Garmin’s Swedish Grid Confusion

Posted by Martin R on October 23, 2008
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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.

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  1. #1 DianaGainer
    October 23, 2008

    That reminds me of an old story my granny (now 104 years old) used to tell: A young couple was about to cook a ham for supper. Before putting it in her pot, the bride cut it in half. “Why did you do that?” her husband asked, mystified. “I don’t know,” the young lady answered. “That’s the way my mother always did.”

    So the next time they visited the bride’s mother, they asked her, “Why do you always cut your ham in two before cooking it?” The mother thinks a moment and answers, “I really don’t know. That’s the way my mother always did it.”

    The next time the couple visits the bride’s granny, they ask her the question. Granny laughs and says, “Well, honey, my cookpot weren’t big enough! I had to cut it half in two ’cause it wouldn’t fit!”

    So 3 generations of ladies cutting ham for no reason, because Granny had a new pot by then.

  2. #2 Martin R
    October 23, 2008

    So what you’re saying is that my GPS navigator was designed by an old lady? Right?!?

  3. #3 Per Edman
    October 23, 2008

    Anyone who jotted down the old, faulty coordinate of their golden find might not greatly appreciate if this legacy faulty was “corrected” with no hope of recovery.

    A’course, naming the old, faulty readout something like “old, faulty readout” or “Garmin Legacy Interpretation” might have been preferable.

  4. #4 Martin R
    October 24, 2008

    That’s a very good point. I hadn’t thought of that. But the labelling could, as you say, have been better.

  5. #5 David Haskiya
    October 24, 2008

    And to add even more confusion RT90 is no longer the official national grid, but has been replaced by SWEREF99. I´d guesstimate that the National Heritage Board will demand incoming reports of new sites positioned in that grid sometime during next year.

    Unless of course they come to the conclusion that demanding positions in a certain grid presupposes a hierarchical relationship between the reporter of a new site and the recipient of said report.

  6. #6 Martin R
    October 24, 2008

    David, haha! But please don’t tell me that SWEREF coordinates can be mistaken for RT90 ones? Please?

  7. #7 David
    October 24, 2008

    No, they differ a lot from RT90, but SWEREF99 lat/long can be very similar to WGS84 – they differ very little. The mathematics involved make my head spin, but there´s quite a lot of info on grids and projections at the Nationa Land Survey´s site.

    Web 2.0 is all about user-generated content, so expect to see Bob adding his own “archaeological sites” to FMIS soon! ;-)