This may depend on the resolution of the dem when you generated it from your contour lines and how much you trust those.
Since you can set yourself the resolution and scr of your data (either when you interpolated it OR by resampling the whole thing afterwards) i'd say it all depends on how much you trust the sources you used to create it. From what i gathered by your post if said data is every 20 cms, then i'd create a few test dems at multiple resolutions just to check if said dem can be trusted (from 5x5 m up to 25x25). Afterwards just use the one that fits your needs the best.
In short the accuracy of your DGPS survey data is what matters here, start from there and everything'll be a lot easier