Has anybody utilized the Atlas plugin? I am starting to take a look and setting it up on a project. My question is how do you set up the layer that is utilized for the coverage layer. Is there a way to get a polygon the mimics the view of the map area shown in the Print Composer?
The Atlas plugin is great and works very well indeed. I am using it from QGIS 1.9.0 Master and I recommend you update as it is now integrated into the core.
You can use any vector data as a coverage. This is a very important point. There is nothing special about the coverage file (NOT to be confused with the old ArcInfo data format).
Creating a coverage for Atlas and using it in the plugin is probably best described by example.
Work Flow 1 (I have an existing shapefile with areas I am interested in): Let's say I am making a map of my country's National Parks and I have a shapefile of the National Park boundaries.
I will get a set of maps where I have one map per National Park boundary, automatically zoomed to the boundary's extent plus my margin setting (Atlas->Margin Around Coverage - defaults to 10%) in each case.
Work Flow 2 (I have a set of points and want to show some area around them): I repeat the process above and could simply use the points as they are, in which case the "Margin around coverage" setting will not work because points have zero area and a percentage of zero is nothing. In this case, select the "Fixed Scale" option instead and set the scale for all maps in the Map tab in the usual way.
Work Flow 3 (I have points or areas but want to show a variable sized area around them depending on some attribute): I repeat the work flow above but now I create my coverage file by buffering my features. I can use a conventional buffer or perhaps use the "Squares, Ovals, Diamonds" plugin. This time I select my buffer layer as my Coverage Layer in Atlas and will get one map per feature as per Work Flow 1.
Work Flow 4a (I have a large area and want to create a grid of maps at a given scale covering the whole region): By now you've got the idea of using Atlas! So how do you create the coverage grid. I advocate using the Grid for Atlas Plugin and I suspect that your error message may be because it is intended for a later version of QGIS than the one you are using - which is why I recommend you upgrade to 1.9 Master and hopefully this will remove your problem. I have no problems with this plugin in v1.9 whether my Print Composer is open or not.
When using the plugin, setup your print composer and set your scale in the map tab. Then switch back to the main QGIS window, and run the plugin, identifying the layer you want to make the Atlas for. This will create a set of polygons in a grid or overlapping pattern covering the area defined by your selected layer and at the scale defined in the Print Composer for the given map frame. Go back to the Print Composer and select your new file generated by the plugin as your coverage.
Work Flow 4b (I have a large area and want to create a grid of maps at a given scale covering the whole region - without using the Grid Plugin): If you still can't get the Grid plugin to work all is not lost but you will need a calculator. You can create a grid using the standard Vector-Research Tools->Vector Grid tool. This approach does have the limitation of only creating what the "Grid for Atlas" plugin calls a "classic" grid. You will need to manually work out the cell dimensions based on your map frame dimensions and your desired scale (which should be easy enough if you are working in meters). Ensure you output your grid as polygons and also ensure that when you use this coverage in Atlas you set the margin to 0% otherwise your scale will be wrong (as you effectively already incorporated it into the calculation of your grid cell dimensions).
There is a handy plugin called Grids For Atlas which you can use to create a grid based on the map view in the composer. It is also handy for generating grids based on another map layer, or grids that follow a feature.
Coverage layers in Atlas can also be point or line layers not only polygon layers.
You should also checkout the latest dev build of QGIS as atlas is now built into the print composer with loads more features like text replacements.
I actually received a response form Oslandia, creators of the plugin. They obviously said to move to 1.9. It looks as though I started the plugin prior to having a Print Composer setup.