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I am completely new to cartography and QGIS - I'm a writer, editor and acceptable designer (Adobe Creative Suite).

I wish to produce PDF tour guides featuring maps that display a route. One version of the PDF will be for print (so high resolution images - 300 dpi) and one version for digital.

I have downloaded Ordnance Survey District Raster data. The data folder I am working with is 75.1MB and contains 68 GeoTIFF Full Colour tiles. The area covered is north west Wales (10,000 square kilometers) but the route I wish to represent passes through a much smaller area and I wish to break the route down into a series of about 10 maps, each covering an area not much greater than 200 square kilometers.

I want to be able to find the areas to be represented, save them as files that I can, if necessary, work on in photoshop or illustrator to show the route i.e. a coloured line overlaid - I have gpx points and am aware they could be loaded via QGIS but perhaps that's a task for later.

To achieve this, my assumption, based on what I can understand from reading QGIS tutorials, is to load all the tiles and then merge them. Once that has been achieved I then clip the area I wish to represent, and save it as a picture file. If that is not correct please tell me what i should be doing.

If it is the correct approach then please can you tell me what the error message "gdal_merge.bat was unexpected at this time" means and what I can do about it? It appears when I try to merge all of the tiles and the merge does not take place.

If I merge half of the tiles then that works so I assume it has something to do with file size.

Also the merged image is a black background (land and sea) and white features (roads, towns etc). As it only merges some of the tiles then some of the resultant map in the QGIS interface is black and white (like a photographic negative) and the rest (non merged tiles I assume) is colour. In the style section of properties I noted that the unmerged tiles are designated 'palette' and the merged part is designated Singleband gray.

Any assistance to successfully get me out of this quagmire and back onto solid ground would be very welcome.

  • I wonder if it's necessary for you to merge all the rasters? If you're making 10 maps, with areas of 200 km^2, could you just capture those areas in QGIS and then export as an image file? Also, producing these maps will probably be much more fun if you convert your coordinates to a vector file for display in QGIS. Thanks – Patty Jula Nov 26 '13 at 18:09
  • Thank you. Much appreciated. I shall give your suggestion a go tomorrow. The data is also available in vector format. The reason I tried to merge all of them rather than capture a few was that I didn't know I could capture before merging. The tutorial I followed didnt' mention it. I shall find a tutorial on capturing areas. – David Nov 26 '13 at 20:39
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The problem description sounds like the Atlas function is going to work great for you. The general workflow is as follows:

  1. Prepare the project by loading and styling all input data. Don't worry about merging raster files. That's not necessary.
  2. Create a polygon vector layer and draw features around the areas which you want to export as images.
  3. Go into Print Composer, prepare the map layout and configure Atlas to use the polygon layer to create the output images.

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