You load a new layer onto your canvas and you expect it to show up adjacent to all the other layers. But it appears in the middle of the ocean, is out of the extent, or covers another country altogether. It could still be good data. Often the reason is simply that the GIS does not understand how the coordinate values of the layer should be interpreted.
First, make sure you zoom into the new layer and see how it relates to a map of which you are familiar. Next, check the coordinate reference system (CRS) of the layer in the layer properties. Also check the CRS of the project. These two do not necessarily have to be the same since most desktop GIS applications can re-project layers on-the-fly, but every layer should have the correct CRS assigned to it.
A coordinate reference system defines the relationship between the numeric values of a coordinate and a real location. Most geographic coordinate systems are based on latitudes and longitudes, where the values are given in degrees. Projected coordinate systems locate places on a flat plane using x and y coordinates, and they typically use meters or feet as units. A coordinate with a longitude of 30 and a latitude of 50 is different from a coordinate with a value of 30 and a value of 50. If your GIS reads a projected coordinate as a geographic one, the feature will show up somewhere far away from where it should.
The difference between assigning a CRS and reprojecting a layer into a different CRS is important. Assigning a CRS tells the software what your coordinates currently represent. Reprojecting converts your coordinates from one CRS to another. You cannot reproject a layer into a different CRS and expect it to end up in the right place. Doing so just changes how your coordinates are interpreted, and it might move your layer once more without solving the problem.
CSV files with coordinate data often pose a problem if the columns are not named and do not indicate the order or units of the coordinates. Your file might have the longitudes listed first, then the latitudes, but the import tool might be looking for latitudes first, then longitudes. This switch can cause your layer to show up in an unwanted location. Double-check that the CSV is importing properly, including decimal separator, minus sign, and field type. Text-based coordinates should be reviewed and cleaned prior to mapping.
Raster data can have a similar issue if it does not have a defined spatial reference, or if it is not georeferenced. Even though your image might look like a map, it will not display correctly unless it has been georeferenced with ground control points or has some sort of geospatial metadata associated with it. You cannot simply assign a CRS to a raster and expect it to display in the proper place. You must georeference the image by associating known points in the image to their proper coordinates and then save the result with the spatial reference included.
Be sure to keep the original data intact and create new data files that have been corrected or reprojected. Once the layer looks like it is in the right place, check multiple recognizable features, and not just one. Roads should align with roads, borders should join with other borders, and known landmarks should still be located in their proper places. If the layer looks correct in one part of the extent but moves around outside that area, it is a sign that the coordinate information requires further review.
