add contour lines from srtm 30m dem data?

[clhenrick]

So after doing some research it looks like contour lines generated from SRTM 30m data could look decent up to about z15 and after that they start looking a little too jagged without smoothing them out.

I took an extract of an SRTM raster for the Mt Whitney area, extracted part of it, converted to EPSG:3857, then generated contours at 50m intervals. The following screenshots show the contours overlaid on top of the og terrain tiles in QGIS:

zoom 14 (~1:36,000)

zoom 15 (~1:18,000)

zoom 16 (~1:9,000)

zoom 17 (~1:4,500)

My general workflow for print cartography is to take the contours after rendering them with GDAL and use illustrator's "smooth lines" to apply bezier curves to them so they don't look so jagged. This could probably get them looking really nice at zooms 16+ but we'd have to figure out how to do it programatically.

[mojodna]

line-smooth is a nice CartoCSS way to smooth out jaggies. I tend to use it on longer lines and higher zooms.

[clhenrick]

thanks @mojodna I'll give that a shot!

[mojodna]

line-simplify-algorithm: visvalingam-whyatt has also been helpful (and line-simplify is pixels in that case, i think) to further smooth lines.

[clhenrick]

The line-smooth & line-simplify CartoCSS @mojodna pointed out is definitely helping, here are some samples rendered in TileMill:

zoom 16

However at zoom 17 even with some line simplification the contours start to look a little jagged / unrealistic:

[almccon]

What's going on with the background there? Is that what you mentioned about the gaps/overlaps between park areas?

[clhenrick]

Yeah, that's where I noticed the seam between "green areas"

[almccon]

Looks like an OSM error: http://www.openstreetmap.org/#map=16/36.5750/-118.2913

[clhenrick]

Almost like someone plopped another polygon or redrew one on top of another?

[almccon]

Or they were imported from two different datasets that had different generalization or different datums.

Or, even more of a nightmare: maybe those are the real boundaries... kind of like how the state borders are totally messed up along the Mississippi as the river moved but the legal boundaries didn't. Maybe there's something equivalent that happens on mountain ridges?

[clhenrick]

Looks like one is an administrative boundary but also a "natural = divide" while another is a wilderness area:

[clhenrick]

Well for the meantime a879104 blurs the green areas' boundaries which should help hide these.

[almccon]

darn this crowdsourced data! I mean, it looks like the wilderness area ought to join seamlessly with the other shapes, but instead the mapper traced it with new boundaries, it seems.

I'm going to mention it on the OSM IRC channels and hopefully somebody who is a better mapper than I am will fix it.

[clhenrick]

Ha, yeah that's a pretty annoying issue!

[clhenrick]

@mojodna anyway we could generate contour lines on the fly from Mapzen DEM data using the AWS workflow? Probably not feasible for the preview release but if possible might be a nice to have.