contours

Generate vector tile contours from USGS data.

Overview

I use OpenMapTiles to create self-hosted vector map tiles. However, I'm interested in building a topographic, outdoors map, and so I need contour lines. The USGS releases 1x1 degree 40' contour line data generated from their 1/3 arc-second seamless DEM. In order to integrate this data with OpenMapTiles, I need to cut the contour lines into vector tiles, and then I can add them as a separate source in my style.json. Something like:

"sources": {
  "openmaptiles": {
    "type": "vector",
    "url": "https://api.maptiler.com/tiles/v3/tiles.json?key={key}"
  },
  "contours": {
    "type": "vector",
    "url": "https://example.com/url/to/tile.json"
  }
}

Where the tile.json should be something like:

{
	"attribution": "USGS",
	"description": "USGS 40' contour lines",
	"format": "pbf",
	"id": "contours",
	"maxzoom": 16,
	"minzoom": 11,
	"name": "contours",
	"scheme": "xyz",
	"tiles": ["https://example.com/url/to/tiles/{z}/{x}/{y}.pbf"],
	"version": "2.0.0"
}

Installation

Clone the repository:

git clone https://github.com/nst-guide/contours
cd contours

This is written to work with Python >= 3.6. To install dependencies:

pip install click requests tqdm

This also has dependencies on GDAL and tippecanoe. I find that the easiest way of installing GDAL and tippecanoe is through Conda:

conda create -n contours python gdal tippecanoe -c conda-forge
source activate contours
pip install click requests tqdm

You can also install GDAL and Tippecanoe via Homebrew on MacOS

brew install gdal tippecanoe

Code Overview

download.py

Downloads USGS contour data for a given bounding box.

python download.py --bbox="west, south, east, north"

This script calls the National Map API and finds all the 1x1 degree contour products that intersect the given bounding box. The script then downloads each of these files to data/raw/. By default, it doesn't re-download and overwrite a file that already exists. If you wish to overwrite an existing file, use --overwrite.

to_geojson.sh

Takes downloaded GeoDatabase contour data from data/raw/ and uses ogr2ogr to turn them into GeoJSON line-delimited data, placed in data/geojson/. By default, it runs ogr2ogr on every downloaded GeoDatabase file, and currently it's not possible to select only specific files to convert.

GeoJSON line-delimited is helpful because that allows Tippecanoe to run in parallel.

tippecanoe

This turns the contour data into Mapbox Vector Tiles, ready to be served.

Options:

• -Z11: Set the minimum zoom to 11. Tippecanoe won't create any overview tiles.
• -zg: Let Tippecanoe guess the maximum zoom level. It seems from testing that it selects 11, i.e. that 11 is high enough to represent the contours
• -P: run in parallel. If the GeoJSON files are not line-delimited, won't actually run in parallel.
• --extend-zooms-if-still-dropping: Not sure if this actually does anything since I'm not using --drop-densest-as-needed.
• -y: only keep the provided attributes in the MVT. I think the only metadata needed for styling is FCode, which determines whether it's a multiple of 200' and should be styled darker, and ContourElevation, which stores the elevation itself. I haven't checked if you can do modular arithmetic on the fly in the Mapbox style specification, but if you can then you could leave out FCode.
• -l: combine all files into a single layer named Elev_Contour. Otherwise, it would create a different layer in the vector tiles for each provided file name
• -o: output file name. If you want a directory of vector tiles instead of a .mbtiles file, use -e
• data/geojson/*.geojson: path to input data

Usage

# Download for Washington state
python download.py --bbox="-126.7423, 45.54326, -116.9145, 49.00708"
bash to_geojson.sh
tippecanoe \
    -Z11 \
    -zg \
    -P \
    --extend-zooms-if-still-dropping \
    -y FCode -y ContourElevation \
    -l Elev_Contour \
    -o contours.mbtiles \
    data/geojson/*.geojson