Web Map Service
An Open Geospatial Consortium (OGC) standard for requesting georeferenced map images from a server over HTTP. It returns pictures of data, not the data itself.
Detailed Definition
A Web Map Service (WMS) is a standard way to ask a server for a map picture. The Open Geospatial Consortium (OGC) describes it: "This standard provides a simple HTTP interface for requesting geo-registered map images from one or more distributed geospatial databases. A WMS request defines the geographic layer(s) and area of interest to be processed. The response to the request is one or more geo-registered map images (returned as JPEG, PNG, etc) that can be displayed in a browser application." OGC's standards page lists the OpenGIS Web Map Service Implementation Specification, version 1.3.0 (OGC 06-042).
OGC adds that the interface lets a client ask for transparent images "so that layers from multiple servers can be combined." That is how one web map shows a basemap from one agency and survey lines from another.
A real capabilities document
A WMS server describes itself in a capabilities document. BLM's national PLSS service, for example, answers as WMS version 1.3.0 and lists three requests:
- GetCapabilities
- GetMap
- GetFeatureInfo
The same document lists the coordinate systems the service can draw in, including CRS:84, EPSG:4326, and EPSG:3857. See Coordinate System.
What WMS does not do
The response is an image. There are no polygons to measure, no attribute table to filter, and nothing to intersect with another layer. For the features themselves, OGC's standards are the Web Feature Service and its successor, OGC API - Features. See Web Feature Service. For pre-rendered basemaps, OGC's tiled standard is the Web Map Tile Service, which OGC says "complements the existing Web Map Service standard." See Tile Service.
Why it matters in mining claim research
WMS layers are useful as reference: the PLSS grid over imagery, or land status behind a project area. They are not a record. What appears depends on the server's symbols and drawing scale, and the source data carries BLM's warning that "These data are neither legal documents nor land surveys, and must not be used as such." Any number used in research, from an acreage to a count of mining claims, should come from the underlying features or the official record, not from a picture of them.
Related Terms
GIS
A geographic information system: software and data that store, analyze, and display information tied to locations on the earth.
Spatial Data
Data that identifies where something is on the earth and what it is like, stored as vector features, raster grids, or coordinates in records.
API
Application Programming Interface: a defined access point that lets one program request data or functions from another. Many government data services publish APIs.
Raster Data
GIS data stored as a grid of cells, each holding a value such as an elevation, a color, or a class. Used for imagery and continuous surfaces.
Tile Service
A map service that delivers pre-rendered map tiles at fixed scales, so a viewer loads only the tiles it needs. OGC standardizes it as WMTS and OGC API - Tiles.
Web Feature Service
An Open Geospatial Consortium (OGC) standard for querying and retrieving geographic features, with geometry and attributes, from a server over the web.