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View Full Version : Image to GIS - what the hell



EricPoehlsen
07-28-2012, 08:06 AM
Hi together, I am experiencing a strange phenomenon while trying to create a real-GIS map for some fantasy project.

I do have multiple raster and vector sourcemaps - my main working map is currently an inkscape svg. which is at 7 units per kilometer - yeah that is a little strange but it is what happened in tracing the original old rasterfile.

It is an island which is roughly 400km north to south and 270 km east to west. measuring in inkscape i get 418kms and 266kms between significant points. The area calculation returns roughly 70.000 km². The Island is situated at the northern hemisphere between 46°N and 50°N.

So I created an image overlay in googleearth and resized it that i measure the significant points at +- 2km of my inkscape measurements. I wrote down the corner coordinates and used them to create an geocoded tiff. I tried tons of tools - the QGIS geocoding returned anything but useful results, the MapWindowGIS geocoder didn't work at all but I finally managed to create a working geocoded tiff with QLandkarteGT

Next step, I imported the geocoded tiff into QGIS and measured the two distances. The distance was nearly what it should be 418 kms and 266kms respective. So I created a new shapefilelayer and did a fast tracing of the outline - went to the measurement-tool and got an area of 154.000 km² :?:

Anyone here an idea what the hell went wrong there?! I did the tiff geocoding again, I did the tracing again and ended up with another shape which has the same east-west north-south dimension but returns more than double the expected area!

EricPoehlsen
07-29-2012, 04:16 PM
No answer here, but problem solved with some folks from the QGIS community - it was a problem with the CRS of the vector layer.
Just to clarify what was going on a little image:
http://www.eric-poehlsen.de/images/andere/why.jpg

su_liam
08-08-2012, 03:17 PM
What with my German being so rusty(pretty much the only thing I can do in any language other than English is get myself punched in the nose), I'm having to make a guess as to what's happening here. It looks like your getting an area measurement of about 68,000 square kilometers, which looks like success.

But what went wrong? What was the solution? If I can figure out how to successfully get vector and raster imagery loaded up and georeferenced in QGIS, I could spend a lot less time using Windows...

EricPoehlsen
08-11-2012, 01:47 AM
The orange polygon had an area (Fläche) of more than 154000 square kilometer left dialog window, while roughly measuring the extent of it with the measuring tool returned the correct 68000 square kilometers. The odd thing was, all layers showed they were in the same CRS - yet somehow they weren't...