Individual Tracking Data Research Articles

Inferring animal population distributions from individual tracking data: theoretical insights and potential pitfalls

Animal population-level phenomena are often inferred from large tracking data sets obtained from only a few individuals. Two key challenges are to understand how these two scales are related, and to identify the factors that influence the extent to which small samples consisting of a few individuals can predict spatial patterns at the population scale. We used a simple spatially explicit theoretical model to explore some of the factors that affect inferences made at the population level from individual tracking data. We adopted a 'mixtures of correlated random walks' approach to simulate two discrete movement modes with different step lengths and turning angles in a hypothetical ungulate population with contrasting population sizes and sampling intensities. Movement state was assumed to be influenced by habitat type (patch or matrix) and social cues. We explored the predictive power of a tracked population subsample by regressing the space-use map generated by a few randomly chosen individuals against the map generated by the entire population (the 'true' map) for different scenarios (e.g. random and clumped habitat distributions) and parameter values. We show that the predictive power of the tracking sample varies nonlinearly and often counter-intuitively with factors such as habitat preference, the spatial context of the landscape and the importance of social interactions. We suggest that movement models coupled with individual tracking data can be used with Monte Carlo simulations to improve tracking studies by better understanding the links between detailed individual movement data and population distributions.

P5-S3.03 Individual tracking among high risk groups for STI management in India: an analysis of 400 000 high risk groups followed from 2004 to 2009

BackgroundAvahan, the India AIDS Initiative of the Bill & Melinda Gates Foundation implemented a large HIV prevention program across six high HIV prevalence states among high risk groups (HRG) consisting...

Large-scale STI services in Avahan improve utilization and treatment seeking behaviour amongst high-risk groups in India: an analysis of clinical records from six states

BackgroundAvahan, the India AIDS Initiative, implemented a large HIV prevention programme across six high HIV prevalence states amongst high risk groups consisting of female sex workers, high risk men who have sex with men, transgenders and injecting drug users in India. Utilization of the clinical services, health seeking behaviour and trends in syndromic diagnosis of sexually transmitted infections amongst these populations were measured using the individual tracking data.MethodsThe Avahan clinical monitoring system included individual tracking data pertaining to clinical services amongst high risk groups. All clinic visits were recorded in the routine clinical monitoring system using unique identification numbers at the NGO-level. Visits by individual clinic attendees were tracked from January 2005 to December 2009. An analysis examining the limited variables over time, stratified by risk group, was performed.ResultsA total of 431,434 individuals including 331,533 female sex workers, 10,280 injecting drug users, 82,293 men who have sex with men, and 7,328 transgenders visited the clinics with a total of 2,700,192 visits. Individuals made an average of 6.2 visits to the clinics during the study period. The number of visits per person increased annually from 1.2 in 2005 to 8.3 in 2009. The proportion of attendees visiting clinics more than four times a year increased from 4% in 2005 to 26% in 2009 (p<0.001). The proportion of STI syndromes diagnosed amongst female sex workers decreased from 39% in 2005 to 11% in 2009 (p<0.001) while the proportion of STI syndromes diagnosed amongst high risk men who have sex with men decreased from 12% to 3 % (p<0.001). The proportion of attendees seeking regular STI check-ups increased from 12% to 48% (p<0.001). The proportion of high risk groups accessing clinics within two days of onset of STI-related symptoms and acceptability of speculum and proctoscope examination increased significantly during the programme implementation period.ConclusionsThe programme demonstrated that acceptable and accessible services with marginalised and often difficult–to-reach populations can be brought to a very large scale using standardized approaches. Utilization of these services can dramatically improve health seeking behaviour and reduce STI prevalence.

Mean sea surface and geoid gradient comparisons with TOPEX altimeter data

Cycles 4 to 54 of TOPEX data have been analyzed through comparisons with the mean sea surface given on the distributed geophysical data record (GDR). Two inverted barometer correction procedures were considered for the data reduction. One used a constant atmospheric pressure for all data while the one adopted for use, for most computations, introduced a cycle average pressure. The maximum difference between the two estimates was 3.0 cm with a clear annual signal. With the modified correction the TOPEX sea surface was compared to The Ohio State University (OSU) mean sea surface, given on the GDR, to estimate three translations (Δx = −2.3 cm; Δy = 25.0 cm; Δz = −0.3 cm) and a bias (43.3 cm) between the two surfaces. The only significant translation is Δy which indicates the reference frame of the TOPEX system differs from that used in the OSU mean sea surface system. The bias between the TOPEX mean sea surface and the OSU mean sea surface was used to estimate an equatorial radius of 6,378,136.55 m based on an 18‐cm bias estimate of the TOPEX altimeter. Examination of the average difference, by cycle, between the TOPEX sea surface and the OSU mean sea surface suggested a bias change of 3.1±2.2 mm/yr with a positive sign indicating the average ocean surface is rising or the altimeter measured distance is decreasing. Models were implemented that solved directly for a bias, bias rate, annual/semiannual, and tide correction terms. These latter two terms were represented by a degree 8 spherical harmonic expansion with the tide corrections being to the components of the Cartwright and Ray tide model given on the GDR. The most complete solution gave a bias rate of 3.6±0.6 mm/yr when the OSU mean sea surface was used and 3.2±0.4 mm/yr when the TOPEX data were used to define a mean sea surface. When the bias and bias rate terms were included in the estimation process, the annual/semiannual terms and the tide terms estimate changed slightly. The greatest change (±1 mm) took place in the annual term. The computations indicated that a simultaneous solution for bias, bias rate, and annual/semiannual terms gave the most accurate results. Nonsimultaneous solutions led to slightly different bias rate values. The root mean square difference between the TOPEX sea surface and the OSU mean sea surface, after translation and bias correction, was ±17 cm for a typical cycle. However, some locations were identified where the difference could reach 2.3 m and were repeated over several cycles indicating errors in the mean sea surface. Most of the large differences occur in regions lacking altimeter data prior to the TOPEX/POSEIDON mission and/or areas of significant bathymétrie signature. Geoid gradients are needed for the reduction of individual track data to a reference track. The accuracy of the determination of such gradients was determined through the comparison of predicted (from a mean sea surface) along‐track gradients to the observed gradients. Among four mean sea surfaces tested the best agreement (±0.9 cm/km) was found with the OSU mean sea surface placed on the TOPEX geophysical data record.