MAPC2MAPC Help pages : JNX
Overview
How to ....
Grids and Datums
Map Images
Uses
System Issues

JNX files are used to hold both a map image and its calibration. The files created by MAPC2MAPC are usable only on certain Garmin units that have an active Birdseye subscription or have an unofficial version of the firmware installed. JNX files can be much larger than Custom Map (KMZ) files, can have several levels (although MAPC2MAPC only creates a single level) and are usually faster to display. More details of the JNX file contents here. And JNXSCALE is a simple program to allow the Product ID field and Scale Factor - which controls when the map is displayed - to be changed.

These files are created for those users who wish to use these facilities. Their creation is not a recommendation!

Note that JNX images have to be aligned North-South. The program will reproject maps as needed to achieve this but this introduces areas of padding - usually thin triangles - into the JNX images.

I am pleased to thank Steve McMahon at vbaccelerator for the CRC32 calculation

MOBAT2JNX is a command line program to create multi-level JNX files from Mobile Atlas sets created as OSMTracker or SAS Planet format. JNXMERGE merges the contents of two JNX files to create one - this is for two maps of the same area but different zoom levels.

JNX2KMZ is a command-line program that converts a JNX file to as many KMZ files as there are levels in the JNX

JNXCustomizer is a program to change the scale and other data in a JNX file - Tutorial (in Spanish)

21 March 2011 - programs now updated for new Garmin firmware.
9 July 2020 - add Birdseye as enabler

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This from a user :

Moving into Basecamp and back again works, as Alex Whiter described, and is necessary unless you have hacked firmware. (Incidentally you can use JNXSCALE afterwards if you wish to adjust the scale, without damaging the binding which Basecamp writes to the file).

But even after that, I was having problems displaying the JNX. Seems my Garmin didn't like a tile size of 1024x1024, or even 1000x1000 px. Makes sense, as if you remember, I was having problems before with 1000x1000px tiles when making KMZ custom maps. In that case, dividing a 4000x4000px tile into 18 rather than 16 was enough to make it work (which suggested 924x924px tiles).

Not sure exactly what the threshold is, but reducing to max 950x950 works so far. Though the size I'm actually using may be less, as I assume MAPC2MAPC divides the tiles as equally as possible.

The JNX works perfectly in the Garmin, I honestly can't see a hint of a join between different JNX's. And fast.