contour-osm

contour-osm is an application to export contour lines to an osm or pbf file. The datasource can be any vector file format supported by GDAL as well as a database.

NOTE: This application is written to export the contour data to a Garmin GPS device. It is not intended to upload such contours into the openstreetmap database. Do not upload the generated data to the OSM server!

History

Phyghtmap

Since the idea for this application is borrowed from Phyghtmap it deserves being credited here. Phyghtmap is a complete package which aims to do 3 tasks:

Download SRTM files from either NASA or viewfinder
Generate contour lines
Convert said contour lines to osm, o5m or pbf

There are a number of advantages to this all-in-one approach, especially for inexperienced users, but it would be interesting to have separate programs for each of the steps above since they could be useful in other contexts as well. As it turns out these applications exist already:

NASA's dba which allows downloading a specific area as well, or any browser if you really want to download manually
The GDAL suite, especially gdal_contour. This library is used internally by phyghtmap, however the contours are not calculated by GDAL.
ogr2osm

Another limitation of Phyghtmap is the lack of support for datasources other than SRTM or viewfinder. As described below in obtaining elevation data there are more accurate datasources which are not necessarily distributed as hgt files.

Also, in some circumstances you may already have converted the downloaded files to contour lines and possibly you even inserted them in a database (for example when you use hikingmap or opentopomap).

Proof of concept using ogr2osm

You can use ogr2osm to achieve the desired result. First you should build a query to fetch the data, or have a shape file ready containing the area you want to export.

Given a table elevation with the elevation in the column height and the according contourline in the column geom, an example SQL statement may look like this:

select height, ST_Intersection(geom, p.polyline)
from (select ST_GeomFromText('MULTIPOLYGON (((6.051 50.5191 0,6.1232 50.5191 0,6.1232 50.4792 0,6.051 50.4792 0,6.051 50.5191 0)))', 4326)  polyline) p, elevation
where ST_Intersects(geom, p.polyline)

Remark: contour-osm contains a Polyfile class to convert osmosis poly-files to WKT, which is usable in your query.

Create a translation for ogr2osm and save as contour-translation.py:

'''
A translation class for contour data
'''

import ogr2osm

class ContourTranslation(ogr2osm.TranslationBase):
    def filter_tags(self, attrs):
        if not attrs:
            return
        
        tags={}
        
        if 'height' in attrs:
            tags['ele'] = attrs['height']
            tags['contour'] = 'elevation'
            
            height = int(attrs['height'])
            if height % 500 == 0:
                tags['contour_ext'] = 'elevation_major'
            elif height % 100 == 0:
                tags['contour_ext'] = 'elevation_medium'
            else:
                tags['contour_ext'] = 'elevation_minor'
        
        return tags

Then, run ogr2osm using the query and translation above:

ogr2osm -t contour-translation.py -o test.osm --sql "select height, ST_Intersection(geom, p.polyline) from (select ST_GeomFromText('MULTIPOLYGON (((6.051 50.5191 0,6.1232 50.5191 0,6.1232 50.4792 0,6.051 50.4792 0,6.051 50.5191 0)))', 4326)  polyline) p, elevation where ST_Intersects(geom, p.polyline)" "PG:dbname=gis user=gis host=localhost"

The resulting OSM file is compatible with the output of Phyghtmap and can be converted to a Garmin map image using mkgmap. For more information on this step see the OSM wiki.

Obtaining elevation data

What you are looking for is often called DEM (digital elevation model) or DTM (digital terrain model). It is important to find a model with an acceptable resolution, generally a precision of 3 arc-seconds is enough, meaning that there is one value for the altitude in each area of 3 arc-seconds squared. This value may not be accurate when it is measured by a satelite, the height of trees, snow or other objects may be measured in stead of the height of the terrain itself.

NASA's Shuttle Radar Topography Mission, version 2.1. For land areas between 60 degrees south and 60 degrees north they offer a resolution of 3 arc-seconds, but for the United States a 1 arc-second version is available.
NASA's Shuttle Radar Topography Mission, version 3. This source covers the same area as version 2.1, but in a 1 arc-second resolution. You need to register first before having access to the EarthExplorer and it is suggested to use the Bulk Download Application to download vast areas. For more information please consult the help index.
However, version 3 of NASA's SRTM seems to contain large voids, a void-filled version is around as well. You need to register first and the user account is not the same as for the EarthExplorer.
Viewfinder Panoramas. This source offers global coverage in a 3-arc second resolution, it is the only option beyond 60 degrees north. There is a limited selection of areas in a 1 arc-second resolution as well.
Resolution does not always equal accuracy, data may have been interpolated from lower resolutions or inserted from other sources. For a limited selection of European countries there is a 1-arc second resolution available with a higher accuracy. The data is obtained using laserscan (LiDAR) in stead of satelites.
Commercial elevation models, which may offer a higher resolution or more accurate data. Keep in mind that the data may be offered in a different projection, use the GDAL tools to reproject the data. Also note that each country has a different definition of sea level, your altitude lines may shift at the borders when combining several elevation models.

Converting elevation data to contour lines

You may be able to download ready-to-use contour lines in the form of shapefiles from some sources, but most of the time you must convert the downloaded raster data to contours yourself. In such a case it is preferred to obtain either HGT or GeoTIFF files, but any other raster format supported by GDAL will do.

Before calculating the contour lines you may need a few preparations. First of all it is common to find raster files in which the elevation is expressed in centimeter in stead of meter. This is generally good from a precision perspective but you will need to convert using gdal_calc.py:

gdal_calc.py -A [infile] --calc "A/100" --outfile [outfile]

Next you can convert to projection of the file, however this is not strictly necessary as contour-osm can do the reprojection as well. Typically an EPSG 4326 projection (WGS84) is desired, you can convert with gdalwarp:

gdalwarp -s_srs EPSG:32628 -t_srs EPSG:4326 [infile] [outfile]

Converting raster data to contour lines can be done with gdal_contour. It may not be immediately clear which params you need but the default command for the SRTM datasource is:

gdal_contour -i 10 -snodata 32767 -a height [infile] [outfile.shp]
shapeindex [outfile.shp]

The -i parameter tells the program to create a contour line for every altitude which is divisible by 10, the -snodata parameter means that the value 32767 should be ignored because it's an indication there is no data available.

NOTE: large areas covered by more than one datafile can either be merged together beforehand using gdal_merge or imported in a database. A detailed tutorial on how to use the GDAL tools is beyond the scope of this document but you can refer to the hikingmap elevation import script for an example implementation.

Requirements

Contour-osm has two requirements:

Usage

usage: contour-osm.py [-h] --datasource DATASOURCE [--tablename TABLENAME]
                      [--height-column HEIGHTCOLUMN]
                      [--contour-column CONTOURCOLUMN] [--src-srs SRCSRS]
                      [--poly POLY] [--osm]
                      OUTPUT

Write contour lines from a file or database source to an osm file

positional arguments:
  OUTPUT                Output osm or pbf file

optional arguments:
  -h, --help            show this help message and exit
  --datasource DATASOURCE
                        Database connectstring or filename
  --tablename TABLENAME
                        Database table containing the contour data, only
                        required for database access.
  --height-column HEIGHTCOLUMN
                        Database column containg the elevation. Contour-osm
                        will try to find this column in the given table when
                        this parameter is omitted.
  --contour-column CONTOURCOLUMN
                        Database column containg the contour. Contour-osm will
                        try to find this column in the given table when this
                        parameter is omitted.
  --src-srs SRCSRS      EPSG code of input data. Do not include the EPSG:
                        prefix.
  --poly POLY           Osmosis poly-file containing the boundaries to process
  --osm                 Write the output as an OSM file in stead of a PBF file

roelderickx/contour-osm