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

The "Humanitarian Field Guide to GPS Technology" is a 6 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.

    Scenario and Application – in the field – security

An aid worker is deployed at short notice to do an assessment of a flood related disaster.  The UNOSAT map for the region shows major features such as main roads, but because of atmospheric and terrestrial conditions the satellite imagery derived map does not show minor roads or topographical features.

The aid worker will require a GPS receiver to navigate to locations shown on the map and to record new locations of importance discovered in the field.  She loads the KMZ version of the UNOSAT map into her map display featured GPS receiver and uses the unit as a navigation aid.

The simplest implementation of the KMZ approach requires only a map display featured GPS receiver (and initially some method of getting the KMZ file into it).  Because it provides second by second map to ground data, this arrangement has security utility – is-this-driver-taking-the-right-route type of security.  However it is not convenient for the production of extensive documentation.

    KMZ and KML files – introduction

KML (Keyhole (a software company) Markup Language) files can carry global position data and references to other files (typically graphics files) in a markup language format (as for example HTML is) and as a result they are text based.  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 KMZ (Keyhole Markup Zip) 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 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 (as for example HTML is) and as a result they are text based.  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 and can be transferred into and out of GPS units.

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

The detail of loading KMZ files is dependant on the handheld device the user has to hand.  Some Garmin GPS receivers (Garmin list of 'custom maps compatible' devices) will load and read KMZ files.  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 Garmin eTrex 10, although not a map display unit, will load GPX files.

After the 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 display quality option; this item should be set to maximum.

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 may have a constraint on the maximum number of tiles they can load.  Garmin map-capable units (with the exception of the Montana series) can hold 100 custom map tiles.  A Montana series unit can hold 500 tiles [Garmin Support, "file limitations"].  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.

A user may generate position data on Google Earth (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 and EasyGPS useful for global position file conversions.

    Other portable devices with GPS capability – smart devices – apps – hints – data transfer

MapToGround is not currently placed to investigate a range of devices and their use of KMZ files.  The following is a summary of comments from a colleague with communication and survey experience in the field.

Smart cell phones, laptops and tablets are available with GPS and mapping facilities.

However, the GPS component of these units is not necessarily as comprehensive as dedicated GPS units.  Commonly position data is presented as a dot on Google Maps with a limited selection of coordinate systems or lacking an explicit location reference.  The internet must be accessible in order to download the map tiles (otherwise the display is just a red dot in the middle of a grey screen).  The waypoint system is limited.  In brief a GPS equipped smart phone is not necessarily equivalent in that regard to a conventional GPS receiver.

However, with the right phone, and a little preparation, you can get software applications that will equip the phone with all the capabilities you would expect from a GPS receiver.


Things to be alert to when selecting a phone

Will the GPS in the phone work without a cell signal?  When implementing the GPS software in a phone, some manufacturers require the phone to have a connection to a mobile service to implement a feature called assisted GPS.  Assisted GPS uses location data transmitted from the cell phone towers, and the phone then triangulates its approximate position from those data.  (Conventional GPS units obtain data via satellites and are independent of the phone network.)  The assisted GPS method provides a very quick response to a location request and can be effective when the satellite signals are poor due to shading from buildings or the like.  However the assisted method will not work where there is no cell phone signal.

The assisted GPS method relies on each cell phone tower being correctly programmed with its location.  This may not be the case, especially in a disaster relief situation.  If you are considering a phone with assisted GPS, check to see if you can turn the assisted GPS off and that the GPS component will function using the satellite signals alone.

Test the display in poor and bright light.  Some touch screens lose sensitivity at temperatures over 40 C.


Features to look for when selecting a smart phone GPS application

Multiple Coordinate Systems – Lat/Long, MGRS, UTM – so that you can convert between coordinate systems.  A single datum, provided it's WGS84, is sufficient.

Map Caching – permits when the internet is available downloading in advance maps of the area of interest.  Thereafter the user will not need signal or internet access for the mapping function to operate.  Take care with zoom levels, a one copy of the each map is required for each zoom level.

Waypoint Recording and Navigation Functions – to be reasonably expected, but surprisingly not available on some applications.

MotionX (links) is an iPhone/iPad application with comprehensive GPS features.  The manual is thorough and readily available for perusal.

Connectivity back to a laptop for some combinations of smart phones and applications is via email, one photo or waypoint at a time – a little painful when you have recorded lots of waypoints.

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

A KMZ file will open in Google Earth 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 then the Google Earth tools, particularly for distance measurement, can be used on the data provided on the map image.

Google Earth will display tracks and waypoints from GPS receivers and GPX files.  There are several ways of loading GPS data into Google Earth, 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 and to then open those files in Google Earth.  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 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 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 screen.

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

Google Earth 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 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 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: 18 June, 2017       MapToGround introduced July 2010.       site management