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I have a shapefile with the attribute "yield". I need to know the math difference between yield (id1) and yield (id2)... then yield (id2) and yield (id3) and so on... I hope you understand what I mean.

In Excel it is easy to handle. But I do not want to export and import the files.

nmtoken
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Pimpel
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  • I think you will need some python scripting for that. – Alexandre Neto Jul 02 '13 at 15:44
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    @Pimpel, make math difference in Excel (save result in csv or dbf table), then Join this table to shape-file (http://gis.stackexchange.com/questions/6398/join-non-spatial-csv-to-spatial-data-in-qgis) – spatialhast Jul 02 '13 at 17:02

1 Answers1

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How about a solution in Spatialite? Import your shapefile into a spatialite DB, lets call it "polys". Add a column to hold the differences:

ALTER TABLE polys ADD COLUMN diffs real;

Now this UPDATE will populate the diffs column with the differences between each consecutive row:

UPDATE polys SET diffs=(
SELECT p1.yield-p2.yield
FROM polys AS p1, polys AS p2
WHERE p1.ROWID=p2.ROWID-1 AND p2.ROWID>=2);
Micha
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