I've created contour lines every 5 meters from a grid DEM with 5x5 meter of resolution. But strangely I've obtained that the contour lines are segmented and rich of lines that goes up and down, especially in the plains areas.

I've then used a focal statistic to smooth the DEM and I've obtained a very good result in term of aesthetic; but is preferable to non alter excessively the original data.

Some suggestions or ideas on what is the problem?!

Contour very irregular

1 Answer 1


What a great question! The problem is that contours are a cartographic lie. They're a convenient way of conveying information about the relief within a landscape. However they don't translate directly to the real-life experience of topography, except perhaps in the heavily modified and engineered landscapes of our urban environments. Topography in reality is not as smoothly varying and regular as contours would lead us to believe. When you have a fine-resolution DEM that shows small-scale topographic variation, like those generated from laser scanning, you either need to smooth the DEM or smooth the contours themselves before you get something similar to what you would typically see on a topographic map. Often this smoothing is built into the contouring algorithm and so the user isn't always aware that it is taking place. I, however, prefer your approach of smoothing the DEM because then the user must explicitly decide about the level of generalization that goes into creating the artificially smooth surface implied by the contours and is more aware of how much that deviates with the reality of the topographic surface. This is also why contours don't always make the best source of data for interpolating DEMs. Thanks for the wonderful question.

  • 1
    Not sure whether to give you +1 for your detailed answer or for your sheer politeness!
    – Joseph
    Oct 20, 2014 at 11:06
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    @Joseph I almost spat my coffee across the room in laughter when I read that comment! Thanks so much for brightening what is otherwise a rather dreary Monday morning. Oct 20, 2014 at 11:39

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