Extract contours from DEMs

[mojodna]

Preliminary approach: use skimage.measure.find_contours on numpy arrays loaded using rasterio. Convert and simplify contours using shapely.

Output Formats

GeoJSON / TopoJSON / MVT vector tiles.

Data Attributes

Data attributes will include an elevation value (an integer), kind value (e.g. contour), and min_zoom value for the zoom when that interval was first included on the map. A 500 meter interval contour would get a min_zoom of 9 in both the zoom 9 tile, and any subsequent zoom 10, 11, 12 & etc tile. A 200 meter interval contour would get a min_zoom of 10 in both the zoom 10 tile and any subsequent zoom 11, 12, 13 & etc tile. The min_zoom value can act as an “index” indicator, but we will not include an index property.

Intervals

Zoom	Interval (Imperial)	Interval (Metric)
9	tk	500m
10	tk	200m
11	1000ft	100m
12	tk	50m
13	200ft	20m
14+	40ft	10m