Lower Bounds for Shoreline Searching With 2 or More Robots

Searching for a line on the plane with $n$ unit speed robots is a classic online problem that dates back to the 50's, and for which competitive ratio upper bounds are known for every $n\geq 1$. In this work we improve the best lower bound known for $n=2$ robots from 1.5993 to 3. Moreover we prove that the competitive ratio is at least $\sqrt{3}$ for $n=3$ robots, and at least $1/\cos(\pi/n)$ for $n\geq 4$ robots. Our lower bounds match the best upper bounds known for $n\geq 4$, hence resolving these cases. To the best of our knowledge, these are the first lower bounds proven for the cases $n\geq 3$ of this several decades old problem.Comment: This is an updated version of the paper with the same title which will appear in the proceedings of the 23rd International Conference on Principles of Distributed Systems (OPODIS 2019) Neuchatel, Switzerland, July 17-19, 201

Remote sensing of tidal networks and their relation to vegetation

The study of the morphology of tidal networks and their relation to salt marsh vegetation is currently an active area of research, and a number of theories have been developed which require validation using extensive observations. Conventional methods of measuring networks and associated vegetation can be cumbersome and subjective. Recent advances in remote sensing techniques mean that these can now often reduce measurement effort whilst at the same time increasing measurement scale. The status of remote sensing of tidal networks and their relation to vegetation is reviewed. The measurement of network planforms and their associated variables is possible to sufficient resolution using digital aerial photography and airborne scanning laser altimetry (LiDAR), with LiDAR also being able to measure channel depths. A multi-level knowledge-based technique is described to extract networks from LiDAR in a semi-automated fashion. This allows objective and detailed geomorphological information on networks to be obtained over large areas of the inter-tidal zone. It is illustrated using LIDAR data of the River Ems, Germany, the Venice lagoon, and Carnforth Marsh, Morecambe Bay, UK. Examples of geomorphological variables of networks extracted from LiDAR data are given. Associated marsh vegetation can be classified into its component species using airborne hyperspectral and satellite multispectral data. Other potential applications of remote sensing for network studies include determining spatial relationships between networks and vegetation, measuring marsh platform vegetation roughness, in-channel velocities and sediment processes, studying salt pans, and for marsh restoration schemes

Coordination between leaf and root traits in Mediterranean coastal dune plants

Plant trait-based functional spectra are crucial to assess ecosystem functions and services. Whilst most research has focused on aboveground vegetative traits (leaf economic spectrum, LES), contrasting evidence on any coordination between the LES and root economic spectrum (RES) has been reported. Studying spectra variation along environmental gradients and accounting for species' phylogenetic relatedness may help to elucidate the strength of coordination between above- and belowground trait variation.center dot We focused on leaf and root traits of 39 species sampled in three distinct habitats (front, back and slack) along a shoreline-inland gradient on coastal dunes. We tested, within a phylogenetic comparative framework, for the presence of the LES and RES, for any coordination between these spectra, and explored their relation to variation in ecological strategies along this gradient.center dot In each habitat, three-quarters of trait variation is captured in two-dimensional spectra, with species' phylogenetic relatedness moderately influencing coordination and trade-off between traits. Along the shoreline-inland gradient, aboveground traits support the LES in all habitats. Belowground traits are consistent with the RES in the back-habitat only, where the environmental constraints are weaker, and a coordination between leaf and root traits was also found, supporting the whole-plant spectrum (PES).center dot This study confirms the complexity when seeking any correlation between the LES and RES in ecosystems characterized by multiple environmental pressures, such as those investigated here. Changes in traits adopted to resist environmental constraints are similar among species, independent of their evolutionary relatedness, thus explaining the low phylogenetic contribution in support of our results

Tectonically controlled subglacial lakes on the flanks of the Gamburtsev Subglacial Mountains, East Antarctica

The morphology of surface lakes strongly influences their ecology and limnology (Wetzel, 2001). This morphology is a result of both the geologic processes that produce topographic basins and the regional climatic and local hydrologic processes that control water depth and sediment infilling (Carroll and Bohacs, 1999). Although basin forming processes range from glacial scour to meteorite impacts (Cohen, 2003), the deepest, oldest surface lakes are tectonically controlled (Meybeck, 1995) and contain diverse exotic ecosystems (Rossiterm and Kawanabe, 2000). Subglacial lakes are also thought to be ancient systems that may contain exotic biota (Bulat et al., 2004; Karl et al., 1999; Priscu et al., 1999). Here we present evidence for the scale and configuration of 2 large subglacial lakes in East Antarctica that together with Lake Vostok define a province of major lakes on the flanks of the Gamburtsev Subglacial Mountains. Spatially-defined in the new Moderate Resolution Imaging Spectroradiometer (MODIS) imagery of Antarctica (T. Scambos et al., A MODIS-based mosaic of Antarctica: MOA, submitted to Remote Sensing of Environment, 2005, hereinafter referred to as Scambos et al., submitted manuscript, 2005), these lakes are aligned parallel to Lake Vostok. Other data shows that they are distinguished by distinct gravity lows, flat ice surface slopes and have estimated water depths of at least 900 m. Surface elevation data indicates that large deep subglacial lakes have a profound influence on the regional ice sheet topography and probably ice sheet flow. These deep subglacial lakes with elongate, rectilinear morphology are tectonically controlled features. Unlike the shallow lakes in West Antarctica and beneath Dome Concordia, these deep subglacial lakes remained stable environments through many glacial cycles since their origin 10-35 Ma enabling the development of novel ecosystems

Marine record of late quaternary glacial-interglacial fluctuations in the Ross Sea and evidence for rapid, episodic sea level change due to marine ice sheet collapse

Some of the questions to be addressed by SeaRISE include: (1) what was the configuration of the West Antarctic ice sheet during the last glacial maximum; (2) What is its configuration during a glacial minimum; and (3) has it, or any marine ice sheet, undergone episodic rapid mass wasting. These questions are addressed in terms of what is known about the history of the marine ice sheet, specifically in Ross Sea, and what further studies are required to resolve these problems. A second question concerns the extent to which disintegration of marine ice sheets may result in rises in sea level that are episodic in nature and extremely rapid, as suggested by several glaciologists. Evidence that rapid, episodic sea level changes have occurred during the Holocene is also reviewed

Tidal flood water withdrawal, with special reference to Jupiter Inlet, Florida (M.S.Engineering Thesis)

The focus of this study was the flow patterns of a flood tide near an inlet. The objectives were to examine flood flow patterns with particular reference to non-uniform or selective withdrawal as influenced by bottom topography and longshore currents, and to test the applicability of conceptually simple analytic solutions to realistic sandy inlet bottom topographies, which often include an ebb shoal. Specifically, the applicability of three analytic solutions, two of which include offshore selective withdrawal, to modeling of tidal water withdrawal during flood tide under variable bottom topography and varying ratios of longshore current to inlet velocity, was examined. The three analytic solutions, including those for a horizontal (flat) bottom, a linearly sloping bottom and a logarithmically sloping bottom, together with a uniform longshore current, were derived using potential flow theory. These solutions exhibit uniformly distributed flows, selective offshore withdrawal, or an exaggerated offshore withdrawal, respectively, depending on the bottom slope. In order to investigate the flow patterns that exist during flood flow at a real inlet, experiments were conducted in a fixed bed hydrodynamic model of Jupiter Inlet, Florida. Measurements were made to determine streamlines and velocities. A field study at the prototype also tracked drogue patterns to determine streamlines and velocities. The physical model tests compared well with the field data. Comparison of the laboratory and field data was then made to the analytic solutions to determine whether the topography at Jupiter Inlet, which includes a well-developed ebb shoal, simulates a flat, mean linearly or logarithmically sloping bottom. By comparing velocities at six selected points, a significant relationship between the physical model and field data to the flat bottom analytic solution was evident. The physical model tests and field data suggested that the flood tidal prism was drawn from the region predominantly shoreward of the ebb shoal, thus implying a nearshore selective withdrawal. Because the flood tidal prism was drawn from the nearshore, the flow patterns at Jupiter Inlet did not resemble the analytic solutions of a linearly or logarithmically sloping bottom, even though over a relatively long distance offshore, the bottom topography does slope offshore at this inlet. In general, different inlet topographies would lend themselves to different analytic solutions, two examples being 1) the linearly sloping bottom of Koombana Bay Inlet, Australia, which shows an offshore selective withdrawal and 2) the basin-like nearfield topography of Jupiter Inlet which shows a more uniform nearshore withdrawal. The implications of this study are relevant to inlet management issues such as the mining of an ebb shoal for use as a source of beach sediment and changes in larval transport patterns due to jetty modifications. (Document has 97 pages.

Climigration? Population and climate change in Arctic Alaska

Residents of towns and villages in Arctic Alaska live on “the front line of climate change.” Some communities face immediate threats from erosion and flooding associated with thawing permafrost, increasing river flows, and reduced sea ice protection of shorelines. The term climigration, referring to migration caused by climate change, originally was coined for these places. Although initial applications emphasized the need for government relocation policies, it has elsewhere been applied more broadly to encompass unplanned migration as well. Some historical movements have been attributed to climate change, but closer study tends to find multiple causes, making it difficult to quantify the climate contribution. Clearer attribution might come from comparisons of migration rates among places that are similar in most respects, apart from known climatic impacts. We apply this approach using annual 1990–2014 time series on 43 Arctic Alaska towns and villages. Within-community time plots show no indication of enhanced out-migration from the most at-risk communities. More formally, there is no significant difference between net migration rates of at-risk and other places, testing several alternative classifications. Although climigration is not detectable to date, growing risks make either planned or unplanned movements unavoidable in the near future

Interaction Templates for Multi-Robot Systems

This work describes a framework for multi-robot problems that require or utilize interactions between robots. Solutions consider interactions on a motion planning level to determine the feasibility and cost of the multi-robot team solution. Modeling these problems with current integrated task and motion planning (TMP) approaches typically requires reasoning about the possible interactions and checking many of the possible robot combinations when searching for a solution. We present a multi-robot planning method called Interaction Templates (ITs) which moves certain types of robot interactions from the task planner to the motion planner. ITs model interactions between a set of robots with a small roadmap. This roadmap is then tiled into the environment and connected to the robots’ individual roadmaps. The resulting combined roadmap allows interactions to be considered by the motion planner. We apply ITs to homogeneous and heterogeneous robot teams under both required and optional cooperation scenarios which previously required a task planning method. We show improved performance over a current TMP planning approach