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Given a starting point and a travel time, I would like to produce a travel distance polygon. Can I request this from osm2po via http?

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What kind of polygons are you thinking of? ConvexHull, Alphashapes, IsoLines? ... osm2po returns either a cloud of visited vertexes or the convex hull. Here is the exact copy from the osm2po-online-help-page:

It is possible to calculate a route in osm2po,
output the result as GeoJson and to add it as
a layer in QuantumGIS.

* Convert the graph file with [cmd=g]
  and start osm2po with [cmd=r] (Service)

* If you need the street-types (-class)
  enable them by setting
  [graph.support.edgeflags=true] either in the
  osm2po.config file or as additional parameter
  on the commandline.

* Instead of opening the WebGUI in your Browser
  with e.g. [http://localhost:8888/Osm2poService]
  add these Parameters to the address above

  [?cmd=fr&source=100&target=1000&format=geojson]

* The Browser should show you the routing result
  as plain text in geojson format (UTF8).

* If this works, copy the complete address,
  open your QuantumGIS and add the layer under
  [Menu-Layers-Add_VectorLayer]
  A Dialog opens. Set the [protocol] to
  [GeoJSON] and the [Encoding] to [UTF8]
  Finally paste the address into the [URI] Field and
  click [Open]

* The same works with driving circle distance.
  Here osm2po creates a cloud of points (in geojson-format)
  which are reachable in either time or distance from a given
  starting point (source). e.g.:

  1) Show each point which can be reached within a distance of
     5 km from starting point (53.5,10.0):

     [?cmd=fx&source=53.5,10.0&findShortestPath=true&maxCost=5.0&format=geojson]

  2) Show each point which can be reached within a distance of
     12 Minutes (0.2 hours) from starting point (53.5,10.0).

     [?cmd=fx&source=53.5,10.0&findShortestPath=false&maxCost=0.2&format=geojson]

cmd=fx returns the cloud of points. Replace it with cmd=fh to receive the hull.

See also Is it possible to get the points used for the convex hull in osm2po?

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