I’ve been using Garmin’s handheld GPS navigators since the spring of 2005; two models running the same firmware. They have been invaluable in archaeological fieldwork, pinpointing finds and test pits swiftly and accurately in situations where you would once have counted steps to the nearest landmark and put an X on a small-scale map. GPS has also helped me a lot when driving, and lured me to seek out over 600 geocaches.
But recently I discovered a really annoying glitch in Garmin’s firmware, having to do with the coordinate readout.
The machine is able to use many tens of different coordinate systems, allowing me for instance to offer coordinates in the Hungarian national grid. The standard grid system used by Swedish surveyors (and archaeologists) is named “RT90 2,5 gon väst” and commonly known simply as “RT90″ or “rikets nät”, the national grid. The Garmin firmware offers a choice between RT90 and rikets nät. This makes no sense, but as both are indicated as using RT90 when referencing your position to the built-in map, it would appear that both menu alternatives give an identical result.
They don’t. I found out during fieldwork last September that coordinates in the two grids differ by about nine meters at Sättuna and about seven at Djurhamn. This is a problem because it means that when you write coordinates on a find bag or in a list of test pits, as I have for four whole fieldwork seasons, you must also make note of which mode your machine is working in at that moment. And I switch a lot to and fro because geocaches are usually pinpointed in good old long-and-lat.
Garmin’s Swedish representative hasn’t been able to explain this to me. But Mät-Niklas has.
Apparently, the first handheld GPS navigators came equipped with a slightly faulty geometrical model of the RT90 grid. After a few years, this was corrected. What has happened in the Garmin case is apparently that they added the corrected model (“RT90″ in the menu) but forgot to delete the faulty one (“Rikets nät”). And so their machines come equipped with a built-in boobytrap for anyone who records coordinates in the Swedish national grid.