4

I have a PNG image which looks like this and has some lines which I need to trace (the green and red lines):

enter image description here

The coordinate system of the map from which the image is taken is EPSG:3034. How can I position the image correctly on a shapefile such as this one (a shapefile with coordinate system EPSG:32632) using QGIS:

enter image description here

I tried to georeference the PNG file (with the Georeferencer plugin) by defining the coordinates of four points, but somehow I am getting a distorted final image. As an example, when I am inputting the East and North coordinates in the georeferencer (QGIS) I use the E/N coordinates of the form 44.214555, 22.67459 (WGS84).

How do I solve the problem?

The idea is to be able to extract the information regarding the location and length of the lines from the PNG file in a new EPSG:32632 shapefile.

The result I get, when georeferencing 4 points (geographical extremities of the country. i.e. most northern point, most southern point, etc.):

enter image description here

  • QGIS has a great georeferencing tool, but I think you'll need a bit more reference data such as City points - then you simply click the PNG, then 'add point from map' and click the corresponding city point on the QGIS map... do this several times (but not too many), run the georeferencer (aligns the image to the points you add) and you should be good to trace your lines into a new line dataset... – DPSSpatial Jan 5 '16 at 18:48
  • What type of transformation did you use? these two grids are different types and a linear transformation - the default - won't work. Also, where did you place your 4 GCPs? evenly spaced around the perimeter or bunched together? – HDunn Jan 5 '16 at 18:49
  • It is not clear from your description what is the problem. What 'distorted final image' means, can you show? Why do you think that entering coordinates as DD.DDDD is a problem? – Maxim Dubinin Jan 6 '16 at 3:06
  • for the four points, I used the geographical extremities of the country . The image I get is included in the question now – ELD Jan 6 '16 at 9:36
1

Since you have the border shapefile in QGIS, you can use the From map canvas option of the georeferencer to pick border points in your image and the shapefile. You should set the project CRS and target CRS to the same as the shapefile.

Your attempt failed because you entered lon/lat degrees, not meters as required for EPSG:32632. I suspect that you have used Set Layer CRS for the shapefile wrongly. Try a dataset from GADM or Natural Earth which comes in EPSG:4326.

You should set the project and target CRS to EPSG:3034, and georeference to that using all points where three countries join in one point with the from map canvas icon:

enter image description here

enter image description here

enter image description here

The scale is now about 1:3 000 000. If not, check the dst coordinates.

Using 255 as additional transparent value, changing project CRS to EPSG:3857 and adding an Openstreetmap background, you see the lines on it:

enter image description here

  • I followed your advice and set the target and project CRS to EPSG:3034. I geo-referenced the extremities of the country (most northern point, most eastern point, etc.). The result I get is in the question above – ELD Jan 6 '16 at 9:30
  • I think you are still entering lat/long coordinates in degrees, which is wrong for EPSG:3034, or the shapefile data is corrupted. See my extended answer on that. – AndreJ Jan 6 '16 at 10:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.