6.4 KML and GPX

Key takeaways

• KML (Keyhole Markup Language) is an OGC standard built around Google Earth's 3D presentation.
• GPX is the de facto standard for GPS tracks, routes, and waypoints.
• Both are XML-based and interoperable, with known quirks.

Introduction

Most modern GIS uses binary or JSON formats, but you'll still encounter plenty of KML and GPX in the wild — especially from consumer mapping tools, navigation devices, and fitness apps. This short lesson covers the essentials of both.

KML

KML was developed by Keyhole Inc., acquired by Google, and published as an OGC standard in 2008. It's XML that describes:

• Placemarks (points) with names, descriptions, and styled icons.
• Paths and polygons.
• Overlays (ground, screen, photo).
• Folders and network links (referenced KML resources).
• Camera views and tours.

A tiny KML file:

1<?xml version="1.0" encoding="UTF-8"?>
2<kml xmlns="http://www.opengis.net/kml/2.2">
3  <Placemark>
4    <name>Atlas HQ</name>
5    <Point>
6      <coordinates>12.5683,55.6761,0</coordinates>
7    </Point>
8  </Placemark>
9</kml>

Notice the coordinate order: longitude, latitude, altitude — comma-separated.

KMZ

A KMZ is a ZIP archive containing a KML plus referenced resources (icons, images). It's the preferred distribution format — one file, compressed.

Strengths

• 3D-ready (altitude, extrusions, models).
• Rich presentation (styling, labels, tours).
• Consumer-friendly (Google Earth opens it directly).

Weaknesses

• XML bloat (files are much larger than binary equivalents).
• Presentation is mixed with data (styling in the file).
• Not ideal for analysis workflows.

Converting to/from KML

1ogr2ogr -f KML out.kml parks.gpkg
2ogr2ogr -f GPKG out.gpkg trail.kml

GPX

GPX (GPS Exchange Format) is an open standard for representing GPS data:

• Waypoints — named points of interest.
• Routes — ordered lists of waypoints (planned).
• Tracks — recorded sequences of positions (actual).

A GPX file for a hike:

1<?xml version="1.0" encoding="UTF-8"?>
2<gpx version="1.1" creator="GPS device" xmlns="http://www.topografix.com/GPX/1/1">
3  <trk>
4    <name>Sunday hike</name>
5    <trkseg>
6      <trkpt lat="55.6761" lon="12.5683">
7        <ele>3</ele>
8        <time>2025-04-06T09:00:00Z</time>
9      </trkpt>
10      <trkpt lat="55.6762" lon="12.5685">
11        <ele>4</ele>
12        <time>2025-04-06T09:00:10Z</time>
13      </trkpt>
14    </trkseg>
15  </trk>
16</gpx>

Strengths

• Native support in almost every GPS device and app (Garmin, Strava, Komoot, Apple Health, Polar).
• Time-indexed — the default representation of a GPS track.
• Small schema — easy to parse.

Weaknesses

• XML verbosity; GPS traces can be large.
• Limited attribute flexibility — add custom tags via extensions.
• Single CRS only (WGS 84).

GPX processing

import geopandas as gpd

Modern GPS tools often also emit FIT files (binary, Garmin-specific). Libraries like fitparse (Python) convert FIT ↔ GPX.

When to use each

Common pitfalls

• Axis order: KML is lon, lat, alt; GPX uses lat and lon as XML attributes. Don't confuse.
• Altitude modes in KML (clampToGround, relativeToGround, absolute) matter for 3D display.
• Large GPX files from minute-interval trackers: 40 000+ points in a long hike; thin before analysis.
• KML embedded imagery: KMZ can balloon to hundreds of megabytes; extract before processing.

Self-check exercises

Summary

• KML is for presentation (Google Earth, consumer maps); KMZ is its compressed cousin.
• GPX is the GPS-track lingua franca.
• Both are XML, both are universally supported, both have WGS 84 as the only CRS.
• Convert to a modern binary format (GeoPackage, GeoParquet) for analysis.

Further reading

• OGC KML 2.3 specification.
• Topografix GPX 1.1 schema.
• gpsbabel — swiss-army knife for GPS format conversion.
• GDAL drivers for KML and GPX.