What is the Garmin 275... There isn't such a model AFAIK
... The 276 is an older model and uses the BlueChart software for marine charts... and the charts have to be unlocked area, by area, and downloaded into the unit using a PC. Areas not downloaded only have general marine data.
The 378 and 478, are two generations newer, since the 276 was followed by the 376, which was the first unit to support the XM radio weather.
From Garmin's website:
Preloaded and Portable
From curbside to dockside, the GPSMAP 478 has you covered — no downloads or computer required. Its built-in marine charts utilize the latest BlueChart® g2 technology and feature coverage for all U.S. coastal areas, including Alaska and Hawaii. Our g2 mapping technology offers a smoother, more fluid presentation on your screen.
The 378 has the inland data preloaded.
However, all three units, the 276, 378 & 478 can also accept BlueChart data that is unlocked from the CD and downloaded on to their data cards. You do need a proprietary Garmin Data Card programmer to do this... I'm not sure how good the 276's compatibility with the newer BlueChart G2 data formats is though.
Remember, that the charts pre-loaded on to the ROM in the 378 and 478 are from a fixed point in time and do not include any NTMs after the date the data was fixed, so may not be completely accurate to the areas you are sailing in.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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