Often you need reference data layers for your basemap or to display your
results in the context of other datasets. Many organizations publish datasets
online that can be readily used in GIS. A popular standard for publishing maps
online is called WMS (Web Map Service). This is a better choice for using
reference layers as you get access to rich datasets in your GIS without the
hassle of downloading or styling the data.
Name your connection. This is not the name of the layer but the name of
service which is offering the WMS layer. A single service usually offers
multiple layers that can be added to your project. The URL that you need to
access a WMS layer is called GetCapabilities. When you access a WMS server
with this parameter in the URL, it returns a list of layers available along
with various metadata. In this case, name the connection as MRDATAUSGS
and the GetCapabilities URL as
http://mrdata.usgs.gov/services/ca?request=getcapabilities&service=WMS&version=1.1.1&.
Click OK.
Next, click on the Connect button to fetch the list of layers available. You
will notice different IDs listed next to the layers. ID 0 means you get a
map of all the layers. If you do not want all the layers, you can expand the
list by clicking on + icon and selecting the layer of interest. Select the
layer 0 for this tutorial.
In the Image encoding section, you need to choose an image
format. Image formats matter a great deal and which one you choose depends on your use case. Here
are some pointers
Quality: PNG is a lossless compressed image format. JPEG is lossy
compressed format. TIFF can be either. That means the quality of PNG
images will be better compared to JPEG. If your main purpose is to print
a map, use PNG.
Speed: Since PNG images are uncompressed and thus larger in size,
they will take longer to load. If you are using the layer in your project
as a reference layer and need to zoom/pan a lot, use JPEG.
Client Support: QGIS supports most of the formats, but if you are
developing web applications, browsers usually do not support TIFF,
so you should choose another format.
Type of data: If your layers are primarily vector, PNG will give better
results. For imagery layers, JPEG is usually a better choice.
For this tutorial, choose JPEG as the format. Change the
Layer name if you wish and click Add.
You will see the layer loaded in the QGIS canvas. You can zoom/pan around
just like any other layer. The way WMS service works is that every time you
zoom/pan, it sends your viewport coordinates to the server and the server
creates an image for that viewport and return it to the client. So there
will be some delay before you see the image for the area after you have
zoomed in. Also, since the data you see is an image, there is no way to
query for attributes like in a regular vector/imagery layer.
You can, however, see some metadata about the layer. Right-click the layer
and choose Properties.
You will notice that the Properties dialog looks different and has fewer
tabs. You can go to the Metadata tab to learn more about the WMS service
and the layers.
