The Difference Between Coverage and a Shapefile in Terms of Data Structure

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In geographical information systems -- systems for storing, editing, and displaying geographical information on a computer -- coverages and shapefiles are represented as basic vector data models. In terms of data structure, they both consist of a table that lists the coordinates of a start node, an end node and any vertices in between and then indicates which lines go through which points. However, despite their similarities, some fundamental differences exist between them.

Coverage

  • A coverage is what is known as a topological data structure. The coverage data structure consists of a folder that contains multiple so-called feature classes, such as arc, label and polygon. Each feature class does not correspond directly to a geographical feature, but each geographical feature -- land use, street, railroad, as a few examples -- corresponds to a coverage folder. A coverage stores information about neighboring geographical and the relationships between and among them in attribute tables. The attribute tables are linked to hidden binary files containing the geometric information for each feature class.

Coverage Benefits

  • A coverage stores information about shared vertices of adjacent geographical features once rather than twice and so should, at least in theory, produce a file size that is up to 50 percent smaller than a shapefile. However, this is rarely the case in practice, because coverages require additional files to store topological information. Note that although attribute tables represent a large proportion of the overall file size, the proportion is the same regardless of whether geographical features are stored as coverages or shapefiles.

Shapefile

  • The shapefile data structure also links geometric files to attribute files, but each shapefile corresponds directly to a feature class. In a shapefile, geographical features are represented by closed rings, or loops, that allow complex, disconnected and overlapping features to be represented. A geographical feature that is bisected by a street or railroad can be represented as two rings corresponding to two records in the attribute table or as a single ring with two parts corresponding to a single record in the attribute table.

Shapefile Benefits

  • The principal advantage of the shapefile data structure is its simplicity, which allows geographical features to be drawn faster than those stored as a coverage. Furthermore, shapefiles can be copied easily without needing to be explicitly imported or exported and the shapefile specification is supported by numerous software packages. Shapefiles are typically larger than coverages, but as the cost of storage space has fallen the relative file sizes have become less of an issue among GIS users than they once were.

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