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At 3.8.3, when I Export > Save As a Geopackage point layer to a shapefile, fields with type = double are output with 15(!!!) significant digits. Here are the before and after field widths:

Input:
Type:    double
Length:   10
Precision:  2

Output:
Type:    double
Length:   23
Precision: 15

Furthermore, the output values are modified by a slight amount. Here are some examples:

Input    Output
150.62   150.620000000000005
150.51   150.509999999999991
221.86   221.860000000000014

Unfortunately, based on the comment by @user30184, the QGIS shapefile output appears to be the default action.

Now I'm curious about the passage that @user30184 quoted, particularly the phrase "...Real (floating point) fields without an explicit width..." This implies that my fields did not have explicit widths.

How do I make my real field widths explicit, either by modifying the source layer, or in the GDAL output process, or some other method?

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    Precision value seems to be close to GDAL default value gdal.org/drivers/vector/shapefile.html Real (floating point) fields without an explicit width are treated as width 24 with 15 decimal places of precision..
    – user30184
    Commented Nov 14, 2019 at 8:14
  • 1
    You can use the Vector - Table - Refactor Fields tool to change field attributes like precision. When adding a new field using the Field Calculator you tell it the field type, length and precision.
    – John
    Commented Nov 14, 2019 at 20:57

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