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Newbie GIS user here. Basic question about getting my raster (DEM) to align with my basemap. I've read similar threads but none of them make sense to me. In short, I have added a bing aerial layer in QGIS 2.10.1 and want to overlay four raster files. I can't get them to align properly, with the DEMs currently floating off the coast of West Africa. I've assigned the CRS for the project as WGS 84 / Pseudo Mercator, but can someone give me a guide on reprojecting the raster data to match the basemap.

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    I recommend to upgrade your QGIS software first because it is very old. – ahmadhanb Sep 1 '17 at 12:48
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Layers in QGIS may have different projections and if they are correctly assigned, the layers will automatically align because QGIS reprojects all layers on the fly to match the "Project CRS" (the Projection shown on the lower left (in your case EPSG:3857)

When you add a layer to the map and a dialog appears to choose a coordinate system means that no CRS system is assigned or the data is not georeferenced at all.

To see the assigned layer projection you can right click on the layer -> Layer properties -> General.

"currently floating off the coast of West Africa" means that they are located close to the maps origin (0, 0). That usually happens when the layer has no georeferencing because then the pixels are taken as the coordinates and they will always start at the origin OR the correct CRS has small numbers compared to the assigned CRS. That could be WGS84 (EPSG:4326).

Try assigning EPSG:4326 and see if the data matches. If not, compare Layer properties -> Metadata -> Layer Extent and Dimensions to verify.

  • Thanks RoVo for your answer. I'm still not quite following the process properly. I've tried changing to WGS84 (EPSG: 4326) as suggested but get an python error message everytime I try to zoom to layer.I also can't see the layer properties for the bing aerial as it doesn't have a properties dialog. The Metadata for one of the raster dems is x = 2000, y = 2000. I've tried converting the asc format to tiff but to no avail. I'm not doing very well at this projection area. – Jeap Sep 1 '17 at 19:40
  • Start researching "georeferencing". You're going to need to identify the corner coordinates in a 'known' coordinate system. If your raster extents are 0,0 or 0.5,0.5, to 2000,2000, you're seeing the pixel count, not actual GIS-usable coordinate values. – mkennedy Sep 1 '17 at 20:06
  • Cheers Mkeenedy - I've researched and applied georeferencing to one of the DEMs and it's worked a treat. Just clarifying that I need to do this for every DEM I've got and there's no batch method that I can apply? Additionally, I've only done it briefly to make sure I understand it, therefore, only using 4 GCP's (I know that more GCPs generally means better accuracy) but wht would be considered an acceptable Mean error? My DEM has a Mean error of 5.85 (pixels). Thanks again for your help both. – Jeap Sep 2 '17 at 11:14
  • The amount of acceptable error in your georeferencing is completely dependent upon your goals / purpose. The best thing to consider is what you are doing against the error. Example: If you have a pixel resolution of 1m then 5.85 pixel mean error means that some pixels are 5.85m away from where they 'really should be'. Now compare this to your use case. If you are using a visual map of terrain it may not matter. If you are using this data to tell a drone where to fly in a city - then you could easily run into a building with 5m of error! – A.A Sep 21 '17 at 13:00

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