Abstract
Map browsers currently in place present maps and geospatial information using common image formats such as JPEG or PNG, usually created from a service on demand. This is a clear approach for a simple visualization map browser but prevents the browser from modifying the visualization since the content of the image file represents the intensity of colors of each pixel. In a desktop GIS, a coverage dataset is an array of values quantifying a certain property in each pixel of a subdomain of the space. The standard used to describe and distribute coverages is called web coverage service (WCS). Traditionally, encoding of coverages was too complex for map browsers implemented in JavaScript, relegating the WCS to a data download, a process that creates a file that will be later used in a desktop GIS. The combination of a coverage implementation schema in JSON, binary arrays, and HTML5 canvas makes it possible that web map browsers can be directly implemented in JavaScript.
TopBackground
Coverages represent homogeneous collections of values located in space/time, such as spatio-temporal sensor, image, simulation, and statistics data. Common examples include measures by a static sensor of a variable over time (1-D time series), optical satellite imagery (2-D imagery), series of geometrically corrected satellite images (3-D x/y/t time series), a interpolation representing the temperature distribution of the atmosphere (x/y/z geophysical voxel models), and a simulation representing the evolution of the temperature of the atmosphere over time (4-D x/y/z/t models). Coverages encompass multi-dimensional regular and irregular grids, point clouds, and general meshes. [Baumann et al, 2017]
Often the word “coverage” is used a synonymous of “gridded data”, “raster data” or “imagery”. Even if the three expressions are examples of coverages, this is not the whole picture. For example, non-gridded data (like a river gauge time series) can also be modeled as a coverage. Generally, the concept of coverages encompasses spatio-temporal regular and irregular grids (both discrete and continuous), point clouds, and general meshes.
The OGC Geography Markup Language (GML) (Portele, 2007) mainly focuses on feature data but also provides foundational elements to describe coverages in XML. Following literally the common practices for feature data, it requires the creation of a GML Application Schema (and specialized form of XML Schema) for each coverage type. The definition of Application Schemas has some degrees of freedom and, if we combine this with the lack of easy to find GML Application Schemas catalogues, the risk is that descriptions of the same kind of information done by different organizations result in different data structures, preventing interoperability. It also makes the implementation of applications more difficult because software programmers need to anticipate all possible variations of the Application Schemas.
Key Terms in this Chapter
Geographic information: Information concerning phenomena implicitly or explicitly associated with a location relative to the Earth.
Coordinate Reference System: Coordinate system that is related to the real world by a datum.
Coordinate System: Set of mathematical rules for specifying how coordinates are to be assigned to points.
Irregular Grid: A grid whose grid lines have individual distances along each grid axis.
Coverage: A feature that acts as a function to return values from its range for any direct position within its spatiotemporal domain.
Regular Grid: A grid whose grid lines have a constant distance along each grid axis.
Client: Software component that can invoke an operation from a server.
Partition (of a Coverage): A separately stored coverage acting, by being referenced in the coverage on hand, as one of its components.
Service: Distinct part of the functionality that is provided by an entity through interfaces.
Displaced Grid: A grid whose direct positions are topologically aligned to a grid, but whose geometric positions can vary arbitrarily.
MeSH: A coverage consisting of a collection of curves, surfaces, or solids, respectively.