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A frustrated newbie with QGIS Lisboa version in Windows 7. I want to clip a soil map (multiple polygon vectors) with a single polygon (rectangular polygon vector). The project projection is set at NAD27 UTM 12N. The soil map is Lambert Conformal Conic NAD83 and the clipping rectangular polygon is NAD27 UTM 12N. In the project setting I have checked the box "Allow on the fly transformation. The operation: Vector --> Geoprocessing Tools -->Clip --> input vector file (the soils)--> clip layer (the rectangle)--> output shalefile (new name)-->OK. Results: The layers section lists my new file BUT the file does not display and when I click on the file listed and look at the attribute table the FIELDS from the soil map are shown but there are no records in the table.

  1. What am I doing wrong?
  2. What coordinates will the new map be in? That of the soil map or that of the clipping layer?

Thank you. AQJO

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3 Answers

I believe the answer is that your clipping polygon needs to be in the same projection as the data you are clipping it with. When geoprocessing, especially for accurate results, it's best to make sure all of your data is in the same projection.

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You need to reproject your data to be in the same CRS before performing this operation.

Try reprojecting your data in the same CRS then try again.

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you and @chris henrick suggest that the two layers be in the same projection. But what is the point of checking the project setting box to "Allow on the fly transformation"? –  Aqjo Apr 3 '13 at 23:28
    
qgis when doing those kind of operations is in fact calling gdal's ogr routines. and as input it gives the original files which are in different planes, hence the empty results. –  nickves Apr 3 '13 at 23:37
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The on the fly transformation is purely for display purposes. The clip operation is performed through a separate script which operates purely on geometries ignoring the project. This would be a bug in the script, it should warn the user when operating on files in different projection. –  whatnick Aug 2 '13 at 2:43
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As commented above the bug is in the script performing the clip not verifying uniformity of projection. To reproject via tha GUI try the following:

  1. Set the on-the-fly reprojection to either 1 of the 2 projections
  2. Right click on the layer and select save as and select the same projection as the other layer. Reprojection dialog
  3. Save the new file and progress with the clip.
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It is better to turn on-the-fly-reprojection off before doing geoprocessing across different layers. Then you see easily whether they align or not. –  Andre Joost Dec 14 '13 at 15:49
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