Tuesday, July 5, 2022
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 data.

Many apps that report exercise routes and may export them as GPX information embody extra information than the plain GPS coordinates. As an example, a GPX file from my favourite recording app, Guru Maps, appears to be like 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>
    <title>Barnimer Dörferweg</title>
    <sort>TrackStyle_FF7F00C8</sort>
    <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><!-- hundreds 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 <sort> attribute to encode the colour of the observe as displayed within the app in a non-standardized format (TrackStyle_FF7F00C8).

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

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

  • The one actually essential items of knowledge are the coordinates and presumably the elevation.
  • Timestamps are non-public information. I don’t wish to share these.
  • The opposite measurements are largely irrelevant.

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

Necessities

  1. XmlStarlet

    I take advantage of Xml to do many of the XML processing. On macOS, you may set up XMLStarlet by way of Homebrew:

  2. xmllint

    One non-obligatory 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.

    Unique supply: Dimitre Novatchev on Stack Overflow.

Working the command

Assuming your supply file is known as enter.gpx and the XSLT file you downloaded above is within the present listing, that is the complete command to course of the GPX file and save the end result to output.gpx:

xmlstarlet ed 
  -d "//_:extensions" 
  -d "/_:gpx/_:metadata/_:time" 
  -d "/_:gpx/_:trk/_:sort" 
  -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> components.
  • Delete the timestamp from the file’s <metadata> part if current.
  • Delete the <trk><sort> factor.
  • Delete the <time>, <hdop>, <vdop>, and <pdop> components 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 essential for my use case.

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

The processed GPX file appears to be like 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>
    <title>Barnimer Dörferweg</title>
    <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 take advantage of. Your mileage could range in case your instruments add different information to your GPX information. Be happy to edit the command accordingly. XmlStarlet makes use of XPath syntax to pick out which components 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 towards the official GPX schema:

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

Glad processing!


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

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