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Thread: GPS question: Why don't some computer maps agree

  1. #1
    Moderator Peakbagr's Avatar
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    GPS question: Why don't some computer maps agree

    ....Why don't some routes you draw on your computer(N Geo) always agree with the mapping software in your GPS?

    Twice on bushwhacks in the last 3 weeks I've drawn a route on the computer using Nat'l Geographic's latest program. In the woods, on the Garmin 60CSx, the summit, or connecting spots appear in a different place than on the computer?

    In the first instance, the "X" marks the summit was quite a distance from the actual summit. It happened again today. In all cases we walked all over the large summit plateaus, but the GPS was showing the summit to be a distance away. Over by the edge of the summit in the previous example, and down a contour on today's bushwhack.

    When downloading the tracklogs back to NG, it show we were over the true summits.
    Is this a case where the NG raster maps don't always translate to the digitalized Garmin maps in the GPS?
    Most of the time, what I load as a route or waypoint is where it shows up on the GPS. In these 2 instances, they did not.

    Ideas, guys?

  2. #2
    Senior Member DougPaul's Avatar
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    Maps have errors and differ between each other.

    Your GPS (assuming you have good signals) is often more accurate than your maps.

    Doug

  3. #3
    Member Garry G.'s Avatar
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    Could you have a datum problem? You could be off as much as 100 meters by not using the correct datum.

  4. #4
    Senior Member CaptCaper's Avatar
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    Quote Originally Posted by Peakbagr
    ....Why don't some routes you draw on your computer(N Geo) always agree with the mapping software in your GPS?

    Twice on bushwhacks in the last 3 weeks I've drawn a route on the computer using Nat'l Geographic's latest program. In the woods, on the Garmin 60CSx, the summit, or connecting spots appear in a different place than on the computer?

    In the first instance, the "X" marks the summit was quite a distance from the actual summit. It happened again today. In all cases we walked all over the large summit plateaus, but the GPS was showing the summit to be a distance away. Over by the edge of the summit in the previous example, and down a contour on today's bushwhack.

    When downloading the tracklogs back to NG, it show we were over the true summits.
    Is this a case where the NG raster maps don't always translate to the digitalized Garmin maps in the GPS?
    Most of the time, what I load as a route or waypoint is where it shows up on the GPS. In these 2 instances, they did not.

    Ideas, guys?
    It belive it's because the NG Topos are Raster Scanned and the Garmins are Vector Scanned.

    I've always found the Vector ones off. Such as Delorme Topo,etc. as well as Garmins. I wasted over $200 to get into the Delorme Topo with Satelite,etc. maps...only to have the program to be as bad as Garmins....

    I use NG's for all routes or waypoints to be taken and loaded into GPS for navigating.
    I find after the hike the NG shows the track more to the trail and summits in NG better if not right on. W

    When on the ridges and at the summit both the GPS and NG's Topo are right on...... In the woods down along the trails they can be off because of trail changes and signal loss. But along the ridges and at the summit they are Soooooooooo close if not right on....


    I only use Mapsourse to save,edit,load stuff back into my GPS. Never for planning a route,etc.

  5. #5
    Senior Member DougPaul's Avatar
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    Quote Originally Posted by CaptCaper
    It belive it's because the NG Topos are Raster Scanned and the Garmins are Vector Scanned.
    Not necessarily. Either DRGs (raster scan) or DLGs (vector) can be arbitrarily accurate.

    In either case, one can make approximations to reduce the data size of the map. These approximations introduce different kinds of errors.

    In this specific case, are the maps the same scale? The NG TOPO! state series gives multiple scales, the most detailed is an image of a paper 1:25K map*. Garmin Mapsource US Topo is a 1:100K map (displayable at a range of scales). (You have to get the regional Garmin topos to get 1:25K for selected areas.)

    And it is always possible that differing errors (eg rounding or bugs) in the two packages cause the same track to be displayed differently.

    * The USGS had the paper topos scanned at 300 DPI. My version of NG TOPO! rescanned them to 175 DPI. This reduces the data size at the cost of lower resolution. If you want to see the original 300 DPI scans, go to http://libremap.org/.

    Doug

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