for a plane view of the same figure.

Two factors contribute to the locational error (size of error polygon) associated with the signal from a particular point. The first is the precision of the aerial - this is obtained from the standard deviation of the replicate bearings - and secondly the geographical characteristics of the point relative to the receiving towers. Each point and receiver combination has a confidence arc associated with it but it is the distance and surrounding topology of the point from the receivers that primarily determines the size and shape of the error polygon. Table 2.3 summarises mean error polygons for the 3 groups of locations.

Table 2.3 Error polygon areas (square metres) - means of groups

Number Mean Std.Dev. Minimum Maximum

Group 1 19 134.82 97.12 19.56 339.93

Group 2 9 97.92 51.59 33.43 183.88

Group 3 15 299.39 240.61 36.46 995.91

Group 1 points have the largest error arc from J-1 station and the smallest from E-4. However these points included some of the closest to J-1, almost all were on relatively flat ground, and are line-of- sight even though at greatest distance from E-4. Overall the result is an average sized error polygon compared to group 3, which, although having small error arcs from both stations, includes many of the points on steep terrain. As the error polygons calculations are based on plane geometry this results in a relatively larger surface area for a confidence arc. For example, several of the locations were on ridges. A polygon which has 4 corners, each 30 co-ordinate units apart, centred on a point on the ridge extends either side of the ridge. This effectively increases the ground area of that polygon as it drapes down the hillside. Also, many of the points in Group 3 were farthest away from J-1 station and this resulted in maximum width of the error arcs at these points. Point locations in group 2 had average error arcs, and the smallest mean error polygon. These points were equidistant from both stations, and bearings to them often intersected at right angles, both of which factors helped to reduce errors. Although Group 2 locations were spread over dissected terrain (several creeks run through the area) and on a steep hillside, the size of the polygons was little affected because the short distance from the points to either station diminished the width of the error arc compared to locations in the other groups. There are also half the number of locations in the group and this may have contributed to the results.

2.5.5. Summary

Error values found in this study were consistent with those from published reports (Mills and Knowlton, 1989; Springer, 1979; and Lee et al., 1985). Although observer accuracy was not specifically tested during training sessions, both observers took bearings on transmitters and after becoming familiar with the techniques there was seldom any actual difference between bearings and no statistical difference. Cochran (1980) warned against testing system accuracy solely on stationary transmitters if movement of the collars is expected. In this study blind testing of operators with moving transmitters resulted in comparable accuracy to stationary ones. With little experience the observer could tell that the collar was in fact moving, particularly in thick bush. The resulting signal was not as consistent as stationary transmitters but, except for a few areas which had difficult signals anyway, location and determination of an accurate (within system error) azimuth was possible. Radio triangulation problems in this study area resulted from a combination of the confined nature of the site, characteristics of possum movements, and the topography of the area. Large areas of bush and slight, though significant, ridges between the transmitter and receiving aerials produced considerable signal bounce. As experience was gained much of the signal bounce became predictable. As many of the possums were repeatedly tracked, and their movements, from den sites to feeding areas, were consistent over time, signal loss or sudden, large-scale variations could be explained. For example, signals from a possum denning along a steep creek bank may have been easily located before the animal moved. However, as the possum moved the orientation of the

transmitter aerial changed, as did the effect of the terrain on the signal. Confidence in locations made during this time, until the possums signal again became clear, were higher when the animal was later located in a commonly frequented area.

The more subtle problem of refraction could not be overcome. This was associated with rising ground, and heavy bush, between transmitter and receiving station. It was apparent that small movements within an area prone to refraction could have a marked effect on apparent signal direction. As many of these problem areas were feeding areas examination of the distances moved between observations, and consideration of feeding habits led to some of them being excluded when inconsistencies were detected. Movements of possums through areas that were line-of-sight to both stations presented no problems.

Saltz and Alkon (1985) noted 4 principal applications for error estimation within a study:

1. to aid in the determination of equipment and field protocol to achieve study aims 2. to aid in selection of appropriate home range calculations

3. provide a guideline for data inclusion/exclusion decisions and 4. as an essential addition to the overall reporting of results

The data presented fulfil those aims and allows confidence in presenting analyses as well as bringing to attention the weaknesses of the system. A major improvement to the present system would be the inclusion of a third radio tracking station within the paddock based on knowledge and error arcs already obtained. This station would have provided bearings from those areas in the lower part of the paddock.

The difficulties associated with tracking an animal with a relatively limited home range in steep terrain have been indicated, but the importance of quantifying the precision of such a system to account for differences in observers, geography and signal quality is recognised. Assuming transfer of equipment and resolution from one study to another without recalibration is unacceptable scientific method.