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This is my first experience of using GIS systems.

Firstly, to give you an overview of my project, I am trying to analyze the obesity rates within the boroughs of London. I have gotten to the point now whereby I am trying to buffer the attribute of fast food restaurants whilst intersecting it with schools.

Unfortunately my unit of measurement for the buffer tool on both schools and fast food restaurants is in degrees. I have tried changing the CRS of both individual layers to a meters based CRS, but the layer just disappears when I do this and won't come back until I revert it to its previous CRS.

Why does that happen?

PolyGeo
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Ryan
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  • Instead of changing the CRS try to reproject your layer: https://gis.stackexchange.com/questions/35590/reprojecting-vector-layer-in-qgis, also check this thread https://gis.stackexchange.com/questions/106988/create-buffer-in-meters – Taras Oct 22 '20 at 09:44
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    Don't ever set the CRS to a different one, but transform/save to a different CRS. – Erik Oct 22 '20 at 09:44

2 Answers2

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A very common misconception when working with spatial data and GIS is the proper application of changes to its coordinate reference.

Very generally speaking, spatial data has two components that come to play here:

  • coordinate numbers; simple scalar values
  • coordinate reference (CRS); a (mathematical) definition of what the above values relate to

For different CRS, the coordinate numbers are different:

  • for degree based CRS (geographic reference system), those scalar values will be in the range of +-180.0 and +-90.0
  • for meter based CRS (planar reference system; projection), those scalars will be in ranges that can be as large as 1*10⁹

Now, as pointed out in comments and answers, if you set the CRS to a different one, you only change the definition of what those coordinate values relate to. So while the scalar values stay the same, they will now be treated as a different unit; 45° N suddenly means 45m N...you see where this is going: your layers disappear, or better, they move.

In order to correctly apply a different coordinate reference to your data, you will have to transform it, meaning that a different CRS will be set and the coordinate values transformed according to the mathematical definition and transformation rules that come with each CSR.

geozelot
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No worries, thats a very common issue for new QGIS users. Note the following: Never SET a CRS unless you really know what you are doing there, instead REPROJECT it.

So, set your CRS back to its original. Then right click on the layer, choose "Export" and "Save features as...":

enter image description here

Then choose your desired new CRS and save the file. This new file is now properly reprojected. If you have chosen a CRS with unit meters, you can do your buffer analysis.

enter image description here

To what happened to you: If you set a CRS without reprojecting the layer, you tell QGIS that this layer is in a CRS it actually is not in. Thats why the layer disappears from your canvas and pops up somewhere else on the world, or even somewhere not on our planet earth.

MrXsquared
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  • Thank you everyone this was very helpful! Whilst I have you, I am now trying to intersect the schools with the fast food buffer I have just set up. I am now running into another error message as shown below. Are you able to advise what I should do to get around that? Thanks! – Ryan Oct 22 '20 at 09:57