1.3.2. GetMap

"The GetMap operation returns a map. Upon receiving a GeetMap request, a WMS shall either satisfy the request or issue a service exception." (OGC 2004)

The following line shows a GetMap request (the URL is all one line, broken up here for readability):

The parameters "http://www2.dmsolutions.ca/cgi-bin/mswms_gmap? indicate the location of the map server. The parameters for the GetMap request follow after the server location definition.

A GetMap request consists of the following mandatory request parameters

Request Parameter	Description
VERSION=version	Request version
REQUEST=GetMap	Request name
LAYERS=layer_list	Comma-separated list of one or more map layers.
STYLES=style_list	Comma-separated list of one rendering style per requested layer.
CRS=namespace:identifier	Coordinate reference system.
BBOX=minx,miny,maxx,maxy	Bounding box corners (lower left, upper right) in CRS units.
WIDTH=output_width	Width in pixels of map picture.
HEIGHT=output_height	Height in pixels of map picture.
FORMAT=output_format	Output format of map.

There are six more GetMap parameters that can be declared in the URL, but those parameters are optional. Please have a look at the following pdf file (page 26) if you are interested in those parameters: OGC Web Map Service Interface

The result of a GetMap request is a raster image.

Result of a GetMap request (UMN MapServer)

GetMap Exercise

Check out the GetMap request by typing the request string in the address bar of your browser. Watch out that there are no spaces within your URL string. You will get a raster image that you can save on your computer.
To get a new image, you have to change the parameter values of the GetMap request. The parameters version, request, and coordinate system are fix for each request. All other parameters are changeable. But since at the moment, you do not know neither the minimum and maximum coordinates nor the names of the available layers or the available image formats, you can only change the width and height of the requested map (you will be able to change all values after the GetCapabilities chapter). Try it out!

You have no idea how to do it? (Click here for more information)

Take into account that the changing of the width and height parameters only change the width and height of the output image. If their proportion (width / height of output image) is different from the proportion of the original bounding box (width / height of bounding box measured in real coordinates) the content of the output image is distorted. Try it out by taking arbitrary numbers for width and height!