Converting Between KML and GPX: A Comprehensive Guide

In the vast realm of geospatial data, KML (Keyhole Markup Language) and GPX (GPS Exchange Format) are two of the most prominent file formats. This guide delves deep into the nuances of converting between these formats, ensuring you have all the knowledge at your fingertips.

Understanding KML and GPX

KML (Keyhole Markup Language) is an XML-based format tailored for visualizing and annotating geographic data, predominantly with Google Earth. It can represent points, lines, polygons, and 3D models.

GPX (GPS Exchange Format), another XML-based format, is designed for exchanging GPS data between software. It’s a favorite for sharing routes, tracks, and waypoints.

Step-by-Step Conversion Guides

Using GDAL

• Single Conversion:
• KML to GPX:
  bash ogr2ogr -f "GPX" output.gpx input.kml
• GPX to KML: ogr2ogr -f "KML" output.kml input.gpx
• Batch Conversion: Use a shell script or batch file.

  for file in *.kml; do
      ogr2ogr -f "GPX" "${file%.kml}.gpx" "$file"
  done

Using QGIS

• Single Conversion:

1. Open QGIS.
2. Import the KML or GPX file.
3. Right-click the layer.
4. Select “Export” > “Save Features As…”.
5. Choose the desired format and save.

• Batch Conversion: Use the “Batch Processing” tool in QGIS.

Using ArcGIS Pro

• Single Conversion:
• Use “KML To Layer” or “Layer To KML”.
• For GPX, use “GPX To Features” and “Features To GPX”.
• Batch Conversion: Use the “Batch” option in the tool dialog box.

Using Python

Single Conversion:

  import geopandas as gpd

  gdf = gpd.read_file("input.kml", driver='KML')
  gdf.to_file("output.gpx", driver="GPX")

Batch Conversion:

  import os

  directory = "path_to_directory"
  for filename in os.listdir(directory):
      if filename.endswith(".kml"):
          gdf = gpd.read_file(os.path.join(directory, filename), driver='KML')
          gdf.to_file(filename[:-4] + ".gpx", driver="GPX")

Using R

Single Conversion:

  library(sf)
  library(XML)

  kml_data <- st_read("input.kml")
  gpx_data <- as(kml_data, "GPX")
  writeOGR(gpx_data, "output.gpx", driver="GPX")

Batch Conversion:

  files <- list.files(pattern="*.kml")
  for (file in files) {
      kml_data <- st_read(file)
      gpx_data <- as(kml_data, "GPX")
      writeOGR(gpx_data, sub(".kml", ".gpx", file), driver="GPX")
  }

Differences Between KML and GPX

Feature	KML	GPX
File Structure	XML-based with nested elements	XML-based with a flatter structure
Geometry Types	Points, Lines, Polygons, 3D Models	Waypoints, Tracks, Routes
Attributes	Rich attribute set with custom data	Primarily for waypoints with limited custom data
Styling	Extensive with color, width, and icon support	Limited, primarily for waypoints
Coordinate System	WGS84	WGS84
Use Case	Visualization in Google Earth	GPS data exchange, tracking, and navigation

Challenges in Conversion

1. Geometry Support: KML supports a wider range of geometries, including 3D models, which GPX does not. When converting from KML to GPX, these additional geometries might be omitted or simplified.

2. Attribute Handling: KML allows for a richer set of attributes with custom data. GPX, being primarily for waypoints, has a limited set. This means some data might be lost when converting from KML to GPX.

3. Styling: KML has extensive styling options, from colors to icons. GPX has limited styling capabilities. This means when converting from KML to GPX, the styling information will likely be lost.

4. Coordinate Systems: Both use WGS84, but there might be subtle differences in how they handle altitude or depth. Always verify post-conversion.

5. File Structure: KML has a nested structure, allowing for hierarchies of folders and elements. GPX has a flatter structure. This can lead to challenges in maintaining the same organizational structure during conversion.

6. Lossy Conversion: Due to the differences in attributes, styling, and geometry support, converting from KML to GPX (or vice versa) can be lossy. This means some information might be lost in the process.

Practical Advice for Smooth Conversion

• Always Backup: Before starting any conversion, always keep a backup of your original files.
• Verify Post-Conversion: After conversion, always check the output file to ensure all necessary data is present and correctly formatted.
• Use Dedicated Tools: For complex conversions, tools like GDAL or QGIS provide more control and flexibility.
• Understand the End Use: If you’re converting for a specific purpose, like GPS navigation, ensure the converted file is compatible with the device or software you’re using.

FAQs on Conversion