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I am using the scale bar of the QGIS composer (ver 1.8) and I don't understand how the whole thing works.

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Where can I set the units properly? The layers have idential CRS (WGS84), the settings for map tools is in meters.

I have tried to get it correctly by changing segment size and map units per bar unit, but it doesn't work. However, I have to admit that I don't understand what these two fields actually mean.

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WGS84 uses degrees as units. It is not possible to add a meaningful scalebar with meters/feet to a map which is projected to WGS84. Choose a different CRS (with units in meters/feet) and the scalebar should make more sense. If you want to stick with WGS84, just get rid of the scalebar. What would it be useful for anyway?

This thread discusses CRS for the northern Atlantic What's the best projection for rasterization of random latitude and longitude data in the northern Atlantic?

  • Ah, okay, I didn't know that. Thanks! Can you suggest any good CRS? And, maybe a stupid question, can I just change the CRS like that, won't it mess around with my maps? But you are right, scale bars are not too useful... feels incomplete without one, though. – Kristina Aug 29 '13 at 16:23
  • @Kristina Do you want to show the whole world or just the extent which we can see in the map you posted in the question? – underdark Aug 29 '13 at 16:56
  • Just the extent I depicted, Northeast Atlantic and bits of Europe. – Kristina Aug 29 '13 at 17:00
  • In 1.8, if you have google print as background to the map of the composer, the canvas crs must be set to google mercator, which creates the same problem of units as degress as @underdark notes. Otherwise the background will be offset from the layers. For background of Bing and Openlayers you can use the PopularMercator/WGS crs which has less noticeable problem of the offset. – Willy Aug 30 '13 at 4:30
  • The problem in the composer with offset layers may not be apparent until you output the pdf. – Willy Aug 30 '13 at 4:36
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To print a map from composer with meaningful units, the canvas (main window) crs ought to be set to a UTM crs, for instance GDA94/MGA55 EPSG:28355 which is metres. Most static layers will plot correctly with this, regardless of their crs.

When you use layers which are made of tiles (web sourced for intance) then there is a problem and you have to set the canvas crs to something like what the tiles are made in, see comment.

If you make your layer (reproject or save as) and your canvas crs both in a UTM crs, you can have confidence your outputs will be accurate, both the scale bar in the canvas and in the composer and the output pdf.

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