Using Garmin GPS units with Custom Maps

Garmin® GPSMap64s

Garmin devices support the following map formats (though the extent of support varies by model):

  • IMG (.img) Proprietary Garmin vector map
  • JNX (.jnx) Garmin BirdsEye high resolution raster map imagery (locked to a specific device). JNX format maps are available via the Garmin BirdsEye subscription service. They are processed by Garmin’s BaseCamp software which is used to upload them to a GPS. Being a raster format JNX also supports satellite imagery.
  • GPX (.gpx) GPs eXchange format is a universal open source XML data format used to store waypoints, routes and tracks. The format is open and can be used without the need to pay licence fees. Location data (and optionally elevation, time, and other information) are stored in tags and can be interchanged between GPS devices and software. Common software applications for the data include viewing tracks projected onto various map sources, annotating maps, and geotagging photographs based on the time they were taken.
  • KMZ (.kmz) User created raster map (not locked to device). A compressed format incorporating a files folder and a KML (.kml) file. The files folder contains map tiles which when assembled form a complete map. The KML file contains the latitude and longitude of each side of each tile contained in the files folder. Also stored is the rotation angle of each tile.

These files are stored in the following locations on a Garmin GPS:

IMG: [GPS]\Garmin\ or [microSD]\Garmin\
KMZ: [GPS]\Garmin\CustomMaps\ or [microSD]\Garmin\CustomMaps\
JNX: [GPS]\Garmin\BirdsEye\ or [microSD]\Garmin\BirdsEye\
GPX: [GPS]\Garmin\GPX\ or [microSD]\Garmin\GPX\

An IMG format BaseMap is included with each Garmin GPS and additional maps may be purchased directly from Garmin or via third party sellers. Originally the delivery method was CD or DVD but these days they come pre-loaded on an SD card or as a download from the Internet. There are two series of outdoor maps available in the UK – TOPO Pro & TOPO Light, the former (more expensive!) versions including contours and active routing. Other maps that may be purchased include the City Navigator series (title self-explanatory). Maps of variable quality based on OpenStreetMap data are also readily available. Garmin IMG maps are generally routable as are many of the OpenStreetMap versions.

User created raster maps (KMZ)

Garmin supports raster imagery like aerial photos and topo maps for their newer product lines such as the Colorado, Oregon, and Dakota series. It requires conversion of the raster images to an open format KMZ image overlay, which can be created in Google Earth:

  • Zoom to the desired location in Google Earth
  • Load the map or aerial image as an Image Overlay
  • Calibrate the image by manually positioning, rotating and resizing it until it corresponds to the underlying Google Earth imagery.
  • Save it as a KMZ file.
  • Copy it to the \Garmin\Custom Maps folder on your Garmin GPS unit. (Alternatively, import the KMZ file into Garmin’s BaseCamp software and transfer it to the GPS using the tool provided. Such imported files are displayed as overlays in BaseCamp).

There are some drawbacks:

  • The image must be no larger than one megapixel in size (1024 x 1024; 2048 x 512; etc).
  • You can chop an image into subsections/tiles and combine them into a single overlay file, but this requires individually calibrating each tile.
  • You can have no more than 100 such tiles loaded onto your GPS unit at any one time.

To get round this problem various software packages are available – almost exclusively at a cost! Indeed, the only free package we are aware of that converts KMZ files into a tiled format that can be uploaded to a Garmin GPS unit is known as “KMZ Factory“.

Note that KMZ files do not have to be tiled for them to be displayed in BaseCamp – tiling is necessary only if they are to be uploaded to a Garmin GPS.

File name restrictions

Below is a partial list of common map file names reserved for use by Garmin devices:

  • gmapprom.img | Primary or Pre Installed Map
  • gmapprom1.img | Secondary Map
  • gmapbmap.img | Device Base Map
  • gmapoem.img | OEM Installed Map (can not be disabled in Map Settings)
  • gmapsupp.img | Map written to device or microSD card by MapSource
  • gmaptz.img | Time Zone Map
  • gmap3d.img | Primary 3D Buildings Map
  • gmap3d1.img | Secondary 3D Buildings Map