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Needed geometric tools to draw lots of parallel lines representing flight paths for taking overlapping aerial pictures - Lambert Conformal Conic was ideal (I thought). But saving to my project's Geographic CRS didn't preserve dimension accuracy so the lines are about 20% closer together.

I drew one line 15km long, pasted it and then dragged it vertically (y) to an approximate location, then used the node tool to change the final y position at each end of the new line so it was exactly 400m away from the first line, horizontal position is not that critical so I just eyeballed it.

I was using a fresh install of QGIS 2.14 Desktop on a Windows 7 PC.

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  • A GCS doesn't preserve any map properties. You likely want a Mercator to preserve constant bearing.
    – Vince
    Commented Apr 6, 2016 at 16:50
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    If you use a custom Mercator definition, if there's a standard parallel parameter, set it to the center of your area of interest. That will minimize distance distortions. You might also have had a problem with 2D planar versus geodesic (ellipsoid surface) distance calculations based on the CRS in use.
    – mkennedy
    Commented Apr 6, 2016 at 18:12
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    What GIS software are you using?
    – PolyGeo ♦
    Commented Apr 6, 2016 at 20:02
  • You might need to densify the points along the lines before reprojecting to another CRS.
    – AndreJ
    Commented Apr 7, 2016 at 5:54
  • I created the lines by drawing one, copying it, pasting it and then dragging it vertically (y) to an approximate location, then using the node tool to paste in the final y position for the ends of the new line so it was exactly where I wanted it, horizontal position is not that critical. Each line was 10 miles long and my intent was to place them 790 feet apart.
    – B.Briskey
    Commented Apr 10, 2016 at 5:27

1 Answer 1

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Lambert Conformal Conical has one or two standard parallels (latitudes), on which the scale is true. Mercator has one meridian (longitude) with true scale.

So depending on the extent of your study area (more along the latitudes or longitudes) one of the two projections has less distortion.

Once you have chosen a CRS, set the project CRS to it.

For taking exact distances, take these steps in QGIS:

  • Install the CADTools plugin
  • Create a new layer with the chosen CRS
  • Draw a line in the layer
  • leave the editing mode
  • From the CadTools Toolbar, choose Select one Line segment left of the Create parallel line icon
  • Select your line
  • Click on the Create parallel line icon
  • Enter a distance (in meters, positive or negative for left or right)
  • choose the same CRS for the new layer CadLayer Lines
  • continue for more parallel lines (you will see them after a redraw)
  • Use Vector -> Geometry Tools -> Densify Geometry, Verticed to Add: 100, Save to new file with Browse using still the same CRS
  • Change the project CRS to EPSG:4326
  • Save the densified layer with Save As ... to a new file with EPSG:4326

You will note that the densified line will be bended (if it is long enough), while the CadLayer Lines still is a straight line from start to end.

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