3

I'm currently trying to produce simplified field slips on QGIS for an undergraduate mapping project in Villamanin, Northwest Spain. I've imported vector map layers from the download centre from the IGN for Spain and have been working through them to produce slips suitable for mapping contacts/etc onto. When starting to render slips I've noticed I have a small coordinate offset between raster maps downloaded from Iberpix and the corresponding slip I've produced on QGIS (see image). Does anyone have any ideas about this?

enter image description here

Whilst the offset is small I'm concerned it will introduced real errors into my final project map...

  • 1
    Can you measure the offset in real world meters? It might help if you can point to the files you downloaded from IGN (raster and vector). – AndreJ Jun 25 '17 at 5:46
  • Hi AndreJ, The vector files downloaded were from: centrodedescargas.cnig.es/CentroDescargas/index.jsp , from the 1:25,000 vector maps in .dgn format. The raster files were from: ign.es/iberpix2/visor , in image format saved as PDFs. I'm not entirely sure how to measure the offset in real world metres. – Lydia Marsden Jun 25 '17 at 13:28
  • I did not get from the second link to a download, but I'm sure the first image is wrong. Assuming the red lines indicate the same point, you can calculate deltX and deltY, around 200m. see my answer below. – AndreJ Jun 25 '17 at 17:24
  • Thanks Andre - sorry for the slow response, currently mapping in Skye - we've managed to work this part out thanks! – Lydia Marsden Jul 3 '17 at 23:06
1

The IGN offers two vector datasets: MTN25 and BTN 25.

Compared to an Openstreetmap background or SRTM height data, the second one aligns as ETRS89 UTM 30N, while the first one only fits if set to ED50 UTM 30N.

You can find in the enlocesd LEEME.txt:

Sistema de Referencia Geodésico: ED50 o ETRS89 según fecha de la hoja

translated: "Geodetic Reference System: ED50 or ETRS89 according to date of the leaf"

The difference ist just the offset of about 230 meters you get between your images.

See also Projection of Shapefiles for Spain (INE)

  • Hey AndreJ, thanks so much for your response! It seems you're on the right track, do you know how I can rememdy this in QGIS? – Lydia Marsden Jun 26 '17 at 8:13
  • Set CRS for Layer lets you change the CRS. If you added the dgn file, you were prompted for a CRS, because dgn has no CRS information. So you could have set ED50 UTM 30N already there, instead of ETRS89 UTM 30N. Once set correctly, you can use Save As .. to a new filename and reproject to ETRS89 UTM 30N if you want to. – AndreJ Jun 26 '17 at 9:31
-1

It is passable that you are having this problem because of the CRS you are working with. The common ones are using some kinds of an oval projection. When you are working with a raster and shp file like in your project it can make this offset you experience. Try to change all of the CRS into this projection enter image description here. When you measure distance in this sphere projection it will give you the "true" distance between points on a rounded surface. Don't work with the original layers. Instead make new ones withe the new CRS so if this will not work or cause some unexpected problem you will have the option to return to your previous work. Please let me know if this answer was helpful. Nitzan

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.