MapToGround
GPS use and mapping in humanitarian applications

About KMZ and GPX files

KMZ and GPX files have application with handheld GPS units, smart devices and Google Earth.

A Field Guide to the GPS and Associated Technology   August 2021

The "Humanitarian Field Guide to GPS Technology" is a 7 page PDF document that covers GPS basics, waypoints, tracks, scale, coordinate systems, and datums.  It discusses the interaction of KMZ files, GPX files and GPS data with Google Earth.  It includes links to mapping and GPS resources.

    KMZ and KML files – introduction

Keyhole (a software company) Markup Language, KML, files can carry global position data and references to other files (typically graphics files) in a text based format.  Thus, for example, a KML file on its own could contain a set of waypoints but not an associated map image.  Any map image would be on a separate file.

A Keyhole Markup Zip, KMZ, file is a zipped (using the conventional zip protocol) compilation of a KML file and any associated files.  The zipping and unzipping processes are generally performed by the application programs and are transparent to the user.

KMZ files can be displayed in Google Earth Pro and loaded into some map-capable GPS units.

    GPX files – introduction

GPX files carry GPS data (waypoints, routes, and tracks) in a markup language format and they are in a text based format.  The intention of the GPX format is to provide an informal standard (and it certainly has been picked up) that allows the exchange of GPS data across platforms and devices.  Extensive material on GPX files can be found on http://www.topografix.com/gpx.asp.

GPX files can be displayed in Google Earth Pro and can be transferred into and out of GPS units.  GPX files can be edited.  The author has found GPS Utility and EasyGPS useful for editing GPX files.

    KMZ and GPX files on handheld GPS devices – loading – map tiles – file limitations – Garmin Basecamp – file conversion

The detail of loading KMZ files is dependent on the handheld device the user has to hand.  Some Garmin GPS receivers will load and read KMZ files (Garmin list 'custom maps compatible' devices on the support pages of its website).  A user can deposit KMZ files into the Garmin\CustomMaps folder in the same manner as she would manipulate files on a USB memory stick.  Note, typically only the graphical data (a map), not any global position related data (waypoints, routes, ...) that the KMZ file might contain are loaded by this process.  Global position data generally must be loaded separately as a GPX file.  For Garmin units GPX files carrying waypoint data can be loaded into the Garmin\GPX folder.  After a Garmin unit is restarted the KMZ files will appear on the map list and may be enabled or disabled by the user.  Some GPS units have a map display quality option; this item should be set to maximum.  The majority of GPS units will load CPX files.

KMZ files for handheld devices are usually tiled.  The map image is comprised of sections, or tiles (Garmin sometimes calls them 'images').  For MapToGround produced KMZ files the maximum size of a tile is 1024 kb as this is a requirement of some Garmin GPS units.  GPS units have a constraint on the maximum number of tiles they can load.  For some Garmin map-capable units (for example the Dakota 20) the limit is 100 custom map tiles, other units (from the Montana series) can hold 500 tiles.  The Dakota 20 (and other units?) produces a message too many custom map images, they will not be displayed on start-up if the total tile complement exceeds the limit.

The number of tiles in a KMZ file can be determined by expanding the file into its descendants in the places section of the Google Earth sidebar.  MapToGround produced KMZ files have the tile count displayed in the description field.

The Logistics Cluster produces some large pixel dimension maps with contours and contour shading.  These maps can have (as KMZ files) a tile count in excess of 100.  However it is possible in Google Earth to (judiciously ;-) delete individual tiles to bring the tile count to less than 100 and then to save the file.  The reduced file will function in at least a Garmin Dakota 20 unit.

The image on the right is a screen shot from a Garmin Dakota GPS unit loaded with a KMZ file produced by MapToGround from a MapAction JPG file.  The waypoints (ID-6, ID-11, ID-13) were loaded with a GPX file produced by MapToGround from tabulated MapAction data.

KMZ files can be imported into Garmin Basecamp and then uploaded to an appropriate Garmin device.  Importing Garmin Custom Maps

A user may generate position data on Google Earth Pro (routes, waypoints) and then wish to load these data into a GPS unit.  This process may require the user to convert a KMZ file (saved from Google Earth) to another format (most likely GPX).  There are websites where this task can be done online (GPS Visualizer provides a KMZ to GPX service) or suitable computer application programs are available.  MapToGround has found GPS Utility useful for global position file conversions.

    Navigating with personal smart devices – onboard GPS – apps – creating custom maps

Onboard GPS

For secure navigation in locations where the smart device (mobile phone or tablet) where interaction with phone towers or Wi-Fi networks is not possible the device must an inbuilt or "onboard" GPS receiver.  It could prove useful to know if your device has an onboard GPS receiver.  If in doubt check the device specifications or seek advice.  Apps are available that provide the user about the presence and performance of an onboard GPS receiver.  MapToGround found GPSTest(https://github.com/search?q=GPSTest Sean Barbeau) to be very effective when tested on a tablet running the Android operating system.


Navigation Assistance Software

The range of navigation apps that can be obtained for personal smart devices is extensive, and a potential user could spend a considerable amount of time investigating different options.  Relevant field advice for a particular assignment could be very valuable.  There are two broad classes of mapping apps.  One class uses dedicated base maps, for example Google Maps or apps using OpenStreetMap.  Apps in the other class provide a selection of maps and also permit the user to load custom maps.  Examples of the latter class are Avenza Maps and OziExplorer for Android.  Another consideration when selecting a mapping app is the ability (or not) of the app functioning without internet connection.


Creating a Custom Map

Producing a georeferenced map of a remote location to run on a personal smart device is not a trivial exercise.  The OziExplorer method would be a reasonable choice for the task as it is flexible in the range of image files and coordinate systems it can work with.  If you have a way of producing geo-TIFFs then the Avenza route may be more straightforward though the outcome has less utility.  The following tools may be useful:

Google Earth Pro, www.google.com.au/earth/versions/#download-pro

Source of images that could used as base maps.  Source of identifiable points that could assist geo-referencing.  Display of KMZ files for testing.


IrfanView www.irfanview.com/

For conversion of PDF files to raster files.  Options are available that will determine the size and resolution of the output file..


Mapwel, www.mapwel.net

Mapwel will generate KMZ files from an image and matching OziExplorer *.map file.  The user can select opacity (why not 100%) and draw order.


OziExplorer, www.oziexplorer4.com/au/

OziExplorer generates calibration *.map files for OziExplorerPC and hence also, via Img2Ozf.exe files for OziExplorer for Android.


Quantum GIS, qgis.org/en/site/

Quantum GIS will generate geo-TIFF files suitable for Avenza from raster files.  Take care not to make excessively large TIFF files.

    KMZ files, GPX files, GPS data and Google Earth – Google Earth Versions – loading files – documentation – saving files

Currently (August 2021) there are 3 active versions of Google Earth.

1) Google Earth on web – Currently browser dependent; presumably Google Chrome is a safe choice.  Appears to be directed towards entertainment rather than utility.

2) Google Earth on mobile – Appears to be directed towards entertainment rather than utility.

3) Google Earth Pro on Pro – Google Earth Pro on desktop with load/save options and inspection and markup capacity, is the utility version. 

A KMZ file will open in Google Earth Pro as a map overlay (File --> Open (Google Earth Files)).  The transparency (or its complement, opacity) of a map image can be controlled with the slider at the base of the "Places" box in the side bar.  Many KMZ maps are tiled.  Map tiling can be exposed by expanding the map reference and the tile transparency individually managed.  KMZ maps may be overlayed one on another and alignment of features checked by use of the transparency slider.  Perhaps trial these three files to see the effect – Brazzaville RVs, Brazzaville sectors and Brazzaville overview.  If a KMZ map image with large expanses of water has patches of the underlining Google image breaking through, try unchecking the "Terrain" box in the "Layers" section of the side bar.

When a KMZ file with a map image is loaded into Google Earth Pro then the Google Earth Pro tools, particularly for distance measurement, can be used on the data provided on the map image.

Google Earth Pro will display tracks and waypoints from GPS receivers and GPX files.  There are several ways of loading GPS data into Google Earth Pro, however, in general application the most utilitarian approach is to first transfer the data files from the GPS unit to the device hosting Google Earth Pro and to then open those files in Google Earth Pro.  In the case of GPS units with a USB interface generally the files can be dragged and dropped across devices.  (The drag process crosses drives, so it is a copy of the file that is dragged.)  For units not accessible in that manner, a GPS interface program (for example GPS Utility, EasyGPS or OziExplorer) can be used to get data from the receiver and save it as a GPX file.

As the second step of the process open the GPX file in Google Earth Pro through the "File → Open" menu (select files of type "Gps"(sic)).

If there are data missing, try adjusting the history bar which may have appeared in the top left corner of the Google Earth Pro screen.

The image to the above right is an extract from a Google Earth Pro screen.  The terrain is part of Haiti, the waypoints were loaded with a GPX file produced by MapToGround from Logistics Cluster data.

Google Earth Pro provides some documentation capacity.  Areas can be outlined, placemarks inserted and images inserted.  The process improves with practice ;-).  The file UNOSAT Gaza Strip map outlines shows the use of some documentation items.

The total data displayed in the Google Earth Pro window (say – UNOSAT map, waypoints imported from GPS unit, placemarks) can be saved as a single KMZ file.  If the data in the Google Earth Pro window does not contain images these data can be saved as a KML file which can then be converted to GPX file with, for example, GPS Utility, EasyGPS or OziExplorer.

Updated: 11 August 2021       MapToGround introduced July 2010.       site management