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HomeiOS DevelopmentSanitizing GPX information for public sharing – Ole Begemann

Sanitizing GPX information for public sharing – Ole Begemann


GPX is a well-liked XML format for operating or biking tracks with geocoordinates. It is a how-to for cleansing up a GPX file by eradicating undesirable or privacy-sensitive info.

Many apps that report exercise routes and might export them as GPX information embrace extra knowledge than the plain GPS coordinates. As an illustration, a GPX file from my favourite recording app, Guru Maps, appears like this:

<?xml model="1.0" encoding="utf-8"?>
<gpx model="1.1" creator="Guru Maps/4.5.2" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xmlns="http://www.topografix.com/GPX/1/1" 
  xmlns:gom="https://gurumaps.app/gpx/v2" 
  xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd https://gurumaps.app/gpx/v2 https://gurumaps.app/gpx/v2/schema.xsd">
  <trk>
    <identify>Barnimer Dörferweg</identify>
    <kind>TrackStyle_FF7F00C8</kind>
    <trkseg>
      <trkpt lat="52.6254614634" lon="13.4092010169">
        <ele>54.238586451</ele>
        <time>2020-05-10T05:30:38.997Z</time>
        <hdop>4.6875</hdop>
        <vdop>3.375</vdop>
        <extensions>
          <gom:pace>5.5661926282</gom:pace>
          <gom:course>329.1938658731</gom:course>
        </extensions>
      </trkpt><!-- 1000's of observe factors -->

This observe contains the next properties for every observe level:

  • Geocoordinates (latitude and longitude)
  • Elevation
  • Timestamp
  • Horizontal and vertical dilution of precision (hdop/vdop)
  • Present pace
  • Present course/heading

Plus, Guru Maps makes use of the observe’s <kind> attribute to encode the colour of the observe as displayed within the app in a non-standardized format (TrackStyle_FF7F00C8).

Some apps additionally embrace coronary heart fee or different health measurements.

All this knowledge is beneficial for archiving tracks or importing them into one other app. However earlier than sharing this observe publicly, I’d need to clear the information up first:

  • The one actually vital items of knowledge are the coordinates and presumably the elevation.
  • Timestamps are personal knowledge. I don’t need to share these.
  • The opposite measurements are largely irrelevant.

GPX information can change into fairly massive (1000’s of observe factors is frequent), so decreasing the quantity of knowledge can be good for file sizes and parsing efficiency.

Necessities

  1. XmlStarlet

    I exploit Xml to do many of the XML processing. On macOS, you may set up XMLStarlet through Homebrew:

  2. xmllint

    One optionally available processing step makes use of xmllint, which comes preinstalled on macOS.

  3. XSLT file for eradicating unused namespaces

    Lastly, obtain this XSLT file remove-unused-namespaces.xslt, both from this Gist or from my server. We’re gonna use it in a single processing step to strip unused namespaces from the GPX file.

    Authentic supply: Dimitre Novatchev on Stack Overflow.

Working the command

Assuming your supply file is called enter.gpx and the XSLT file you downloaded above is within the present listing, that is the total command to course of the GPX file and save the consequence to output.gpx:

xmlstarlet ed 
  -d "//_:extensions" 
  -d "/_:gpx/_:metadata/_:time" 
  -d "/_:gpx/_:trk/_:kind" 
  -d "//_:trkpt/_:time" 
  -d "//_:trkpt/_:hdop" 
  -d "//_:trkpt/_:vdop" 
  -d "//_:trkpt/_:pdop" 
  -u "/_:gpx/@creator" -v "Shell script" 
  enter.gpx 
  | xmlstarlet tr remove-unused-namespaces.xslt - 
  | xmlstarlet ed -u "/_:gpx/@xsi:schemaLocation" -v "http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd" 
  | xmllint --c14n11 --pretty 2 - 
  > output.gpx

This sequence performs the next steps:

  • Delete all <extensions> parts.
  • Delete the timestamp from the file’s <metadata> part if current.
  • Delete the <trk><kind> ingredient.
  • Delete the <time>, <hdop>, <vdop>, and <pdop> parts from all observe factors.
  • Set the file’s creator attribute.
  • Now that extension fields are gone, take away all unused XML namespaces from the file header.
  • Delete all xsi:schemaLocation entries besides the one for the GPX schema.
  • Run the file via xmllint for formatting. The --c14n11 choice performs XML Canonicalization (C14N). Amongst many different issues, canonicalization replaces numeric character entities within the XML with their regular Unicode characters, which is vital for my use case.

    For instance, the textual content “Dörferweg” within the supply would change into “Dörferweg”. I discovered that among the instruments I exploit insert non-ASCII characters as numeric codes and different instruments don’t show these accurately.

The processed GPX file appears like this:

<gpx xmlns="http://www.topografix.com/GPX/1/1" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  creator="Shell script" model="1.1" 
  xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd">
  <trk>
    <identify>Barnimer Dörferweg</identify>
    <trkseg>
      <trkpt lat="52.6254614634" lon="13.4092010169">
        <ele>54.238586451</ele>
      </trkpt>
      <trkpt lat="52.6255090307" lon="13.4091548326">
        <ele>53.9600219977</ele>
      </trkpt>

The processing steps above are those that work for me given the apps I exploit. Your mileage could fluctuate in case your instruments add different knowledge to your GPX information. Be happy to edit the command accordingly. XmlStarlet makes use of XPath syntax to pick out which parts to function on. The xmlstarlet sel command is beneficial for inspecting a supply file and attempting out the required XPath incantations.

Validation

Lastly, it’s a good suggestion to validate the processed GPX file in opposition to the official GPX schema:

xmlstarlet val --quiet --err --xsd 
  http://www.topografix.com/GPX/1/1/gpx.xsd 
  output.gpx

Comfortable processing!


PS: Should you’re ever in Berlin, this can be a good lengthy bike route (55 km) with minimal automotive site visitors. Begins and ends at Hauptbahnhof. Obtain the (sanitized) GPX file.

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