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Improving OSM lake polygons using LiDAR data

by Jake Low @ State of the Map US 2025

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by rbs10025 (flickr), CC BY-NC-ND 2.0

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Problem: poor quality lake polygons

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Elevation data

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~500 ft

Traditional DEM

LiDAR DEM

~500 ft

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Generating better

lake polygons

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Input DEM

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Hillshaded

gdaldem hillshade -igor dem.tif hillshade.tif

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Slope map

gdaldem slope dem.tif slope.tif

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Flat areas

gdal_calc.py -A slope.tif --calc 'numpy.where(A < 1.0, 1, 0) \

--type Byte --creation-option NBITS=1 --outfile flat.tif

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Dilated

magick flat.tif -morphology dilate octagon dilated.tif

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Flat areas (close up)

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Dilated (close up)

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Dilated

magick flat.tif -morphology dilate octagon dilated.tif

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Vectorized

gdal_contour -fl 0 flat.tif vectorized.geojson

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Vectorized (close up)

gdal_contour -fl 0 flat.tif vectorized.geojson

~100 ft

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Smoothed

cat vectorized.geojson \

| python geosmooth.py taubin \

| python geosimplify.py 0.000005 \

> lakes-final.geojson

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Results

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Importing with JOSM

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https://tasks.openstreetmap.us/projects/585

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Future work

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[sic]

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peaks❓

lakes ✅

waterways❓

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Sample code available here:

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https://github.com/jake-low/detect-lakes-in-dems

​

Contact me on Slack:

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@jake-low (slack.openstreetmap.us)