A sliver polygon, in the context of Geographic Information Systems (GIS), is a small polygon found in vector data that is an artifact of error rather than representing a real-world feature. [1] They have been a recognized source of error since overlay was first invented in the 1970s.
These meaningless polygons were recognized early in the development of GIS. In fact, in 1978 Michael Goodchild found that they constituted the majority of all polygons in some operational GIS. [2] Early programs for performing vector overlay, such as WHIRLPOOL, developed mechanisms for preventing sliver polygons, such as the "epsilon filter" (fuzzy tolerance) [3] In recent decades, most of the focus of software vendors and practitioners has been on improving the quality of GIS data to prevent them.
Sliver polygons are typically created when polygons are automatically generated from lines that should be coincident (e.g., an international boundary following a river de jure, or two adjacent counties) but are not, due to the natural discrepancies that arise from manual or automated digitization. This can occur when a single layer is digitized and adjacent polygons are traced separately, or during vector overlay when separate polygon layers are merged.
They can be prevented during the digitization process by ensuring that lines that are supposed to be coincident actually are coincident. The editing systems in most GIS software provides for snapping, in which a point placed near an existing point "snaps" to the exact same coordinate. The user can usually control the process, such as setting a snap tolerance (the maximum distance to consider "close enough"), and which layers and which parts of those layers to snap to. [4] In addition, many systems have tracing tools, so that newly digitized lines can exactly follow existing lines.
During the overlay process, the preferred mode of sliver polygon prevention is the use of a fuzzy tolerance, which is sometimes called a "xy tolerance" (ArcGIS) or "snapping threshold" (GRASS), and was originally called an "epsilon filter." [3] The user provides a tolerance distance, and as the lines of the input layers are being combined topologically, any lines that are roughly parallel, consistently closer together than the fuzzy tolerance, are combined into a single line. The choice of a proper fuzzy tolerance depends on the situation, especially the scale of the data, and can be a challenge. If it is set too large, narrow but meaningful polygons (e.g., islands in a river) can be collapsed. [5]
If sliver polygons still exist in a dataset, they may be removed manually or automatically. Some GIS systems provide a tool for topological verification, which searches for topological errors in a data set (sliver polygons being one among many potential types of error), flagging them for correction. [6] One option is to attempt to remove them automatically; many GIS systems provide a tool (usually called Eliminate) that finds very small polygons and merges them into one of the adjacent polygons. There is usually a parameter to pre-select, in situations where there is more than one adjacent polygon, either the largest adjacent polygon or the one with the longest common boundary. [7]
A geographic information system (GIS) consists of integrated computer hardware and software that store, manage, analyze, edit, output, and visualize geographic data. Much of this often happens within a spatial database, however, this is not essential to meet the definition of a GIS. In a broader sense, one may consider such a system also to include human users and support staff, procedures and workflows, the body of knowledge of relevant concepts and methods, and institutional organizations.
A GIS file format is a standard for encoding geographical information into a computer file, as a specialized type of file format for use in geographic information systems (GIS) and other geospatial applications. Since the 1970s, dozens of formats have been created based on various data models for various purposes. They have been created by government mapping agencies, GIS software vendors, standards bodies such as the Open Geospatial Consortium, informal user communities, and even individual developers.
A transport network, or transportation network, is a network or graph in geographic space, describing an infrastructure that permits and constrains movement or flow. Examples include but are not limited to road networks, railways, air routes, pipelines, aqueducts, and power lines. The digital representation of these networks, and the methods for their analysis, is a core part of spatial analysis, geographic information systems, public utilities, and transport engineering. Network analysis is an application of the theories and algorithms of graph theory and is a form of proximity analysis.
A GIS software program is a computer program to support the use of a geographic information system, providing the ability to create, store, manage, query, analyze, and visualize geographic data, that is, data representing phenomena for which location is important. The GIS software industry encompasses a broad range of commercial and open-source products that provide some or all of these capabilities within various information technology architectures.
The shapefile format is a geospatial vector data format for geographic information system (GIS) software. It is developed and regulated by Esri as a mostly open specification for data interoperability among Esri and other GIS software products. The shapefile format can spatially describe vector features: points, lines, and polygons, representing, for example, water wells, rivers, and lakes. Each item usually has attributes that describe it, such as name or temperature.
ArcGIS is a family of client, server and online geographic information system (GIS) software developed and maintained by Esri. ArcGIS was first released in 1999 and originally was released as ARC/INFO, a command line based GIS system for manipulating data. ARC/INFO was later merged into ArcGIS Desktop, which was eventually superseded by ArcGIS Pro in 2015. ArcGIS Pro works in 2D and 3D for cartography and visualization, and includes machine learning (ML).
gvSIG, geographic information system (GIS), is a desktop application designed for capturing, storing, handling, analyzing and deploying any kind of referenced geographic information in order to solve complex management and planning problems. gvSIG is known for having a user-friendly interface, being able to access the most common formats, both vector and raster ones. It features a wide range of tools for working with geographic-like information.
QGIS is a free and open-source cross-platform desktop geographic information system (GIS) application that supports viewing, editing, printing, and analysis of geospatial data.
Georeferencing or georegistration is a type of coordinate transformation that binds a digital raster image or vector database that represents a geographic space to a spatial reference system, thus locating the digital data in the real world. It is thus the geographic form of image registration. The term can refer to the mathematical formulas used to perform the transformation, the metadata stored alongside or within the image file to specify the transformation, or the process of manually or automatically aligning the image to the real world to create such metadata. The most common result is that the image can be visually and analytically integrated with other geographic data in geographic information systems and remote sensing software.
In cartography and geographic information systems, rubbersheeting is a form of coordinate transformation that warps a vector dataset to match a known geographic space. This is most commonly needed when a dataset has systematic positional error, such as one digitized from a historical map of low accuracy. The mathematics and procedure are very similar to the georeferencing of raster images, and this term is occasionally used for that process as well, but image georegistration is an unambiguous term for the raster process.
A georelational data model is a geographic data model that represents geographic features as an interrelated set of spatial and attribute data. The georelational model was the dominant form of vector file format during the 1980s and 1990s, including the Esri coverage and Shapefile.
In geographic information systems (GIS) and spatial analysis, buffer analysis is the determination of a zone around a geographic feature containing locations that are within a specified distance of that feature, the buffer zone. A buffer is likely the most commonly used tool within the proximity analysis methods.
A geographic data model, geospatial data model, or simply data model in the context of geographic information systems, is a mathematical and digital structure for representing phenomena over the Earth. Generally, such data models represent various aspects of these phenomena by means of geographic data, including spatial locations, attributes, change over time, and identity. For example, the vector data model represents geography as collections of points, lines, and polygons, and the raster data model represent geography as cell matrices that store numeric values. Data models are implemented throughout the GIS ecosystem, including the software tools for data management and spatial analysis, data stored in a variety of GIS file formats, specifications and standards, and specific designs for GIS installations.
Proximity analysis is a class of spatial analysis tools and algorithms that employ geographic distance as a central principle. Distance is fundamental to geographic inquiry and spatial analysis, due to principles such as the friction of distance, Tobler's first law of geography, and Spatial autocorrelation, which are incorporated into analytical tools. Proximity methods are thus used in a variety of applications, especially those that involve movement and interaction.
Geospatial PDF is a set of geospatial extensions to the Portable Document Format (PDF) 1.7 specification to include information that relates a region in the document page to a region in physical space — called georeferencing. A geospatial PDF can contain geometry such as points, lines, and polygons. These, for example, could represent building locations, road networks and city boundaries, respectively. The georeferencing metadata for geospatial PDF is most commonly encoded in one of two ways: the OGC best practice; and as Adobe's proposed geospatial extensions to ISO 32000. The specifications also allow geometry to have attributes, such as a name or identifying type.
Geospatial topology is the study and application of qualitative spatial relationships between geographic features, or between representations of such features in geographic information, such as in geographic information systems (GIS). For example, the fact that two regions overlap or that one contains the other are examples of topological relationships. It is thus the application of the mathematics of topology to GIS, and is distinct from, but complementary to the many aspects of geographic information that are based on quantitative spatial measurements through coordinate geometry. Topology appears in many aspects of geographic information science and GIS practice, including the discovery of inherent relationships through spatial query, vector overlay and map algebra; the enforcement of expected relationships as validation rules stored in geospatial data; and the use of stored topological relationships in applications such as network analysis. Spatial topology is the generalization of geospatial topology for non-geographic domains, e.g., CAD software.
Vector tiles, tiled vectors or vectiles are packets of geographic data, packaged into pre-defined roughly-square shaped "tiles" for transfer over the web. This is an emerging method for delivering styled web maps, combining certain benefits of pre-rendered raster map tiles with vector map data. As with the widely used raster tiled web maps, map data is requested by a client as a set of "tiles" corresponding to square areas of land of a pre-defined size and location. Unlike raster tiled web maps, however, the server returns vector map data, which has been clipped to the boundaries of each tile, instead of a pre-rendered map image.
The Harvard Laboratory for Computer Graphics and Spatial Analysis pioneered early cartographic and architectural computer applications that led to integrated geographic information systems (GIS). Some of the Laboratory's influential programs included SYMAP, SYMVU, GRID, CALFORM, and POLYVRT. The Laboratory's Odyssey project created a geographic information system that served as a milestone in the development of integrated mapping systems. The Laboratory influenced numerous computer graphic, mapping and architectural systems such as Intergraph, Computervision, and Esri.
Vector overlay is an operation in a geographic information system (GIS) for integrating two or more vector spatial data sets. Terms such as polygon overlay, map overlay, and topological overlay are often used synonymously, although they are not identical in the range of operations they include. Overlay has been one of the core elements of spatial analysis in GIS since its early development. Some overlay operations, especially Intersect and Union, are implemented in all GIS software and are used in a wide variety of analytical applications, while others are less common.
A spatial join is an operation in a geographic information system (GIS) or spatial database that combines the attribute tables of two spatial layers based on a desired spatial relation between their geometries. It is similar to the table join operation in relational databases in merging two tables, but each pair of rows is correlated based on some form of matching location rather than a common key value. It is also similar to vector overlay operations common in GIS software such as Intersect and Union in merging two spatial datasets, but the output does not contain a composite geometry, only merged attributes.