Tiles#
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import holoviews as hv
from holoviews import opts
hv.extension('bokeh')
The Tiles element represents a so called web mapping tile source usually used for geographic plots, which fetches tiles appropriate to the current zoom level. To declare a Tiles element simply provide a URL to the tile server, a standard tile server URL has a number of templated variables which describe the location and zoom level. In the most common case that is a Web Map Tile Service (WMTS) source which looks like this:
'https://tile.openstreetmap.org/{Z}/{X}/{Y}.png'
Here {X}, {Y} and {Z} describe the location and zoom level of each tile. Alternative formats include bbox tile sources with {XMIN}, {XMAX}, {YMIN}, {YMAX} variables, and quad-key tile sources with a single {Q} variable.
A simple example of a WMTS source is OpenStreetMap:
hv.Tiles('https://tile.openstreetmap.org/{Z}/{X}/{Y}.png', name="OSM").opts(width=600, height=550)
One thing to note about tile sources is that they are always defined in the pseudo-Mercator projection, which means that if you want to overlay any data on top of a tile source the values have to be expressed as eastings and northings. If you have data in another projection, e.g. latitudes and longitudes, it may make sense to use GeoViews for it to handle the projections for you.
Note:
Holoviews provides functions to project longitude, latitude into Web Mercator coordinates. See
hv.util.transform.lon_lat_to_easting_northing(longitude, latitude)
Both HoloViews and GeoViews provides a number of tile sources by default, provided by CartoDB, OpenStreetMap, and Esri. Stamen tile sources are also available but require a Stadia account when not running locally; see stadiamaps.com.
Tile sources can be imported from the holoviews.element.tiles module and are provided as callable functions which return a Tiles element:
hv.element.tiles.EsriImagery().opts(width=600, height=550)
The full set of predefined tile sources can be accessed on the holoviews.element.tiles.tile_sources and holoviews.element.tiles.stamen_sources dictionaries.
hv.Layout([ts().relabel(name) for name, ts in hv.element.tiles.tile_sources.items()]).opts(
opts.Tiles(xaxis=None, yaxis=None, width=225, height=225)).cols(4)
For full documentation and the available style and plot options, use hv.help(hv.Tiles).
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