In computer graphics terminology, a "texture map" is an image that is displayed on a 3D object in order to simulate the surface appearance or texture of the object. The most common method to create a 3D model of the Earth is to "wrap" a 2D image or map of the entire Earth around a sphere.
In order to use an image as a texture map, the exact correspondence between pixels in the image and points on the sphere must be known. If the image of the Earth is an Equirectangular map (also known as Platte-Carree or Equidistant Cylindrical), then the pixels of the image are laid out in a regular longitude-latitude grid and mapping the image to a sphere is easy.
In order to facilitate both the use of our images as texture maps and overlaying multiple images, all images served by the SVS Image Server use the same projection, an Equirectangular projection based on the WGS84 datum. In WMS 1.3.0, this is indicated by the line
<BoundingBox CRS="CRS:84" minx=minLongitude miny=minLatitude maxx=maxLongitude maxy=maxLatitude/>
within a Layer definition in the Capabilities XML, which becomes
in the GetMap query to the server. In WMS 1.1.1, the syntax is somewhat different, becoming
<BoundingBox SRS="EPSG:4326" minx=minLongitude miny=minLatitude maxx=maxLongitude maxy=maxLatitude/>
The SVS Image Server supports both WMS version 1.1.1 and WMS version 1.3.0. When requesting an image with a GetMap request, one of these two versions must be given (with VERSION=). When requesting an image with WMS 1.1.1, the parameter SRS=EPSG:4326 is required, while CRS=CRS:84 is required when using WMS 1.3.0.
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