How to measure without coordinates?

My current Project is to "map" a turntable in qgis. I know, very unlikely but that's what my teacher wants.

My teacher also wants me to measure something in this project (Very specific, isn't it?). And that's where the problem starts. Qgis wants me to define geographical coordinates or something like that. The problem is, that i don't have any coordinates because it's a picture of a turntable and not an actual map.

Any ideas?

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I think we have a problem with this question. What is a turntable? Does the turntable exist in the real word, can you see it in Google Earth? – nhopton Sep 10 '12 at 21:15
Any Projected Coordinate system will work for you. Now, what do you mean by mapping, do you have an imagem of your turntable? Or are you going do design it in QGIS? – Alexandre Neto Sep 10 '12 at 22:00
Is this a railway turntable? – blah238 Sep 10 '12 at 22:40
I think she meant a phonograph turntable. – R.K. Sep 11 '12 at 5:12
nhopton: A turntable - this device for playing this old black big CDs ;) Just like R.K pointed out. Alexandre Neto: Yes, I have an image of the turntable. And on top of that image I create Shapelayers. I define what's inside the turntable. My rivers are the the cables of the turntabe and my citys are the circuit boards. blah238: Not really no. – Eleuteria Sep 11 '12 at 10:35

Qgis wants you to define a projection system to be used in your project. It wants it so if you're going to import something from the "real" plane (remember that the earth is spherical) to know where it should go in the Cartesian plane.

There are numerous projection definitions ready to be used that were tailor made as solutions for specific problems. Proj4 lib (the underline horsepower) that makes the reprojection from one plane to the other let's you even define your own projection.

A very simplistic projection for you to use is the "Equidistant Cylindrical Projection" and you can define it in Qgis at Settings -> custom CRS . There add :

``````+proj=eqc +lon 0=90w +no_def
``````

as parameters

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That's what i wanted to read... but unfortunately I can't get it to work. As soon as I enable it, every Layer disappears. – Eleuteria Sep 11 '12 at 10:36
First you have to define the projection and then mark your points. Otherwise proj will try to re-project on the fly from an arbitrary projection that doesn't have much sense to the one you've defined. – nickves Sep 11 '12 at 11:20

If you want to measure distances, you first have to georeference your image of the turntable to "transform" the pixel coordinates of your turntable image into real world coordinates. After that, you can use the measuring tool of QGIS.

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I would say you cannot georeference this or use geografical coordinates. It doesn't matter where in the room (and in the world) this turntable lays, right? I think you need to create your own local plane coordinate system, measuring by a ruler the size of the turntable, then referencing (scaling) the image to this distance (presented as a vector line or a polygon for example). You can start 0,0 coordinates in the corner of the image. Basically, this is a usual task for Autocad, not GIS.

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yes i agree its more cad exercise. But also its a nice exercise to learn how to use proj4 parameters to define your own projection. – nickves Sep 10 '12 at 22:27
Yeah you get it, it think ;) It really doesn't matter where on earth this turntable is located, so there no need for geografical coordinates and plus I don't have them (unless Google scans my appartment). "measuring by a ruler the size of the turntable" I have already done that ! ;) "then referencing (scaling) the image to this distance (presented as a vector line or a polygon for example). You can start 0,0 coordinates in the corner of the image." And that's the point were I got stuck. Any tutorials for that? – Eleuteria Sep 11 '12 at 10:36

i think you need to set "layer CRS" too

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Oh Yeah. ok i've done that now. Now everything is displayed again. and when i want to measure QGis don't displays the error message that i should define a georeference. BUT: The results for the measurement are still wrong. I get 2,658 km instead of like 46 cm. – Eleuteria Sep 11 '12 at 11:10
The scale is wrong. Start again, define the projection, and set the scale at about 1:5 (1 meter = 5 meters) or 5:1 (5 meters on your screen is 1 meter in the world). Then start hitting your points again – nickves Sep 11 '12 at 11:22
Okai i tried that, too. I works better I have to admit although it's still wrong. The scale is automaticlly choosen to 1:193. the measurements are not really right but almost. I get 48m instead of about 46 cm. is there a way of getting this without starting all over again? – Eleuteria Sep 11 '12 at 14:05