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I'd like to create a vector layer by digitizing from an image. How can I get the (nearly) correct longitude and latitude when digitizing from an image from Google Map and from an image from a satellite?

For example, I would like to digitize the following image. I know a lat- lng pair of one location and I know the scale. How can I create the vector layer to match this? What CRS should I use?

enter image description here

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  • Your question does not make sense to me. What is an "image" and what are real world coordinates? Do you mean just some raster file imported into QGIS without being georeferenced?
    – Bernd V.
    Jan 3, 2016 at 12:34
  • @BerndV. I edited the question. Hope it is clearer.
    – wannik
    Jan 3, 2016 at 13:22

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The best way I've found to do this in the past is to use a known source of spatial data and the QGis geo-referencer.

The process in general go's something like this.

  • 1) Find a known source of geographic data that's correctly registered and geo-referenced for the area you wish to digitize. If your in the UK for instance, you can download shape files from the OS national mapping agency for the UK road network.

    The data you obtain, should be loaded into a layer in QGis, that is set for the correct spatial co-ordinate system for the area you wish to work in. In my case, I've loaded the previously mentioned UK road network into a postgres database, then added that layer to my QGis project. How you add it, is entirely down to you, just as long as it's already spatially enabled.

  • 2) Goto Raster->Georeferencer->georeferencer in QGis, you should see the following window open:

enter image description here

When this window opens, use the "Open Raster" tool, to open the image you want to geo-reference.

enter image description here

  • 3) In the Geo-Referencer, use the "Add Point" tool, and add locations to your Image in the Geo-referencer. When the dialog to add the location opens, choose to pick the location from your map-canvas.

enter image description here

From your map canvas, click on the location in the already geo-referenced data that corresponds to the same location on your image. I find that road junctions are particularly well suited to this task, but it's entirely up to you what you choose.

Keep adding points to your image, by selecting them from the map, the more points you add, the more accurate your image referencing will be.

  • 4) Once you've added all your points, use file->save to save the GCP points, then click the yellow cog-wheel and set your referencing options. I would recommend that your Target SRS be kept the same as your map layer source, and that your output raster be set to a tiff file so that the geo-reference info is embedded into it.

When your happy with your options, click the green triangle, wait for the operation to complete and you should end up with a raster image of your source map that's fully geo-referenced in the co-ordinate system you choose.

As for answering "Which co-ordinate system should I use", well I'd pick the one that's best suited for the area your referencing, and that would usually be a country specific or UTM projection, normally in meters.

For the UK (Where I'm from) I would normally use 27700 (OSGB36) , try NOT to use the world WGS84 lat/lng system if you can help it, as the error margin is usually too great.

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