I have a rather large map that I want to break down into smaller parts, or sheets. So I'm going to use the 'atlas' feature of QGIS Print Layouts. My layers in QGIS have a CRS of GDA94 (degrees), where as my map layout will be projected to the specific UTM zone for that area (meters).
What I want to do is create a coverage layer containing a grid of polygons. This will essentially become the extents for each individual map sheet (as well as have other attributes such as map name, region etc). The issue I'm having is trying trying to determine the size of my polygons so that I can get it to be a specific size to match the size of the 'Map' object on my print layout at a specific scale, if that makes sense?
For example, my map sheets are standard A2 page size, which is 594mm x 420mm (portrait). Accounting for margins, let's say that the 'map' object on my print layout is 580mm x 360mm. What I need to do is calculate a grid of polygons where each polygon will be 580x360mm at scale exactly 1:50000 for that particular UTM zone. The scales are important so that I can utilise a grid where 2cm = 1km.
So far, when I try to generate the polygon grid, I only have the option of setting the size in degrees (as this is the unit of the layer CRS). I can get it close to 1.50000, but I dont want close enough, as this will not be precise enough.
Is there a way to calculate this or is my understanding of what I want completely off the path and I should be looking at a different solution?
Best Answer
If you choose a UTM crs in the Create Grid tool dialog, you would then be able to use meters as units, which will make matters much simpler. The fact that you had degree units suggests that a geographic coordinate system was chosen for the grid creation, so make sure that a "UTM Zone xx" crs is chosen in the tool dialog.
The height and width of your pages then become 580mm x 50000 = 29,000m and 360mm x 50000 = 18,000m. In the Greate Grid tool you can also set an overlap if you desire. For the grid extent, make sure to check beforehand the min/max coordinates that your atlas will cover, in UTM coordinates. You can do this by setting your project crs to the desired UTM zone.
Also, in the Layout view, the map object properties enable you to set a scale directly, and use it for your atlas in the "Controlled by atlas" section by checking "fixed scale".