Lake Boundary Research Articles

Sedimentation flux in a large subtropical lake: Spatiotemporal variations and relation to primary productivity

Spatial and temporal heterogeneity in sedimentation of particulate organic material (POM) was studied in a large subtropical lake, and its export from the upper mixed layer was quantified. Sedimentation fluxes were measured over 4 yr with traps deployed in pelagic and littoral areas, the benthic boundary layer (BBL), and the lake interior. Analysis of fluxes and composition of collected material showed that traps deployed at the lake periphery and in the BBL notably overestimate the export flux of newly produced POM. The best estimation of the primary production (PP) export from the upper mixed layer was achieved when traps were deployed in the quiescent hypolimnion, where the effect of lake boundaries is negligible. The proportion and composition of POM exported from the upper productive stratum was dependent on lake thermal and chemical structure and on the dominant phytoplankton species. The dynamics of photopigments (chlorophyll and β‐carotene) collected in hypolimnetic traps reflected the composition and abundance of phytoplankton in the upper mixed stratum. Despite large variation in algal community composition, the ratio of POM sedimentation flux to PP (export ratio) changed only slightly (average ~ 20%) throughout the stratified period. The observed temporal and spatial variability of sedimentation flux in response to ambient conditions was influenced by secondary processes (resuspension at the lake boundaries and oversampling under turbulent conditions) as intensely as by the actual export of POM.

Assessment of Three Mapping Techniques to Delineate Lakes and Ponds in a Canadian High Arctic Wetland Complex

Recent evidence points to warmer and wetter conditions for Arctic regions. It is not clear how High Arctic environments, in particular extensive wetlands, will respond to these rapid changes. Sustainability studies of wetland complexes will require accurate mapping of water bodies (lakes, ponds, streams) over time. This information is critical to obtain reliable estimates of water inputs (e.g., snowmelt), outputs (e.g., evaporation), and storage, and to assess environmental change (e.g., pond desiccation, expansion, or both). Numerous tools and techniques are available to provide this information, though each approach has its benefits and limitations. In this study, we systematically explore the differences and similarities inherent in three different techniques used to delineate lakes and small ponds at an extensive wetland area near Creswell Bay, Somerset Island, Nunavut (72°43' N, 94°15' W). The mapping techniques compared are satellite mapping (extraction from a satellite-based land cover map), black-and-white aerial photography, and a topographic map sheet. Results indicate that while all three techniques could delineate large lake boundaries and their positions successfully, they differed in the number of ponds they delineated, as well as their boundaries and positions. A misrepresentation of water bodies can hinder hydrologic studies of Arctic water resources, and the results of this study emphasize the need to apply appropriate mapping techniques at different scales. The three mapping techniques are based on data from three different dates over several decades, so the different results also may indicate short- and long-term environmental change.

Spatial and temporal variations of the water–sediment thermal structure in shallow ice-covered Lake Vendyurskoe (Northwestern Russia)

The thermal structures in the vicinity of the ice–water and water–sediment boundaries of a shallow lake, L. Vendyurskoe (Northwestern Russia) during four winter seasons are described. The heat flux at the water-ice boundary was 0.1–0.2 W m−2 during winter. The maximal heat flux at the water–sediment boundary was 4.5 W m−2 at the beginning and 0.5 W m−2 at the end of winter. The daily average value of the solar radiation penetrating into the water was 0.5 W m−2 during main part of winter and 2–50 W m−2 during April. During winter, temperature showed an oscillation in the vicinity of the sediment-water interface. Most periods corresponding to the main oscillation frequencies in the near-bottom water layer (0–0.4 m) and upper layer sediment (0–0.35 m), identified by FFT analysis, fall within the scale of synoptic variations (3–10 days), and in a number of cases were equal to 1 day. The theoretical periods of the first baroclinic seiche mode of Lake Vendyurskoe are 4.5–8.5 days that compares well with identified temperature oscillation periods. The comparison between the rate of heat content change in a water column and the difference of vertical heat fluxes from sediment to water and from water to ice show that the horizontal heat transport takes place in the lake during winter as a result of heat advection along the bottom.

LANDSAT Sub‐pixel Analysis in Mapping Impact of Climatic Variability on Prairie Pothole Changes

AbstractThe Prairie Pothole Region in the United States contains millions of seasonal, semi‐permanent, or permanent lakes and wetlands that typically range in size from 0.1 to 10 ha. These lakes and wetlands are vulnerable to climate change, especially in our study area in South Dakota, in which a period of deluge following a sharp drought considerably expanded the areal extent of prairie pothole lakes during the last decade of the twentieth century. Preliminary estimates of lake areas, determined using LANDSAT 5 and 7 images, had appreciable errors especially for the smallest of these lakes. This article describes a new sub‐pixel approach integrated with a CART (Classification and Regression Tree) model using a GIS (Geographical Information System) to quantify mixed water pixels along lake boundaries to improve area estimations for pothole lakes. Errors in estimated area were typically 10% or less for lakes greater than 1 ha in size. An analysis of lakes in our study area demonstrates how lake area changed with the transition from drought to deluge. Small lakes exhibited a distinct seasonal variation in contrast to large lakes that tended to follow precipitation trends more broadly. The total water area of lakes is consistent with broad variation in rainfall.

Fusion algorithm for the information of lake boundary integration from multi-temporalremote sensing images

为了有效改善遥感影像提取湖泊边界信息的可靠性和精度,减少人为误差,提出了一种利用多时相遥感影像提取边界信息的加权平均融合算法以及误差的域法修正处理方法.结果表明,该方法能有效融合各时相影像信息,提高湖泊边界信息提取的可靠性,并且对融合时相变化较大的湖泊边界都有一定的普适性.通过融合算法提取的呼伦湖面积为1928.35km²,修正后的面积为1929.85km².通过利用地统计学理论对算法的验证及误差分析,得出相对误差空间变异拟合模型的块金方差与基台值之比都小于25%,具有很强的空间相关性,修正后的数据空间相关性要优于融合数据,空间变程也得到了有效地降低,显示修正后的数据对半方差函数理论模型的拟合程度更好.;For improving the reliability and accuracy of lake boundary integration information from remote sensing images andreducing human error, this paper put forwards a weighted average algorithm for integrating of the border information extracted byusing the multi-temporal remote sensing images, and a processing approach of error interzone correction which can integrate thevarious temporal information effectively and improve the reliability of extracting the lake border information. According to thisfusion algorithm the area of Lake Hulun is 1928.35km². Using the geostatistics theory to validate the errors, the area is 1929.85km².The results illuminated that the ratio of nugget to sill is less than 25%, and the spatial correlation of the corrected data is superior tothat of the integrated data. Furthermore the spatial variation range has been reduced effectively, and the extent of fitting thetheoretical model is better in the corrected data than the original data.

Object-level change detection based on full-scale image segmentation and its application to Wenchuan Earthquake

Aiming at object fragmentation and poor detection results caused by discontinuous segmentation scale in object-level change detection, a new object-level change detection method based on the full-scale object tree is presented in this paper. The core idea of this new algorithm is to establish the full-scale object tree based on convexity model theory and integrate full-scale image segmentation techniques and change detection into the whole process. Some Wenchuan Earthquake images are taken as an example to discuss the new method for earthquake damage detection and evaluation in urban area, landslide detection, and extraction of barrier lake boundary. The application shows that the new method is robust and it provides an advanced tool for the quantitative detection and evaluation of earthquake damage.

An automatic cloud mask algorithm based on time series of MODIS measurements

Quality of aerosol retrievals and atmospheric correction over land depends strongly on accuracy of the cloud mask (CM) algorithm. The heritage CM algorithms developed for AVHRR and MODIS use the latest sensor measurements of spectral reflectance and brightness temperature and perform processing at the pixel level. The algorithms are threshold‐based and empirically tuned. They do not explicitly address the classical problem of cloud search, wherein the baseline clear‐skies scene is defined for comparison. Here we report on a new land CM algorithm, which explicitly builds and maintains a reference clear‐skies image of the surface (refcm) using a time series of MODIS measurements. The new algorithm, developed as part of the multiangle implementation of atmospheric correction (MAIAC) algorithm for MODIS, relies on the fact that clear‐skies images of the same surface area have a common textural pattern, defined by the surface topography, boundaries of rivers and lakes, distribution of soils and vegetation, etc. This pattern changes slowly given the daily rate of global Earth observations, whereas clouds introduce high‐frequency random disturbances. Under clear skies, consecutive gridded images of the same surface area have a high covariance, whereas in presence of clouds covariance is usually low. This idea is central to initialization of refcm, which is used to derive cloud mask in combination with spectral and brightness temperature tests. The refcm is continuously updated with the latest clear‐skies MODIS measurements, thus adapting to seasonal and rapid surface changes. The algorithm is enhanced by an internal dynamic land–water–snow classification coupled with a surface change mask. An initial comparison shows that the new algorithm offers the potential to perform better than the MODIS MOD35 cloud mask in situations where the land surface is changing rapidly and over Earth regions covered by snow and ice.

Anisotropic elastocreep in glacial ice: A mechanism for intergranular melt and recrystallization

[1] Bonded ice crystals under pressure are in a heterogeneous stress state because of the mechanical anisotropy of constituent grains. This condition plays a role in intergranular melt and recrystallization, which in turn influence properties such as permeability and biologic habitability. To examine this, we develop an anisotropic elastocreep model simulating subgrain-scale stresses in polycrystalline ice, choosing in particular the thermal and deformational regime of the Lake Vostok accretion ice. A critical assumption with regard to localized melting is the macroscopic deviatoric stress. Our calculations indicate that for a macroscopic deviatoric stress of 50 kPa, ice at 0.04°C below the bulk melting temperature would contain regions, at scales ∼1% the grain size, where the liquid phase is thermodynamically favored. This translates to scales of ∼5 mm in the lower few meters of the Vostok accretion under a deviatoric stress that is plausible near the lake perimeter. Less internal melting is indicated in analyses of ice relatively far from the lake margins due to the assumed presence of lower macroscopic deviatoric stress. Simulated strain energy jumps of 1–2% across grain boundaries of accretion ice near the lake boundaries are large relative to free surface energy, indicating the importance of the former in boundary migration recrystallization. Collectively, these findings are consistent with measured dislocation densities (>107–1010 m−2) and irregular grain morphologies found in core samples. The calculated melting point depressions associated with anisotropy fall short of experimental values by a factor of ∼2, while the calculated strain rates deviate by ∼20%.

The response of lake-glacier variations to climate change in Nam Co Catchment, central Tibetan Plateau, during 1970–2000

Based upon the 1970 aero-photo topographic map, and TM/ETM satellite images taken in 1991 and 2000, the authors artificially interpreted boundaries of lake and glaciers in Nam Co Catchment, and quantified lake-glacier area variations in different stages by “integrated method” with the support of GIS. Results show that from 1970 to 2000, lake area increased from 1942.34 km2 to 1979.79 km2 at a rate of 1.27 km2/a, while glacier area decreased from 167.62 km2 to 141.88 km2 at a rate of 0.86 km2/a. The increasing rate of lake in 1991–2000 was 1.76 km2/a that was faster than 1.03 km2/a in 1970–1991, while in the same period of time, the shrinking rates of glaciers were 0.97 km2/a and 0.80 km2/a respectively. Important factors, relevant to lake and glacier response to the climate, such as air temperature, precipitation, potential evapotranspiration and their values in warm and cold seasons, were discussed. The result suggests that temperature increasing is the main reason for the accelerated melting of glaciers. Lake expansion is mainly induced by the increase of the glacier melting water, increase of precipitation and obvious decrease of potential evapotranspiration. Precipitation, evaporation and their linkages with lake enlargement on regional scale need to be thoroughly studied under the background of global warming and glacier retreating.

Exchange between littoral and pelagic waters in a stratified lake due to wind-induced motions: Lake Kinneret, Israel

We used field data and three-dimensional numerical simulations to investigate the exchange processes between the lake boundaries (littoral) and lake interior (pelagic) due to wind-induced motions in Lake Kinneret, Israel. The field data showed large-scale metalimnion oscillations with amplitudes up to 10 m in response to westerly diurnal winds, the existence of a well-defined turbidity intrusion into the metalimnion of the lake and a benthic boundary layer (BBL). The observed vertical and horizontal movements of the turbidity were explained by the vertical and horizontal advection associated with basin-scale wave motions and a residual circulation set up by the basin-scale motions and the wind field. The horizontal advective transport in the metalimnion, associated with the velocities induced by the basin-scale mode-two Poincare wave, provided the primary control for the exchange between the lake boundaries and lake interior on daily time scales. Detailed comparison of simulation results with both temperature and velocity profiles revealed excellent agreement for time scales from hours to days. The validated numerical model was used to extract the residual motions that provided the secondary exchange flows, in the various regions of the lake on time scales from days to weeks. The residual motion in the surface layer consisted of a combination of an anticlockwise gyre in the western half of the lake and a weak clockwise gyre along the eastern boundary that were forced by the curl of the wind field. In the metalimnion, residual motions were predominantly forced by the basin-scale internal wave motions and an anticlockwise gyre established itself throughout the whole basin. Lastly in the BBL, residual motions consisted of an anticlockwise gyre, but the geographic distribution was patchy. Together these results provide new insight into a proper characterization of the processes underlying the flux paths of water and particles between the lake boundaries and lake interior.

1,000 wells for Darfur

A new humanitarian mission called “1,000 Wells for Darfur” grew out of the discovery from recent space data of an ancient megalake in a large basin in that region. Eman Ghoneim, a research professor at Boston University's Center for Remote Sensing, and Center director Farouk El‐Baz mapped the ancient lake's boundary using Landsat, RADARSAT, and Shuttle Radar Topography Mission (SRTM) data.The newly mapped lake once occupied 30,750 square kilometers and would have contained approximately 2530 cubic kilometers of water when full during historically humid climate phases.

An Automated GIS Procedure for Delineating River and Lake Boundaries

AbstractDigitization of river and lake boundaries is an essential task in several hydrologic studies, which, in most cases, is performed manually by using aerial photographs. This paper presents an automatic geographic information system (GIS) procedure for delineating river and lake boundaries using aerial photographs. The GIS procedure is based on supervised classification of aerial photographs by using the National Hydrography Dataset (NHD) for creating training areas. Four study areas – Lake Palestine in Texas, Brazos River in Texas, Neuse River in North Carolina, and Snake River in Idaho – are used to demonstrate the approach. The results from the automatic boundary delineation process are evaluated by comparing them to manually digitized boundaries. The boundaries delineated by the automatic procedure matched the manually digitized boundaries with an overall difference of less than five percent in terms of area measurement. Besides saving about 85 percent of manual labor, the GIS procedure provides a consistent way of delineating water bodies, and provides estimation of river channel widths, which, in conjunction with existing NHD data, can be used to estimate average flow depths. Supplementing the regional scale NHD data with local scale attributes such as channel width and depth may lead to increased use of NHD in local scale studies.

Assessment of impact of aquaculture on Kolleru Lake (India) using remote sensing and Geographical Information System

Aquaculture is one of the fastest growing food industries and the rapid growth of aquaculture worldwide has resulted in growing concerns about its impact on important ecosystems. Kolleru Lake, India's largest fresh water body, and Ramsar site have undergone tremendous changes due to the development of aquaculture. To assess the impact of aquaculture on Kolleru, satellite data were found appropriate because of the synoptic-detailed overview and accuracy. Satellite data of IRS 1D, LISS III from 2004 and Survey of India topographic maps from 1967 were processed using image processing techniques in erdas imagine and analysed in Geographical Information System (GIS) such as arc gis 9.1. Land use map prepared from the satellite data was verified in the field using Global Positioning System to check the land and water use classes and its areal extent. It was estimated from the topographic maps of 1967 that the total lake boundary area was 180.38 km2, in which 70.70 km2 had water throughout the year and 100.97 km2 had water during the rainy season. The digital image processing of 2004 satellite data revealed that Kolleru was no longer a lake and the lake area of 62.65 km2 (34.73%) only remained in a degraded state, extensively colonized by macrophytes. The total loss of lake area was 109.02 km2 between 1967 and 2004, in which aquaculture was developed in 99.74 km2, which represented 55.3% of the 1967 lake area. The maximum conversion to aquaculture occurred from the lake liable to be inundated during the 1967 rainy season. The area under agriculture was 16.62 km2 in 2004, the increase in lake area for agriculture between 1967 and 2004 being 8.22 km2 (4.55% of lake). If human induced degradation is allowed to continue, the lake will very soon disappear. Now, the Government of Andhra Pradesh, India, has initiated schemes and laws to restore the lake to its pre-development state. Lessons learnt from the Kolleru Lake cautioned that there is a need for regular monitoring of important water resources throughout the world to protect the biodiversity of the earth.

Boundary flow on a lake slope during Ekman layer arrest

The disruption of an arrested bottom Ekman layer on a lake boundary slope is initiated when buoyancy enters the balance between the Coriolis force and the pressure gradient. This process is shown to behave in a lake in a qualitatively similar manner to the ocean. A greater bottom slope and stronger stratification are found to dramatically shorten the shutdown timescale of the Ekman flux compared to typical values on the continental shelf and slope. Convergence of the flow at the density front, where the thermocline intersects the bottom, leads to a detrainment of bottom boundary layer water into the interior of the lake and thus facilitates the transport of mass and heat independent of turbulence generation in the boundary layer by internal wave interactions. Internal waves complicate the flow structure. The boundary water detrainment and gravitational instability due to Ekman layer dynamics and the critical reflection and vertical overturns associated with the internal waves combine in effective boundary mixing.

Characterizing partial upwellings and surface circulation at Lake Tahoe, California–Nevada, USA with thermal infrared images

Thermal infrared data from recently launched satellite instruments provide an opportunity to address key scientific questions and develop new applications, which could not be addressed or developed with data from earlier instruments. The satellite instruments that provide these data include ASTER, Landsat ETM+, and MODIS, and this study demonstrates how data from them can be used to map upwelling and circulation associated with the transport of heat, solutes, and particles in lakes. Upwelling in a density-stratified water body such as a lake or reservoir results from a surface wind stress being balanced by a horizontal pressure gradient, causing denser water to rise at the upwind lake boundary. Upwellings are considered partial when intermediate-depth water reaches the surface and total when bottom water surfaces. Upwellings are an important part of ecosystem functioning, since they transport nutrients from deeper in the lake, where they accumulate, to the surface layer, where they facilitate phytoplankton growth. Thermal infrared images acquired by ASTER, ETM+, and MODIS can be used to observe partial upwelling events in lakes and provide insight into their spatial variability and horizontal distribution, information totally lacking from conventional in situ measurements. At Lake Tahoe, partial upwellings were found to occur every few days throughout the spring and summer, transporting water from 10–30 m below the surface to the surface layer. They commonly display a jet-like appearance, traveling from the upwind to the downwind side of the lake, with current speeds of 12–17 cm/s. Partial upwellings were found to generally decrease lake clarity, although deeper upwelling events can increase clarity. Sinking zones, other convergence areas, and divergence areas were also observed. The temperature variability associated with upwelling, which could be clearly mapped in the thermal infrared satellite images, illustrates the advantage of synoptic thermal infrared satellite measurements over in situ point measurements alone for detecting upwelling events, since, depending on location, an in situ instrument might not capture an upwelling event. The spatial information conveyed by the synoptic satellite measurements can be used to help improve monitoring of the clarity and general water quality of Lake Tahoe.

Impact evaluation of haizuka dam on its up stream: A case study in hiroshima prefecture, Japan

Japan ranks fifth in the world for the number of large dams. Environmental impacts of large dams are known, such as enormous losses of water or disruption of fish spawning, however, impacts of the dams on their up streams are functions of topography of the up stream. Haizuka Dam is located in Hiroshima Prefecture, Japan and its implementation will start in 2006. This large dam influences its up stream through dam making activities, which occurs in order and with different spatial presences that were categorized into chronological and spatial impacts. In this case study, spatial impacts were further divided into horizontal and vertical ones. The horizontal impacts were identified as new roads, diversion tunnel, dam lake, and submerged cultivated land, while vertical impacts were recognized as submerged historical monuments, slope protections, dam body, and deforested area in the reservoir. There were convergences of spatial and temporal impacts, however, the extent of the impacts was limited to the lake boundary.

Invasion of the noxious weed Nicotiana glauca R. Graham after an episodic flooding event in the arid zone of Australia

During November 1997, an extreme rainfall event caused flooding in the Olary Creek and created a terminal lake within mallee vegetation on Nagaela Station, far western New South Wales. To assess the effect of this rare event on native flora, six 25×25 m 2 grazing exclusion plots were established on flooded (4), and un-flooded control (2) areas. We investigated (i) the distribution of Nicotiana glauca in relation to the 1997 flooding event (ii) the variation in density of N. glauca soil-stored seed between soil in control and exclosure plots (iii) the allelopathic effects of N. glauca on germination of Lactuca sativa seeds. Analysis of N. glauca stem sections revealed a maximum of five growth rings, confirming an origin following the 1997 flood event. At a September 1998 assessment, no N. glauca were found across flooded and control plots. In October 2000 N. glauca occurred at densities of 24 and 12 N. glauca seedlings ha −1 in the flooded unfenced and fenced plots respectively. No N. glauca individuals were recorded either from control fenced or unfenced plots. The soil-stored seed study revealed that larger numbers of N. glauca seeds were recovered from the flooded open plot (1936±968) than in flooded fenced plots (856±428.12) but none was found in the control plots. Leachates obtained from dry leaves and twigs had a significantly greater negative impact ( p>0.001) on germination of L. sativa seeds than leachates obtained from fresh leaves and bark. This study showed that the density of N. glauca is high in areas where grazing had eliminated competition from native shrubs and grasses. N. glauca is potentially a serious weed along the Olary Creek and ephemeral lake boundaries. Management strategies such as reducing grazing pressure from native and introduced animals and/or effective eradication programs should be adopted, to minimize further infestation.

A shape-based approach to change detection of lakes using time series remote sensing images

Shape analysis has not been considered in remote sensing as extensively as in other pattern recognition applications. However, shapes such as those of geometric patterns in agriculture and irregular boundaries of lakes can be extracted from the remotely sensed imagery even at relatively coarse spatial resolutions. This paper presents a procedure for efficiently retrieving and representing shapes of interesting features in remotely sensed imagery using supervised classification, object recognition, parametric contour tracing, and proposed piecewise linear polygonal approximation techniques. In addition, shape similarity can be measured by means of a computationally efficient metric. The study was conducted on a time series of radiometric and geometric rectified Landsat Multispectral Scanner (MSS) images and Thematic Mapper (TM) images, covering the scenes containing lakes in the Nebraska Sand Hills region. The results validate the effectiveness of the proposed processing chain in change detection of lake shapes and show that shape similarity is an important parameter in quantitatively measuring the spatial variations of objects.

Contour grouping with prior models

Conventional approaches to perceptual grouping assume little specific knowledge about the object(s) of interest. However, there are many applications in which such knowledge is available and useful. Here, we address the problem of finding the bounding contour of an object in an image when some prior knowledge about the object is available. We introduce a framework for combining prior probabilistic knowledge of the appearance of the object with probabilistic models for contour grouping. A constructive search technique is used to compute candidate closed object boundaries, which are then evaluated by combining figure, ground, and prior probabilities to compute the maximum a posteriori estimate. A significant advantage of our formulation is that it rigorously combines probabilistic local cues with important global constraints such as simplicity (no self-intersections), closure, completeness, and nontrivial scale priors. We apply this approach to the problem of computing exact lake boundaries from satellite imagery, given approximate prior knowledge from an existing digital database. We quantitatively evaluate the performance of our algorithm and find that it exceeds the performance of human mapping experts and a competing active contour approach, even with relatively weak prior knowledge. While the priors may be task-specific, the approach is general, as we demonstrate by applying it to a completely different problem: the computation of human skin boundaries in natural imagery.

Guest editors' introduction to the special section on perceptual organization in computer vision

THIS issue contains the second installment of the special issue on perceptual organization in computer vision. In the April issue of TPAMI, we published 11 papers addressing basic principles, algorithms, and applications. This month’s papers continue along these lines and raising new issues as well. The first paper in the special section contributes to one of the most significant trends in perceptual organization, the use of graph algorithms to efficiently find regions that globally optimize, or approximately optimize, a grouping property. P. Soundararajan and S. Sarkar present an analysis and experimental evaluation of a set of graph algorithms, showing that some of the common algorithms are practically equivalent with respect to performance. In the next paper, J.H. Elder, A. Krupnik, and L.A. Johnston describe a probabilistic formulation for contour grouping that combines Gestalt grouping cues with prior knowledge about the shape of the objects whose boundary is being detected. They apply this approach to the problem of detecting exact lake boundaries in satellite imagery. S. Wang and J.M. Siskind also study graph algorithms as a grouping mechanism. They present a novel algorithm based on the ratio of two properties of graph edges that are cut in a segmentation. This leads to a polynomial time algorithm that finds globally optimal groupings, which they then enhance using a set of heuristics. Finally, it is fitting that our issue concludes with S.-C. Zhu’s paper, which reviews a large set of past work on modeling visual patterns, an area fundamental to perceptual organization. Professor Zhu presents a broad taxonomy of methods, stressing the importance of generative models in image modeling. The 15 papers we have presented provide a wide range of viewpoints and attack a variety of problems using diverse tools. We find this appropriate since the challenges of perceptual organization are great and many angles on the problem should continue to be explored. Hopefully, the special section will provide a useful snapshot of the state of many of these approaches.