On Fri, Jan 1, 2021 at 2:36 PM Stephen Woodbridge < stephenwoodbridg...@gmail.com> wrote:
> Hi all, > > I'm contouring bathemetry data using gdal_contour and it works really > great. The problem I have is that when depth falls off rapidly like at > the continental shelf or into a canyon, I get too many contour lines > that all bunch up. If I change the contour step size to fix this, then > the flatter areas don't get enough lines. > > I wonder if anyone has any ideas on someway to thin these lines or some > way to do adaptive contouring based on maybe something like scanning the > image first to build a masks that represent these rapid changes in depth > and then change the contour levels in these masked areas. > > I currently contour into a postgis database, the render them using > mapserver into a tile cache since they are static once they are computed. > > I would be interested in any ideas you might have on how to tackle this > problem. > > -Steve W > > Hey Steve, Interesting problem and this isn't an answer, just my opinion. I live and play in a mountainous area and frequently use USGS topo maps with contour intervals of 20, 40 and 80 feet. Each map's contour interval was chosen with criteria like yours - flatter land needs smaller contour intervals, but steeper land can become too cluttered with a small contour interval. But as a map user it drives me crazy when I stitch together adjoining maps with different intervals and try to get a sense of the landscape. Like this for example <https://greenwoodmap.com/tetonwy/mapserver/map#zcr=7.279815109511815/2448564.5062904786/1516712.6778719614/0&lyrs=DRG,Roads,ownership> where 20 foot contours adjoin 80 foot. The western half of the map is much steeper than the eastern, but that's not obvious from a quick look. I'd just let the bunched up contours tell the reader that hey, it's really steep here! Best regards, Rich -- Richard W. Greenwood, PLS www.greenwoodmap.com
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