Using WMS with OpenLayers

Introduction

The goal of this project is to use freely available Web Map Service (WMS) layers with OpenLayers to create a dynamic map for use within a web browser. In particular, the hope was to include a land use layer for Canada, and the user could browse to a desired location to determine how the land is being used. If the user knows the coordinates of the place they want to view, they can modify the source code by entering the chosen longitude, latitude, and zoom level. Then when loading the map the images will be centered within the map window. For the following example, only GeoBase and Toporama WMS layers are used, and they can be accessed at no cost and without restrictions.

This tutorial contains a brief outline of Web Map Service (WMS) layers and OpenLayers, and then descriptions of the methods used to create the map displayed in Section 4. This includes discussion of the important aspects of the HTML code that generated the resulting map, as well as why the use of OpenLayers is suitable to achieve the desired goals.

Data and Mapping Tools

WMS Layers

WMS provides a HTTP interface for requesting geo-registered map images from one of more geospatial databases. A WMS request defines the geographic layer(s) and area of interest that will be processed.[1] There is then a response to the request where geo-registered map images (in the following example returned as a PNG) can be displayed within a browser.

If the user has included more than one WMS layer, they have the ability to specify which will be the base layer, and which will be overlay layers. If the user then wants multiple images to be combined, the overlay layers need to be designated transparent. This is not necessary, however, as the user may want only one layer to appear at a time. This can be done by making the transparent flag false for each of the overlay layers, and then the user can manually switch between layers through the web browser. With OpenLayers the ‘LayerSwitcher’ control, which can be added into the code, is a popup that allows the user to do this by simply selecting which layer they want to view.

There are six WMS layers being used in the following example. The first layer, which is being used as the base map is obtained from Geobase, and it contains foreign landmass political boundaries. The additional overlay layers which encompass all of Canada are as follows:

References

OGC WMS Standards [2]