Saving XML files using ElementTree

It seems that you have to declare your namespace, meaning that you need to change the first line of your xml from:

<ns0:trk>

to something like:

<ns0:trk xmlns:ns0="uri:">

Once did that you will no longer get ParseError: for unbound prefix: ..., and:

elem.tag = elem.tag[(len('{uri:}'):]

will remove the namespace.

You need to register all your namespaces before you parse xml file.

For example: If you have your input xml like this and Capabilities is the root of your Element tree.

<Capabilities xmlns="http://www.opengis.net/wmts/1.0"
    xmlns:ows="http://www.opengis.net/ows/1.1"
    xmlns:xlink="http://www.w3.org/1999/xlink"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:gml="http://www.opengis.net/gml"
    xsi:schemaLocation="http://www.opengis.net/wmts/1.0 http://schemas.opengis.net/wmts/1.0/wmtsGetCapabilities_response.xsd"
    version="1.0.0">

Then you have to register all the namespaces i.e attributes present with xmlns like this:

ET.register_namespace('', "http://www.opengis.net/wmts/1.0")
ET.register_namespace('ows', "http://www.opengis.net/ows/1.1")
ET.register_namespace('xlink', "http://www.w3.org/1999/xlink")
ET.register_namespace('xsi', "http://www.w3.org/2001/XMLSchema-instance")
ET.register_namespace('gml', "http://www.opengis.net/gml")

In order to avoid the ns0 prefix the default namespace should be set before reading the XML data.

ET.register_namespace('', "http://www.topografix.com/GPX/1/1")
ET.register_namespace('', "http://www.topografix.com/GPX/1/0")