To export a map as an image, you use QGIS’s print composer. This is also where
you can add titles, compasses, and scales, but due to the nature of our maps,
we usually don’t. It’s pretty obvious which way north is on a globe. SOS
recommends that you keep any text that must be stationary on the map, like
labels, close to the equator, so that it isn’t warped when the map is
transformed onto the sphere. However, the preferred way to put legends, images,
and labels onto the sphere is to make a PIP (PIP stands for ‘picture in a
picture’). For a detailed description of PIPs, please see SOS’s Datasets
Manual. For a description of how to create a PIP, see the
Legends, Color Bars, and Scales section of this
Right-click on your image layer and select Zoom to layer
extent. Then go to ProjectNew Print Composer.
Name it anything you like.
Click on the Add Map button and draw a rectangle that
fills the whole white workspace by dragging your mouse across the workspace.
Try to make this as exact as you can. Whatever is on QGIS’s main screen will
appear in the box.
If you want to change what’s appearing in your map box, you can change it in
the main QGIS window, and then click the Update button
under Item Properties. Whatever you most recently clicked on in
the composer will get updated. For example, if you have a legend and a map
arranged in the composer, only the one that you last clicked on will get
Now look at the menus on the right. First, next to Presets under
the Composition tab will be a box with a preset paper
size in it. Click on it and select Custom. Then change
the width and height to 6.84 inches and 3.41 inches, respectively. This is
because we want our export resolution to be 300 dpi (dots per inch, or pixels
per inch) and we want the final resolution of our map to be 2048x1024. 2048
divided by 300 is approximately 6.84, and 1024 divided by 300 is 3.41. You’ll
probably have to change the units to inches instead of mm. Make sure your
export resolution really is at 300 dpi.
Click the Item
properties tab, then scroll down to Extents. To make
your map fill the screen as much as possible, change the X values to -180 and
180, and the Y values to -90 and 90. You should see the map fill the entire
white space. Check the Scale value under Item
Properties. For a map sized 6.84 by 3.41 inches, resolution 300
dpi, the scale should be 114891360. If this is incorrect, it causes problems
with the export. Then click the Export as Image
button. SOS prefers JPEG or PNG, but can accept most common image formats.
Congratulations, you have an SOS-ready map!