Simplify save_graph_xml output

[davidmurray]

Do not delete the following template. Fill it out completely.

• I have read the OSMnx contributing guidelines
• I have read the OSMnx documentation and usage examples
• I have searched the open/closed issues and pull requests for others similar to mine

Is your feature proposal related to a problem?

The save_graph_xml function adds a lot of (unnecessary?) data to the resulting XML file:

osmnx/osmnx/osm_xml.py

Lines 207 to 212 in 586cf26

	for table in (gdf_nodes, gdf_edges):
	table["uid"] = "1"
	table["user"] = "osmnx"
	table["version"] = "1"
	table["changeset"] = "1"
	table["timestamp"] = "2017-01-01T00:00:00Z"

The resulting XML file has a lot of extra fields compared to regular OSM XML files :

<?xml version="1.0"?>
<osm version="0.6" generator="OSMnx">
  <node id="0" timestamp="2017-01-01T00:00:00Z" uid="1" user="osmnx" version="1" changeset="1" lat="45.5571316" lon="-73.5963673">
    <tag k="highway" v="nan"/>
  </node>
  <node id="1" timestamp="2017-01-01T00:00:00Z" uid="1" user="osmnx" version="1" changeset="1" lat="nan" lon="nan">
    <tag k="highway" v="nan"/>
  </node>
  <node id="2" timestamp="2017-01-01T00:00:00Z" uid="1" user="osmnx" version="1" changeset="1" lat="nan" lon="nan">
    <tag k="highway" v="nan"/>
  </node>
  <node id="3" timestamp="2017-01-01T00:00:00Z" uid="1" user="osmnx" version="1" changeset="1" lat="45.5575365" lon="-73.5889391">
    <tag k="highway" v="nan"/>
  </node>
  ...
</osm>

In addition, what's the point of the timestamp if it's always the same date in 2017?

Describe the solution you'd like to propose

Either:

1. Adding a parameter to save_graph_xml to decide whether or not these extra fields should be added to the xml file, or:
2. Removing this code altogether.

Thanks!

[davidmurray]

In addition, it seems that the function adds tags even when the values do not exist:

<node id="2" timestamp="2017-01-01T00:00:00Z" uid="1" user="osmnx" version="1" changeset="1" lat="nan" lon="nan">
    <tag k="highway" v="nan"/>
  </node>

When the values are null/NaN, they shouldn't be written to the xml file in my opinion. Might be worth fixing this as well. :)

[gboeing]

Do the osm_xml_* values in the settings module not allow you to do this as you wish?

[davidmurray]

@gboeing Hi,

Yeah, it seems like it can. ox.settings.osm_xml_node_attrs allows be to specify which keys I want. However, it doesn't let me specify the values. I.e. it's always these:

table["uid"] = "1"
table["user"] = "osmnx"
table["version"] = "1"
table["changeset"] = "1"
table["timestamp"] = "2017-01-01T00:00:00Z"

Since I don't want those it's fine for my use-case, but it could be nice to add a way to set the values.

[gboeing]

@mxndrwgrdnr do you remember the reasoning behind these default values? Are they specified by the API somewhere?

[mxndrwgrdnr]

I don't recall exactly, but I know that at the time I wrote this code I was using a command-line tool (Osmium?) for testing that my outputs were valid .osm files. I'm pretty sure that without these fields present I couldn't get my files to validate, or at least the validator would complain about it.

[gboeing]

Closed by #917

[gboeing]

See enhancements in #1135

User can now specify custom values for each attribute by simply setting those attribute values on the graph nodes/edges before saving as XML.