On Fri, 1 Jan 2021, Stephen Woodbridge wrote:
On 1/1/2021 9:47 PM, Richard Greenwood wrote:
On Fri, Jan 1, 2021 at 2:36 PM Stephen Woodbridge
<[email protected] <mailto:[email protected]>>
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 <http://www.greenwoodmap.com>
Hi Rich,
Yeah, I get your point. And the engineer in me agrees but users of the map
have complained so I have to at least look into the issue.
Swisstopo appears to draw the contours until they are very close together,
then draw *vertical* lines for the steepest sections
https://map.geo.admin.ch/?lang=en&topic=swisstopo&bgLayer=ch.swisstopo.swissimage&E=2643456.18&N=1158412.03&zoom=9&layers=ch.swisstopo.amtliches-strassenverzeichnis,KML%7C%7Chttps:%2F%2Fpublic.geo.admin.ch%2FX-zbcggpRz-GgyBn6mrj2Q,ch.bav.haltestellen-oev,ch.swisstopo.pixelkarte-farbe-pk25.noscale&layers_opacity=0.85,1,1,1&layers_visibility=false,false,true,true&catalogNodes=1396,1397
Of course these maps have the reputation for being hand drawn and the most
beautiful available anywhere; not sure how you could do something similar
automatically.
(I was going to say that these are expensive maps, but I see that
they will be available online free of charge from March 2021
https://www.swisstopo.admin.ch/en/swisstopo/free-geodata.html
)
One thought I had that might work because I'm dealing with ocean bottom
contours is to do something like:
a) take all contours above X
b) take all contours below Y
c) take every Nth contour between X and Y
This would probably work OK for the drop off on the continental shelf at
least for the East coast, I'd have to look at other areas since this is a
global map, but 98% of the users are on the East coast currently but that is
expanding.
Anyway, it is an interesting problem, I'd like to find a simple solution that
I can build into the postGIS database where I have all the contour lines
stored. Or find a solution that handles the generation of the contour lines
with some kind of adaptive thinning. My guess is that it will not be easy to
do it at the generation level, so I'll probably only be able to do the
thinning during the rendering of the tiles.
Thank you for your thoughts on this,
-Steve
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--
Andrew C. Aitchison Kendal, UK
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