Extracting contours

One of my contour datasets has areas that are covered by a 10m contour interval whereas the majority is only covered by a 20m contour interval.

Contours - 10m versus 20m

So if I want to make a consistent 20m DEM I need to remove these pesky 10m contour intervals.

QGIS 1.9 > Select features using an expression

Select using expression

The expression used requires some regex (^[-+]?[0-9]*\\\.?[0-9]+$) to find any contours that have an altitude value that becomes a floating point number when divided by 100 and then halved.

The contours can then be deleted or the expression changed to regexp_match(((“ALTITUDE”/10)/2), ‘^[-+]?[0-9]*\\\.?[0-9]+$’) = 1 to select the desired contours and saved out as a new file.

Sources: http://www.regular-expressions.info/floatingpoint.html, http://gskinner.com/RegExr/


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