Handheld GPS Receivers: Variables Affecting Accuracy and Implications for Geocaching

Here’s the final instalment…

After researching how GPSrs work, designing an experiment to assess which variable most affects the accuracy of coordinates calculated by a handheld GPSr and analyzing almost 2000 observations taken under a wide range of conditions, it was time to consider the implications of the results when it comes geocaching.

Here’s a short list of things to keep in mind when you’re placing a new geocache:

  • Make sure your GPSr has been turned on for at least 20 minutes before your start marking coordinates.
  • Check the DOP readings before you head out, because if the level is higher than about 5, you should hold off to another day. There are several websites that can provide the DOP information – in advance – for a general area. Oldest GeoKid used the Planning Software available free of charge on Trimble’s website.
  • This sample DOP almanac chart was generated by Trmble's Planning Sofware and appears on the University of Washington's Department of Natural Resource Sciences GIS website
  • Leave WAAS turned off unless you’re fairly close to a WAAS ground station.
  • Don’t hide a geocache when it’s snowing heavily.
  • Don’t hide a geocache where there is likely to be a lot of multi-path error. If you do, be sure to provide a good hint, to minimize potential damage to vegetation.
  • Make sure you’re not holding your GPSr close to your body or in a way that covers the antenna.
  • Take more than one set of coordinates and then decide on a “best fit” reading when submitting your geocache location. The geocaching.com page containing tips on Hiding Your First Geocache has some good suggestions on how to average coordinates.
  • In your cache write-up, consider including important information about DOP, weather conditions, etc., so that finders will have some idea how accurate your posted coordinates are likely to be.

And remember, obtaining an accurate set of coordinates is only the first step. You also need to take great care when entering them on the new cache write-up form. Check out softball29’s a ‘lilhoohaa blog post for more thoughts on this key point.

It’s also a good idea to let your GPSr warm-up before you start searching for geocaches. Read the cache description to see if there is any information about how careful the cache hider was to post good coordinates and take that into consideration when deciding whether / when to decipher the hint. It’s our experience that once you’re within 5 to 10 meters (15 to 30 feet) of the posted coordinates, you’re often better off to hold your GPSr at your side and rely on your geosenses for the final stage of the hunt.

As always, we welcome questions and comments. We’d especially like to hear from you if your experiences differ from Oldest GeoKid’s results or if you incorporate some of his findings into the way you geocache.

One thought on “Handheld GPS Receivers: Variables Affecting Accuracy and Implications for Geocaching

  1. I am glad my GPSr does averages. I always let the GPS sit there for 10 minutes or so and average upwards of 200-300 readings so I get the best reading I can. I’ve always had luck that way, thankfully. Good tips here, though. Some to remember for sure!

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