Related

Topo map and GPS position do not match

TIGGERTIMUS
over 12 years ago
I have a GPSMAP 62ST with the preloaded 100k US topo map. My position on the topo map is about 500m displaced from where I actually am. Similarly, if I record a track and load it into Basecamp the track is displaced on the map, but if I export the track to another map program the track matches the other map perfectly. Also, the GPS coordinate of any waypoint match the other map perfectly.

So I'm getting good GSP coordinates but the alignment with the map is off. I've tried resetting the map with the blue triangle in the top left corner but nothing changes.

Has anyone got any ideas what I can do?
  • 0 over 12 years ago
    Sounds like a datum mismatch. In most applications use WGS84. Check what your gps is set to and what Basecamp is set to also, then see the same setting in your other mapping program.
  • Former Member
    0 Former Member over 12 years ago
    The 100k topo maps are notoriously inaccurate; this has been discussed at length on other sites. An easy way to see this would be to download BirdsEye imagery for the same location. You will then be able to view the topo map on top of the aerial photo in Basecamp and see the error.

    If you connect your 62st to your computer, you should be able to download BirdsEye and use it in Basecamp without paying for a subscription (the subscription is only needed to use the imagery on your GPS).
  • 0 over 12 years ago
    I have found the 100K topo to be as much as 100 yds off, but 500 yds is quite an achievement!

    You can upload your track to BreadCrumbs or Track Profiler and see better the actual location.
  • Former Member
    0 Former Member over 12 years ago
    Well Birdseye makes it really easy to see what is happening, and you would need to look at the specific location the OP mentions. I posted some screenshots with Birdseye and the 100k topo awhile ago. There is obviously something very wrong in this one, for example. :)

  • 0 over 12 years ago
    Thanks for the fast replies. I checked and I am using WGS84. I also downloaded Birsdeye and it confirms that the topo is way off, I used Basecamp to measure the difference and it's 508m! So it's obviously just a really inaccurate topo map and not some setting that I haven't got right. Thanks for helping though. I could post a pic of the difference if I could figure out how to do that.
  • 0 over 12 years ago
    Boyd's example above shows how far off maps using old Census Bureau Tiger line files can be. I believe that old Tiger files were used on the 2008 100k topo. If you check Boyd's example you'll see that not only is the alignment bad, but so is the topology. There are more roads on the image than on the map.
  • 0 over 12 years ago
    Well Birdseye makes it really easy to see what is happening, and you would need to look at the specific location the OP mentions. I posted some screenshots with Birdseye and the 100k topo awhile ago. There is obviously something very wrong in this one, for example. :)




    Those people obviously built their houses in the wrong places!
  • Former Member
    0 Former Member over 12 years ago
    If you check Boyd's example you'll see that not only is the alignment bad, but so is the topology. There are more roads on the image than on the map.


    That's just a small screenshot, but IIRC the intersection shown at the top the aerial image near the center is where Patrician Dr and Morristown Dr cross. So the roads are about 1.5 blocks too far South and a little too far East.

    But your points are well taken. Road data is pretty bad on the 100k topo in some places. Then it might be spot on in other places.

    -Boyd
  • 0 over 12 years ago
    Meanwhile, you can download free 24k topo maps from www.gpsfiledepot.com and use those. They are much better than the 100K you have and the price is right.