Importing Vector Map into QGIS / Coordinates / Measurement

[Shaetano]

Hey everbody!

I have started to fiddle around with mapmaking a bit. I started to create a flat map of a fantasy world and soon recognized, that I wanted to drive things a bit further. I tried out some things like climate zones and so on until I realized, that, if I really wanted to use the map for anything, I'd like to be able to define distances/measurements on the map.

So I looked around a bit on the web and found a small tutorial for a rough implementation of a vector graphic into QGIS. I followed it step by step, but really can't get it to work. I have a vector graphic from inkscape, safed as dxf file, which I want to import as a vector layer.
If I do so, the coordinates in QGIS are completely wrong (like -> most western point of it at something like -1000° and so on).

Next I found out about the possibility of georeferencing. I saved the vector as png, opened that in the georef tool in QGIS and worked with it following a tut I found here. Worked fine so far, but once it is finished and brought over to qgis, it is just really really small, which then pixelates everything pretty harshly and completely ruins the work I have done with porting the map to a vector format.


Well..I hope my problem is somewhat understandable. I tried to figure out what to do myself, but after several (video) tutorials (which refer real world data most of the time [i'd like to have my planet somewhat bigger than earth]) and searching for help here, on a german board, the qgis site + reddit I am left completely confused.

Thanks in advance for any bit of help! If something is unclear, I will to my best to give all details and information need.

[Hai-Etlik]

Probably your easiest solution would be to adjust the coordinate system in Inkscape before exporting to DXF. Set the document size in pixels to whatever the extent of your data is, then scale your data to fit, then export to DXF with the base unit to pixels.

If you need negative coordinates, the origin is the lower left corner. For instance to get latitude and longitude, you could set guides at X: -180 px, X: 180 px, Y: 90 px, and Y: -90 px. Use that box instead of the usual "page". Again, export with the base unit set to px.

[Hai-Etlik]

Also, to measure distance on a non-earth planet, you'll have to, at a minimum, set up a custom ellipsoid for the measurement tool. Under Project Properties select General then for Ellipsoid select "Parameters" and fill in the semimajor (radius at equator) and semiminor (radius at poles) axes in metres.

You may also want to define custom CRSes with those parameters for their ellipsoid, particularly if you plan to reproject your map.

[Shaetano]

Thank you for your help! Sorry for the late answer, but had not enough time to try your tips until know. Adding the map to qgis works now like I want it to (dumb mistake I made, I think I am just not made for working with these programs..^^").

But there is one thing that still troubles me. I can't get qgis to do correct measurements with a custom CRS. I made a simple one (rougly double the size of earth, following your guide I found here. Once I enter the radii in the project properties and try to measure something afterwards, the measured distance reduces once I cross the middle of the map.

[Hai-Etlik]

That sounds like the correct behaviour for geodetic distance. Ignore the line being shown by the distance tool, that's just to give you an idea of which points are being measured. The actual distance measured is whatever the shortest line between those points is. Unless you are using a Gnomonic projection, that line would probably appear as a curve on the map if it were drawn correctly (Drawing that path would be much slower than just figuring out its length) So when you get half way around from where you started, it will start measuring the distance "the other way" by wrapping around the discontinuity in the map although it won't draw the line that way.