1,720,956 research outputs found
Utilising optical satellite imagery to derive multi-temporal flow-fields for the Tasman Glacier, New Zealand
No full textThe Tasman Glacier is New Zealand’s longest and largest, representing almost a third of New Zealand’s glacier ice by volume. A relatively long observational record exists for the Tasman Glacier. Velocity measurements are present throughout this record, and generally reflect the large mass loss that has occurred through the twentieth century. Recent studies have applied digital image matching techniques to measure flow velocities on the surface of the Tasman Glacier from repeat satellite imagery. These studies have, however, utilized temporally limited data sets. Additionally, precise quantification of uncertainties is not common; with an accuracy of ± 1 pixel (15 m where ASTER imagery is used) assigned in earlier studies, while no previous work has accounted for inevitably anisotropic uncertainties. Large and ambiguous uncertainties make significance assessment of small inter-annual velocity changes difficult.
This thesis provides a decade long (2000–2010) record of flow velocities, derived for the Tasman Glacier from optical satellite imagery. This record has been derived from a series of annually acquired Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery (2000–2010). Flow-fields were derived from pairs of consecutive images using Correlation Image Analysis Software (CIAS). Repeat GPS measurements of markers installed on the glacier surface were used for optimisation of image matching and flow-field validation. Co-registration of each image pair was carefully quantified at a sub-pixel level, enabling the calculation of confidence intervals for each map of flow velocity. Uncertainties presented here are calculated so as to be unique to each individual velocity measurement. Additionally, uncertainties are anisotropic, accounting for unequal co-registration variance between images in the x and y direc tions. The significance of velocity changes could thus be assessed and the spatial distribution of these changes be interpreted.
The results show two major units of flow, with an apparent disconnection between the upper Tasman and Hochstetter Glaciers. Furthermore, the high temporal resolution velocity record revealed marked oscillations between significant acceleration and deceleration of the glaciers surface flow. Analysis of climatic data suggests that observed velocity changes may be driven by climatic and hydrological forcings. The observations indicate that while a discontinuity exists between the upper and lower parts of the glacier, a previously suggested, de-coupling between ice masses at the Hochstetter Confluence is non-existent. The techniques employed by this study reveal new insights into the dynamic behaviour of the Tasman Glacier, previously regarded as a stagnating glacier undergoing slow thinning and retreat. This thesis highlights the advantages of long term and ongoing monitoring, and the development of continuous records, rather than intermittent and isolated measurements, for the detection and interpretation of variability in the flow regime of large temperate valley glaciers. The findings presented here offer improvements to the measurement of glacier flow velocities from optical satellite imagery, and provide new insights into the response of the Tasman Glacier to climatic and environmental change. Such insights may help improve understanding of temperate, debris covered, valley glacier behaviour, and strengthen predictions of their response to future climate change
A multi-scale approach to assessing the spatio-temporal variability of seasonal snow in the Clutha Catchment, New Zealand
No full textSeasonal snow is an important, but under-observed component of New Zealand's hydrological cycle. Measurement and characterisation of seasonal snow is complicated because it varies over a range of spatial and temporal scales. This makes spatially distributed in situ observations difficult to acquire, and limits efforts to scale point-based observations up to larger areas. Sparse observations of seasonal snow lead to reduced understanding of seasonal snow processes, and subsequent uncertainty in efforts to model seasonal snow. This thesis addresses these issues within the Clutha Catchment, New Zealand's largest, by leveraging remote sensing and geospatial approaches to map and characterise seasonal snow both regionally, and at very high spatial resolution over a small alpine basin.
A daily snow covered area (SCA) time series and regional scale snow cover climatology is derived from MODIS imagery for the period 20002016. Metrics including annual snow cover duration (SCD) anomaly and daily SCA and snowline elevation (SLE) were derived and assessed for temporal trends. On average, SCA peaks in late June (~30 % of the catchment area), with 10 % of the catchment area sustaining snow cover for > 120 d yr -1. A persistent mid-winter reduction in SCA is attributed to the prevalence of winter blocking anticyclones in the New Zealand region. No significant decrease in SCD occurred over the period 20002016, but substantial spatial and temporal variability was observed. Raster principal component analysis (rPCA) identified distinct modes of spatial variability within the time series. Spatio-temporal variability extends beyond that associated with topographic controls, which can result in out of phase snow cover conditions across the catchment. Specific spatial modes of SCD are associated with anomalous airflow from the NE, E and SE. Furthermore, it is demonstrated that the sensitivity of SCD to temperature and precipitation variability varies significantly across the catchment. In order to resolve sub-MODIS scale processes, the potential of remotely piloted aircraft system (RPAS) photogrammetry to map snow depth was evaluated within an alpine catchment of the Pisa Range. Differencing between snow-covered and snow-free digital surface models (DSMs) acquired during 2016 provided high resolution snow depth maps. The accuracy of snow depth maps was thoroughly assessed with in situ snow probe measurements, and by analysing residuals for snow-free areas between DSMs. This accuracy assessment demonstrated repeatability and revealed substantial departures of errors from a normal distribution. This reflects the influence of DSM co-registration and terrain characteristics on vertical uncertainty. Error propagation provided lower uncertainties for snow depth (±0.08 m, 90 % c.l.) than the characterization of uncertainties on snow-free areas (±0.14 m). Comparisons between RPAS and in situ snow depth measurements confirm this level of performance. Semivariogram analysis revealed that the RPAS outperformed systematic in situ measurements in resolving fine-scale spatial variability.
Following the successful evaluation of RPAS photogrammetry for mapping snow depth, further snow depth maps were acquired for 2017. A total of six snow depth maps that resolved fine scale spatial variability in snow distribution facilitated the assessment of topographic controls on snow depth and snow water equivalent (SWE) distribution. Topographic controls were assessed via regression tree analysis between snow depth and terrain indices, including the kernel density of tussock vegetation (KDtussock), elevation (ELEV), the topographic position index (TPI), a Shade index (Shade), and Sx (maximum upwind slope). Despite substantial differences in both total snow volume and spatial distribution, the range of spatial-autocorrelation for snow depth was comparable for both winters at 20 – 30 m. Regression tree modelling reproduced some of the observed spatial structure, and demonstrated temporal variability in the relative importance of controlling parameters. The impact of varying wind regimes on the spatial distribution of snow was highlighted. These findings illustrate the complexity of atmospheric controls on SCD within the Clutha Catchment and support the need to incorporate atmospheric processes that govern variability of the energy balance, as well as the re-distribution of snow by wind in order to improve the modelling of future changes in seasonal snow. Despite limitations accompanying RPAS photogrammetry, this study demonstrates a repeatable means of accurately mapping snow depth for an entire, yet relatively small, hydrological catchment (∼0.4 km2) at very high resolution. Snow depth maps provide geostatistically robust insights into seasonal snow processes, with unprecedented detail. This thesis demonstrates the utility of mapping snow at differing spatial scales for improved understanding of seasonal snow processes and highlights the need to robustly capture dynamic processes in spatial snow models
A multi-scale approach to assessing the spatio-temporal variability of seasonal snow in the Clutha Catchment, New Zealand
Get PDFSeasonal snow is an important, but under-observed component of New Zealand's hydrological cycle. Measurement and characterisation of seasonal snow is complicated because it varies over a range of spatial and temporal scales. This makes spatially distributed in situ observations difficult to acquire, and limits efforts to scale point-based observations up to larger areas. Sparse observations of seasonal snow lead to reduced understanding of seasonal snow processes, and subsequent uncertainty in efforts to model seasonal snow. This thesis addresses these issues within the Clutha Catchment, New Zealand's largest, by leveraging remote sensing and geospatial approaches to map and characterise seasonal snow both regionally, and at very high spatial resolution over a small alpine basin.
A daily snow covered area (SCA) time series and regional scale snow cover climatology is derived from MODIS imagery for the period 20002016. Metrics including annual snow cover duration (SCD) anomaly and daily SCA and snowline elevation (SLE) were derived and assessed for temporal trends. On average, SCA peaks in late June (~30 % of the catchment area), with 10 % of the catchment area sustaining snow cover for > 120 d yr -1. A persistent mid-winter reduction in SCA is attributed to the prevalence of winter blocking anticyclones in the New Zealand region. No significant decrease in SCD occurred over the period 20002016, but substantial spatial and temporal variability was observed. Raster principal component analysis (rPCA) identified distinct modes of spatial variability within the time series. Spatio-temporal variability extends beyond that associated with topographic controls, which can result in out of phase snow cover conditions across the catchment. Specific spatial modes of SCD are associated with anomalous airflow from the NE, E and SE. Furthermore, it is demonstrated that the sensitivity of SCD to temperature and precipitation variability varies significantly across the catchment. In order to resolve sub-MODIS scale processes, the potential of remotely piloted aircraft system (RPAS) photogrammetry to map snow depth was evaluated within an alpine catchment of the Pisa Range. Differencing between snow-covered and snow-free digital surface models (DSMs) acquired during 2016 provided high resolution snow depth maps. The accuracy of snow depth maps was thoroughly assessed with in situ snow probe measurements, and by analysing residuals for snow-free areas between DSMs. This accuracy assessment demonstrated repeatability and revealed substantial departures of errors from a normal distribution. This reflects the influence of DSM co-registration and terrain characteristics on vertical uncertainty. Error propagation provided lower uncertainties for snow depth (±0.08 m, 90 % c.l.) than the characterization of uncertainties on snow-free areas (±0.14 m). Comparisons between RPAS and in situ snow depth measurements confirm this level of performance. Semivariogram analysis revealed that the RPAS outperformed systematic in situ measurements in resolving fine-scale spatial variability.
Following the successful evaluation of RPAS photogrammetry for mapping snow depth, further snow depth maps were acquired for 2017. A total of six snow depth maps that resolved fine scale spatial variability in snow distribution facilitated the assessment of topographic controls on snow depth and snow water equivalent (SWE) distribution. Topographic controls were assessed via regression tree analysis between snow depth and terrain indices, including the kernel density of tussock vegetation (KDtussock), elevation (ELEV), the topographic position index (TPI), a Shade index (Shade), and Sx (maximum upwind slope). Despite substantial differences in both total snow volume and spatial distribution, the range of spatial-autocorrelation for snow depth was comparable for both winters at 20 – 30 m. Regression tree modelling reproduced some of the observed spatial structure, and demonstrated temporal variability in the relative importance of controlling parameters. The impact of varying wind regimes on the spatial distribution of snow was highlighted. These findings illustrate the complexity of atmospheric controls on SCD within the Clutha Catchment and support the need to incorporate atmospheric processes that govern variability of the energy balance, as well as the re-distribution of snow by wind in order to improve the modelling of future changes in seasonal snow. Despite limitations accompanying RPAS photogrammetry, this study demonstrates a repeatable means of accurately mapping snow depth for an entire, yet relatively small, hydrological catchment (∼0.4 km2) at very high resolution. Snow depth maps provide geostatistically robust insights into seasonal snow processes, with unprecedented detail. This thesis demonstrates the utility of mapping snow at differing spatial scales for improved understanding of seasonal snow processes and highlights the need to robustly capture dynamic processes in spatial snow models
Utilising optical satellite imagery to derive multi-temporal flow-fields for the Tasman Glacier, New Zealand
No full textThe Tasman Glacier is New Zealand’s longest and largest, representing almost a third of New Zealand’s glacier ice by volume. A relatively long observational record exists for the Tasman Glacier. Velocity measurements are present throughout this record, and generally reflect the large mass loss that has occurred through the twentieth century. Recent studies have applied digital image matching techniques to measure flow velocities on the surface of the Tasman Glacier from repeat satellite imagery. These studies have, however, utilized temporally limited data sets. Additionally, precise quantification of uncertainties is not common; with an accuracy of ± 1 pixel (15 m where ASTER imagery is used) assigned in earlier studies, while no previous work has accounted for inevitably anisotropic uncertainties. Large and ambiguous uncertainties make significance assessment of small inter-annual velocity changes difficult.
This thesis provides a decade long (2000–2010) record of flow velocities, derived for the Tasman Glacier from optical satellite imagery. This record has been derived from a series of annually acquired Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery (2000–2010). Flow-fields were derived from pairs of consecutive images using Correlation Image Analysis Software (CIAS). Repeat GPS measurements of markers installed on the glacier surface were used for optimisation of image matching and flow-field validation. Co-registration of each image pair was carefully quantified at a sub-pixel level, enabling the calculation of confidence intervals for each map of flow velocity. Uncertainties presented here are calculated so as to be unique to each individual velocity measurement. Additionally, uncertainties are anisotropic, accounting for unequal co-registration variance between images in the x and y direc tions. The significance of velocity changes could thus be assessed and the spatial distribution of these changes be interpreted.
The results show two major units of flow, with an apparent disconnection between the upper Tasman and Hochstetter Glaciers. Furthermore, the high temporal resolution velocity record revealed marked oscillations between significant acceleration and deceleration of the glaciers surface flow. Analysis of climatic data suggests that observed velocity changes may be driven by climatic and hydrological forcings. The observations indicate that while a discontinuity exists between the upper and lower parts of the glacier, a previously suggested, de-coupling between ice masses at the Hochstetter Confluence is non-existent. The techniques employed by this study reveal new insights into the dynamic behaviour of the Tasman Glacier, previously regarded as a stagnating glacier undergoing slow thinning and retreat. This thesis highlights the advantages of long term and ongoing monitoring, and the development of continuous records, rather than intermittent and isolated measurements, for the detection and interpretation of variability in the flow regime of large temperate valley glaciers. The findings presented here offer improvements to the measurement of glacier flow velocities from optical satellite imagery, and provide new insights into the response of the Tasman Glacier to climatic and environmental change. Such insights may help improve understanding of temperate, debris covered, valley glacier behaviour, and strengthen predictions of their response to future climate change
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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