DEM and vector data won't match using QGIS

Question

I have just started using QGIS for my research at university. I have a DEM comprising Italian regions of Lazio, Tuscany and Umbria. I then have a set of vector data from surveys carried out in these territories. When I upload the DEM and survey data, I expect them to be on top of one another, but they are not. I tried everything but they just won't match! If I put a base map from Google Earth or Maps, the survey data matches the Italian regions they refer to, while the dem data ends up corresponding to Spain for some reason. I changed the coordinate references, to Monte Mario Italy 1, but nothing really changes: actually, the survey data just disappears from the map! It's probably something really stupid that I am messing up, but I tried every possible combination and I am getting quite frustrated!

Does anyone know what I can do to fix this?

Answer 1

This is the classic "new to GIS" problem. It happens to everyone, and it may or may not be because of something you did. Sometimes GIS data just doesn't come with the correct projection data encoded in it. You're doing well so far. Don't panic.

Good job using a base map to figure out which layers are correctly located. You've narrowed the problem down enough that we can be sure the DEM has the wrong CRS.

Here's the next question: At any point, did you reproject the DEM, either by using the reproject tool or by save as... and choosing a different CRS? Or have you just been changing its CRS in the layer properties?

If you did reproject the DEM, it will be very tricky to fix. If at all possible you should get the original source data and start over.

If you didn't reproject the DEM, the next step is to figure out what coordinate reference system it's supposed to be in.

That information should be readily available with the DEM, but many data providers are sloppy about this, so sometimes you have to search for it. Note that it might be labeled as "coordinate reference system," "spatial reference system" or "projection" and "datum." If you're lucky, it's unambiguously defined with an EPSG code. Or it might be split up into several pieces of information like "units" or "UoM," "datum," "projection" and so on.

• If you downloaded the DEM from a website (or you know where it came from), go back to the original page and look for a link or section of the page called "metadata" or "other information" or "FAQ" or "General information." Sometimes the coordinate reference information is provided for each dataset. Sometimes it's in a general section that tells you that all GIS data on the website is in one standard CRS.
• If you just have the DEM in a file on your computer, go the the file folder that it's in and look for a text file called METADATA.TXT or README.TXT.

• If all else fails, you have to resort to trial and error. Here are some reasonable/common CRSs to try:

  • EPSG:4326 (this is the most common geographic coordinate system)
  • EPSG:3857 (this is EPSG:4326 projected into meters; many web maps, including Google Maps, use this projection)
  • UTM projections - Use this interactive map to find your UTM zone. Italy is split between UTM zones 32N and 33N (and the very tip of the boot heel is in UTM zone 34N). There are lots of options in QGIS for each UTM zone, but the most normal ones to use would be the ones with either the WGS84 datum or the ETRS89 datum. That would be "WGS 84 / UTM zone 34N" (EPSG:32634), "WGS 84 / UTM zone 33N" (EPSG:32633), and there are several options in ETRS89.
  • Projections in the ETRS89 datum for all of Europe: "ETRS89 / LAEA Europe" (EPSG:3035), "ETRS89 / LCC Europe" (EPSG:4839)
  • local CRS - try any CRS with the name of the country where the data is located. Some countries have a lot of options, so it helps to know what's "normally" used in that country. See if the national government has a geospatial agency that publishes standards.

Once you know what CRS the layer is supposed to be in (or you have a guess), change the CRS of the layer in the layer properties dialog.