@bug1

You're not mentioning what BlueChart you try with GPSMAP 521s. If it's a legacy BlueChart (generation 1) then it will not work. Try with BlueChart g2 or BlueChart g2 Vision.
Code:
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If you have a BlueChart (g1) and want to use that in 521s you might try to modify the "gmapsupp.img" file with GMapTool. Enable experimental options. Go to the "Write" tab and adjust the "TRE" paramterers. Some TRE figures to explore here - [Only registered and activated users can see links. ]