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Garmin eTrex 30: Changing Map Data Fields

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  • 17-07-2014 2:04pm
    #1
    Irish Stones
    Registered Users Posts: 2,307 ✭✭✭


    Hello,
    I was reading the user's manual for the Garmin eTrex 30 and at page 28 I found out that it is possible to customize the Data Fields of the Map so that each field can show data and functions selectable from the list at pages 47 to 51.
    I have tried but didn't succeed in getting what I wanted.

    I went to Map -> Menu -> Set Map
    A new page appears and in the third box (Data Field) I selected "4 fields"
    then Back -> Back
    Now on the Map I have for small boxes on the top. The first one shows "Speed", the second shows "Heading", the third shows "Elevation" and the last one shows "Odometer". I suppose these are the default settings for those fields.

    I think I should be able to select each of those fields and assign them a new function chosen from the list but I haven't understood how to do that.

    Would please anyone of you tell how to do this thing? I would really appreciate it!
    Thanks so much ;)


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    #2
    coylemj
    Registered Users Posts: 25,327 ✭✭✭✭coylemj


    While looking at the map screen or any screen with data fields, briefly press the 'Menu' button on the left and a menu will popup which includes the option 'Change Data Fields'. When you select this it will highlight the first cell, press down on the mouse button and you'll get a data menu, scroll to the data you want displayed in that cell, press the mouse button down to select that data and return to the map screen, then scroll to the other cells you want to change and change the data to be displayed in the same way. When you're finished, press the 'Back' button on the upper right and the screen will return to normal.


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    #3
    Irish Stones
    Registered Users Posts: 2,307 ✭✭✭Irish Stones


    Amazing! :D
    Thanks a million!!!
    I did what I wanted, though I can't explain why yesterday I couldn't get that popup menu when I pressed the menu button. Doesn't matter anymore now, I have what I wanted ;)
    Thanks!

    EDIT:
    Now there's a problem...
    In the MAP page I have set four fields. The first and the second fields show the location coordinates. The third field is the elevation, the fourth field is the accuracy. The problem seems the elevation that doesn't match the elevation shown in the SATELLITE page.
    The difference, at the same point, is 27 metres.
    The elevation shown in the MAP page is clearly wrong because I know exactly the elevation.
    Is there an explanation for that?
    Thanks again :)


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    #4
    Alun
    Registered Users Posts: 21,419 ✭✭✭✭Alun


    That's easy. Your etrex 30 has a barometric altimeter which needs calibration before every use. The elevation shown on the satellite page is the GPS derived elevation, whereas the one shown elsewhere in the data fields is the barometric altimeter derived one. You have to go to setup > altimeter and either input the correct elevation at the start of a walk manually or use the gps elevation initially. The altimeter should also be set for auto calibration so that it can detect significant changes in barometric pressure due to weather conditions and compensate for them. Alternatively if you go into settings for the altimeter and change the barometer mode to fixed elevation, it will then show the gps derived elevation instead. Personally I find the barometric altimeter more trouble than its worth, especially in changeable weather conditions where the autocalibration seems to get confused, so always have it set this way. The sensor also seems to be affected by high winds.


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    #5
    Irish Stones
    Registered Users Posts: 2,307 ✭✭✭Irish Stones


    Alun wrote: »
    That's easy. Your etrex 30 has a barometric altimeter which needs calibration before every use. The elevation shown on the satellite page is the GPS derived elevation, whereas the one shown elsewhere in the data fields is the barometric altimeter derived one. You have to go to setup > altimeter and either input the correct elevation at the start of a walk manually or use the gps elevation initially. The altimeter should also be set for auto calibration so that it can detect significant changes in barometric pressure due to weather conditions and compensate for them. Alternatively if you go into settings for the altimeter and change the barometer mode to fixed elevation, it will then show the gps derived elevation instead. Personally I find the barometric altimeter more trouble than its worth, especially in changeable weather conditions where the autocalibration seems to get confused, so always have it set this way. The sensor also seems to be affected by high winds.

    I suspected that the barometric reading was shown, but I thought that I was able to choose between the two possibilities in the functions list :rolleyes:
    The altimeter is already set for auto calibration.
    Now I have set the Mode in Fixed Elevation and if I have understood it well, now it should show the GPS elevation in both pages.
    I'll try it and let you know ;)
    Thanks a lot!


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    #6
    coylemj
    Registered Users Posts: 25,327 ✭✭✭✭coylemj


    Make sure you have WAAS/EGNOS switched on, it's in Setup/System. This is an augmentation system which involves the handset receiving correctional data which significantly enhances the accuracy of the altitude as calculated based on satellite data - nothing to do with the barometric functionality which is purely based on atmospheric pressure.

    To check to see if you're receiving the data, stand outdoors with a good view of the southern sky, look at the satellite screen and make sure that you can see satellite 33 which is the main satellite providing correctional data in this part of the world. After a minute or two you should see the letter 'D' at the bottom of most of the satellite signal bars, that means you have received correctional data for those satellites. Check the altitude now and compare it to the value you expect.

    http://www.esa.int/Our_Activities/Navigation/The_present_-_EGNOS/What_is_EGNOS
    http://en.wikipedia.org/wiki/European_Geostationary_Navigation_Overlay_Service


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    #7
    Alun
    Registered Users Posts: 21,419 ✭✭✭✭Alun


    As I explained above, unless you have changed the settings from their default, the elevation shown in the data field is the barometric altimeter derived elevation, auto corrected by an algorithm that compares it with the GPS derived elevation, and corrects it if significant differences are found, but with quite a slow convergence algorithm. In other words, if you have just switched the unit on, and the sea level barometric pressure is significantly different from what it was the last time you used it, there will be a significant difference between the two.

    Normal practice is to calibrate the altimeter at the start of an activity either to a known elevation from a map, to a known reference sea level pressure from a nearby weather station, or just to accept the GPS derived elevation. If you don't calibrate it you'll have a significant error until the autocalibration algorithm settles down, and even then in times of rapidly changing sea level barometric pressure, e.g. when a storm front is approaching it may never converge properly at all.

    The elevation shown on the satellite overview screen is always the GPS elevation.

    Now, whilst it's always a good idea to enable WAAS/EGNOS anyway, with a good clear view of the sky, it's highly unlikely that this will make as big a difference as 27m as the OP observed.


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    #8
    coylemj
    Registered Users Posts: 25,327 ✭✭✭✭coylemj


    Could the altitude calculated from satellites be affected by which datum he's using? If he used the Irish Grid and Ireland 1965 datum, would he get a more accurate satellite altitude?


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    #9
    Alun
    Registered Users Posts: 21,419 ✭✭✭✭Alun


    No, both are displayed according to the currently selected datum i.e. corrected for differences between the WGS84 and local reference ellipsoid, Ireland 1965 in this case.


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    #10
    Irish Stones
    Registered Users Posts: 2,307 ✭✭✭Irish Stones


    Alright, I followed both advices and I can tell you that the accuracy has improved from 3 m to 2 m, the elevation now is correct in both screens, so I could say that at the moment I'm more than satisfied :D

    I think that both of you have missed the fact that I am not in Ireland ;), so Ireland 1965 does not apply to me at the moment, but it does when I spend my holidays in your country (I have just returned).
    My profile says where I live, my signature tells you who I am.
    I follow Boards.ie because it's the best place I have found so far to look for (and find) all the answers I need, whatever the issue is.
    God bless you all!


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    #11
    coylemj
    Registered Users Posts: 25,327 ✭✭✭✭coylemj


    Glad you found the advice useful. One modification to my advice above given that you are based in Italy - EGNOS data will be supplied by satellites 37 and 39 which are in geostationary orbit at 21.5 E and 25 E respectively. This means that on your satellite screen they will always be at the same location.


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    #12
    Irish Stones
    Registered Users Posts: 2,307 ✭✭✭Irish Stones


    coylemj wrote: »
    Glad you found the advice useful. One modification to my advice above given that you are based in Italy - EGNOS data will be supplied by satellites 37 and 39 which are in geostationary orbit at 21.5 E and 25 E respectively. This means that on your satellite screen they will always be at the same location.

    You must be an expert, all your words have been extremely helpful.
    Thank you again, very kind of you! :)


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    #13
    coylemj
    Registered Users Posts: 25,327 ✭✭✭✭coylemj


    Irish Stones wrote: »
    You must be an expert, all your words have been extremely helpful.
    Thank you again, very kind of you! :)

    'Expert' would be stretching things but glad to be of assistance. I'm very familiar with the Garmin menus because I'm now on my 4th eTrex (20) having started with the original yellow model and the EGNOS system was 'coming soon' for so long that we all had plenty of time to read the details of the system before it eventually went live a few years ago.


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