[Qgis-user] Primitive Troubleshooting

[Árni Geirsson]

I'm not sure what went wrong in your process but I would think that the
safest route, to avoid CRS related problems, is to start a project in your
Roads CRS, include the Roads layer, then georeference your scanned map to
that same CRS and ultimately use the same CRS when you save the
georeferenced map. If you need to have the georeferenced map in a different
CRS, then use the Warp tool afterwards.

Arni


Árni Geirsson
*Alta ehf* // +354 582 5000 // +354 897 9549

On 27 February 2017 at 18:09, Stephen Wandling <swandling at gmail.com> wrote:

> Earlier I was trying to figure out why Composer wanted to export raster
> files, and I wanted vector.  I finally reduced my 'map' to the raster base
> map and one local shape file, Roads.  With this I found that selecting the
> layer Roads and using "Set Project CRS to this Layer" was causing Composer
> to default to raster output.  A clue!
>
> I had noticed that the raster base map rotated 2 or 3 degrees CW when I
> used "Set Project CRS to this Layer", above.  I also noticed that in
> Composer, if I had not used "Set Project CRS to this Layer", placing a UTM
> grid on my map resulted in the grid being rotated 2 or 3 degrees CCW.  So,
> it appears that the grid is based on the base map?
>
> In reading today's posts, I noted mention of Georeferencer.  Digging into
> this, the tutorials, etc seemed to require having another input, to compare
> points.  I thought I might be able to accomplish this by comparing my Roads
> layer with the raster map.
>
> I loaded my local Roads shape file into a new canvass in QGIS and my
> raster base map into Georeferencer.  I would click a particular
> intersection in georeferencer and it would produce the 'enter coordinates'
> or 'get from map' options.  Selecting the latter, it switched to the map
> canvass and I could zoom in and click the same intersection.  This would
> send me back to the previous menu where the coordinates were filled in and
> I would select OK.  I did this on 4 points.  On the fourth point, I noted
> the red line angled 2 or 3 degrees CW, but quite long.
>
> I saved the points in a file and Started Georeferencing.  The completed
> raster base map came up on the QGIS canvass, along with the Roads shape
> file.
>
> What I saw did not please me.  In some places, the Roads were now falling
> at least 1/2 kilometer off from the base map road locations.  This put them
> in creek ravines and other hillshade elements.  The results were unusable
> for my purposes.
>
> When I had used "Set Project CRS to this Layer", the Roads layer fell onto
> the roads on the raster map.  Streams fell onto their ravines.
>
> Maybe I have to get another base map?  Or maybe my georeferencing method
> was faulty?
>
> Stephen
>
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