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I am using QGIS (2.18 or 3.0) to create nautical charts drawing upon NOAA raster tiles and other info (e.g., lines showing magnetic north). I now want to save PDF files for different subareas using map composer. The subareas are quite varied but at times overlap slightly -- not a set of touching rectangles that could be done using the altas option.

To do this systematically, when completing a map composition I need to record the decimal degree coordinate for each corner of each map. I was hoping that the "coordinate capture" plugin worked in the map composer, but I don't see that as being an option (e.g., icon). If the coordinates in map composer showed as long/lat units, that would be sufficient, but clumsy. However, MM are the units on the composer window even when the corresponding QGIS window shows long/lat coordinates.

Can you suggest methods to capture the long/lat coordinates of the corners of maps in composer?

Even better, any suggestions on how to easily transfer the coordinate data to a spreadsheet?

  • You can use the atlas generator with overlapping rectangles. – csk Jun 13 '18 at 17:12
  • The coordinates of the corners of the map can be found under item properties > extents > X Min, X Max, Y Min, Y Max. They are in the same units as the map CRS. So if the map CRS uses decimal degrees, its extent is also in decimal degrees. – csk Jun 13 '18 at 17:47
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My solution was not elegant and is a bit time consuming, but ...

In print composer I output a tiff file showing the composed map. I then opened qgis a second time (leaving the first open) and imported the newly created tiff file. With that I read off the latitudes and longitudes for the four corners of the map, using coordinate capture, and entered the data in a spreadsheet. I then reloaded the CSV in the original QGIS running to show the added vertices.

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In QGIS 3.0, if your map is in WGS84 the map extents in the print composer map item are in latitude and longitude. enter image description here

In this example, the map extent is displayed as:

  • X min -157.296
  • Y min 6.718
  • X max -93.254
  • Y max 51.676

Therefore, the (latitude,longitude) coordinates of the corners of this map are as follows:

  • Lower left corner (6.718,-157.296)
  • Upper left corner (51.676,-157.296)
  • Lower right corner (6.718,-93.254
  • Upper right corner (51.676,-93.254)

To create points from these values, either:

  1. Copy and paste the corner coordinate into a spreadsheet, save it in CSV format, and import the CSV into QGIS.
  2. Or, click the "view extent in map canvas" button in the print composer. Back in the map canvas, add a point near each of the four corners of the map. Use the Vertex editor to change the point coordinates to the exact desired values (copied from the print composer map extent)

enter image description here

  • This does not work when the image in composer has been rotated. The extent data does not uniquely imply the coordinates for the corners of the map. – Hamp Jun 15 '18 at 15:24
  • If you want a solution for rotated maps, you should include that information in your original question. – csk Jun 18 '18 at 13:37

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