getMap的WIDTH和HEIGHT参数

7.3.3.8 WIDTH, HEIGHT
The mandatory WIDTH and HEIGHT parameters specify the size in integer pixels of the map to be produced.
The Map CS (6.6.1 and B.2) applies to the map.  WIDTH-1 specifies the maximum value of the i axis in the
Map CS, and HEIGHT-1 specifies the maximum value of the j axis in the Map CS.
If the request is for a picture format, the returned picture, regardless of its MIME type, shall have exactly the
specified width and height in pixels. In the case where the aspect ratio of the BBOX and the ratio width/height
are different, the WMS shall stretch the returned map  so that the resulting pixels could themselves be
rendered in the aspect ratio of the BBOX. In other words, it shall be possible using this definition to request a
map for a device whose output pixels are themselves non-square, or to stretch a map into an image area of a
different aspect ratio.
Map distortions will be introduced if the aspect ratio WIDTH/HEIGHT is not commensurate with X, Y and the
pixel aspect.  Client developers should minimize the  possibility that users will inadvertently request or
unknowingly receive distorted maps.
If a request is for a graphic element format that does not have explicit width and height, the client shall include
the WIDTH and HEIGHT values in the request and a server may use them as helpful information in
constructing the output map.
The optional <MaxWidth> and <MaxHeight> elements in  the service metadata are integers indicating the
maximum width and height values that a client is permitted to include in a single GetMap request.  If either
element is absent the server imposes no limit on the corresponding parameter.
If the WMS server has declared that a Layer has fixed width and height, as described in 7.2.4.7.5, then the
Client shall specify exactly those WIDTH and HEIGHT  values in the GetMap request and the Server may
issue a service exception otherwise.