To prepare a map atlas in QGIS we will use a rectangular mesh, which we will generate using a special algorithm. To begin with, however, we need to define the dimensions of the mesh so that they coincide with the extent of the sheet – in this case, it will be an A3 page with a horizontal orientation.
In the beginning, all we need is a QGIS project with any map base and a vectorised mesh layer. The next step is to create a new print layout, select the appropriate sheet format and read out the length/width. We do not need to process the information itself in any way – we will only use it at the grid layer design stage.
Knowing the dimensions of the page, we can prepare a grid of sheets that we will use to generate the atlas. Page dimensions are given in millimeters. The adopted scale is 1:1000, so the page dimensions need to be multiplied by 1000. The result of such an operation expressed in metres is 420 m in width and 297 m in length respectively. To create the mesh, we use the tool from the Vector -> Research Tools -> Create Mesh tab. In addition to specifying the dimensions of the mesh, we also need to specify the spatial extent of the mesh. This can be calculated from the layer representing the grid.
The base grid layer tightly covers the entire area defined by the extent of the grid layer. We, on the other hand, only need those rectangles whose geometries intersect with the mesh elements. We will use the Spatial Extract algorithm to extract the relevant meshes. It can be activated from the Processing Algorithms panel, hiding under the blue cogwheel icon.
The worksheet grid lacks proper numbering – in this case, let’s assume that the highest-positioned worksheet opens the list and the numbers change from left to right. We will use the Sort and Number plug-in to implement the order thus established.
The framework of our atlas is now complete. So we return to the view of the previously created printout and add the map window view to it.
We proceed to activate the atlas. First, we check that the Atlas tab is active. Then we select it, check Generate Atlas, and select the fields containing the name of the page and the data to be sorted. In this case, we will use the order field twice, which was created as a result of the Sort and Number plug-in. This selection will enable the sheets to be displayed in the correct order. Once the atlas has been pre-configured, we go to the Item Properties tab and activate the Atlas Controlled option. We generate a preview of the atlas by clicking on the Atlas Preview icon (looks like a map of the continents with rounded corners). We use the blue arrows to switch between the pages of the sheet.