I am trying to use QGIS to generate OpenStreetMap data in an older format (NTF). Since OpenStreetMap is vector, I am currently using Save Map as Image (I also tried a Print Composer) to generate GeoTIFF, then GDAL to convert between GeoTIFF and NTF.
I understand that DPI (resolution) and map scale are independent variables - i.e. A 1:50K map can be scanned at 72dpi or 300dpi, and it is still 1:50K.
However, it seems that there is some type of correlation, and I can't quite figure out what it is. When I save an image at 1:54168 (Zoom Level 13 according to QGIS), and I save at the default of 72DPI, then open the output TIF in QGIS (and choosing view at Native Resolution) gets me to nearly that (1:54370), however, if I choose a higher DPI the scale becomes proportional (e.g. 1:20360 @ 192dpi). Not so independent.
The reason this is significant is there are styling rules that vary based on scale. So outputting a map through Save Map to Image yields a different result than viewing in QGIS at the same scale.
Can anyone help me to understand this problem set better, or offer a suggestion perhaps for a different workflow? Success to me is a map at 1:X scale, at a decent DPI (169dpi I think is the original standard) that looks the same in QGIS as it does in GeoTIFF/NTF.
