4 replies to this topic

[natcase]

There's probably a simple answer to this and I will be embarrassed. But here goes anyway:

I have DLG data including some very steep cliffs. As is USGS convention, elevation contours are selectively dropped out where they would make a clogged-up mess. The DLG's include elevation attribute data. Is there a quick automated way to create continuous non-overlapping lines (for sake of argument I'm assuming no overhangs here), so I can make everything into closed polygons? I've started in trying to connect them by hand and it's a bear... seems like there ought to be some basic algorithm to do it automatically. Like maybe something developed by raised-relief makers?

[Hans van der Maarel]

I have DLG data including some very steep cliffs. As is USGS convention, elevation contours are selectively dropped out where they would make a clogged-up mess. The DLG's include elevation attribute data. Is there a quick automated way to create continuous non-overlapping lines (for sake of argument I'm assuming no overhangs here), so I can make everything into closed polygons? I've started in trying to connect them by hand and it's a bear... seems like there ought to be some basic algorithm to do it automatically. Like maybe something developed by raised-relief makers?

Well...

What you could do is feed the existing contours into some software (I would personally pick FME for this) and use them to calculate a new DEM, then use that to calculate new contours.

[natcase]

I have DLG data including some very steep cliffs. As is USGS convention, elevation contours are selectively dropped out where they would make a clogged-up mess. The DLG's include elevation attribute data. Is there a quick automated way to create continuous non-overlapping lines (for sake of argument I'm assuming no overhangs here), so I can make everything into closed polygons? I've started in trying to connect them by hand and it's a bear... seems like there ought to be some basic algorithm to do it automatically. Like maybe something developed by raised-relief makers?

Well...

What you could do is feed the existing contours into some software (I would personally pick FME for this) and use them to calculate a new DEM, then use that to calculate new contours.

Hmm. You have a point. I suppose there's really no reason not to use existing DEM data and just generate new contours from that.

[Hans van der Maarel]

Hmm. You have a point. I suppose there's really no reason not to use existing DEM data and just generate new contours from that.

That's an even easier option. There might be a bit of difference between the original contours and the ones generated from a DEM, but I don't expect that to be much of an issue.

[Charles Syrett]

Generating new contours from an existing (or new) DEM can have mixed results, depending on the resolution of the DEM and how it was created. My experience is that if you want the contours to be as close as possible to the original contours, and you have elevation attributes for them, then use Manifold with the DEST algorithm. The resulting contours will be a good match, BUT.....you'll probably see all kinds of little kinks in the lines that weren't there before, as well as a few truncated valleys (which DEST is supposed to eliminate, but doesn't 100%).

So! Now you end up with another "bear" of an editing job. Wasn't that what you were trying to avoid in the first place?

Which way you go on this really depends on the specific situation. Maybe generate the new contours and then cut/paste only the sections you need into your original....?

Charles Syrett
Map Graphics
http://www.mapgraphics.com

Hmm. You have a point. I suppose there's really no reason not to use existing DEM data and just generate new contours from that.

That's an even easier option. There might be a bit of difference between the original contours and the ones generated from a DEM, but I don't expect that to be much of an issue.