Gps Accuracy

[NYJetPhan]

I had been begging my wife for a GPS for GeoCaching, and she has finally come through. I received an Explorist 300 for our anniversary! While obviously not the highest end GPS, it more than adequate for my first unit. I have noticed that If I stand in certain spot a the end of my driveway and make waypoint and call it home, then turn off the unit and return and do it again, I am sometimes 60+ft from the original spot. I currently have 5 WPs called home, and only 2 are within 30 feet of each other. IS this somewhat normal? I just read the thread about maps, which explained why my neighbors house is about 250 feet into the Hudson river...LOL, but this WP inaccuracy bugs me. Is it normal to put in a cache location and be 30 to 60 feet away? if I make a cache, is my so called inaccuracy acceptable?

[+Greymane]

To give you a simple answer, YES, this is normal. There are a lot of factors that affect your readings (the number of satellites you "see", the amount of time the GPS has to access the information, the weather, tree cover, buildings, weak batteries). Any number of things can cause the signal to be inaccurate. The best thing is to let your GPS "bake" for about a half hour in a clear view of the sky to gather a good set of data. This should only need to be done when you use your GPS for the very first time or when you move to an area FAR from your home (several hundred miles).

 

When placing a cache, you want to average your readings (see the FAQs). When finding a cache, remember this problem and when you are about 50 feet from the location, put away the GPS and start looking!

[+reveritt]

Yeah, that's not too bad. For a more reasonable test, however, set the waypoint, then take a walk (with your GPS on) and come back to the same exact spot. How far off are you?

[+Sputnik 57]

I would say that this is not really normal. Most GPSrs should be able to get most waypoints within 20 feet or so. How many satellite locks do you have? Have you left the unit with a clear view of the sky for a couple of hours to make sure that it has a fully loaded almanac of satellite data? 50 feet of error is too high unless conditions for satellite geometry are terrible.

[NYJetPhan]

I've done that, it is much better...but I was just really concerned about the several waypoints called home, but none were at home!! I just can't wait for all this snow to go away, so I can really get out and use this, I just found out there is a benchmark on our property, I can wait to find it.

[NYJetPhan]

I usually track up to 8 satellites, my yard is relatively treeless, when a box says 3 meter accuracy, I expected it, not just under perfect conditions. The more I really read, the more I understand, like I said, once the snow goes, and it gets warmer where I can have the kids out for more than half hour, I'll use it more, and get used to its nuances and learn what to really expect out of it.

[+Prime Suspect]

It's normal, but on the outer edge of normal. What you're seeing is the accumulation of two errors - the error that was present when you initially marked a waypoint, and the current error as you're trying to return to it.

[StanB]

I'd say your accuracy is normal for a GPS without WAAS or an external Differential GPS receiver. With WAAS enabled your errors should average around 18 feet. This assumes your GPS has a clear view of the sky and has locked onto five or more satellites.

[NYJetPhan]

My unit is WAAS enabled, but often WAAS accuracy takes a spell to show, I cannot say for sure how many, if any of my earlier "home" WPs were marked under WAAS, I just stepped out to my home WP area, and am just 8 feet from my only remaining "home" WP. This unit is brand spanking new, as am I at this...it will come.

[+Robespierre]

I really think that "normal" for me is closer to 20', but if you snapped a reading that was 30' Off one direction, and a second reading was off the other direction, this can happen. Approach more than once when setting a cache, and from more than one direction. If possible, choose the nicest day, or check it another day. I've placed 13, and only had any problems with one, and that one continued to be a problem after checking with two units at the same time. Solved it by a better clue.

[+EScout]

As to the original question about accuracy, 30 and 60 feet are not normal. However, maybe the waypoint was not set accurately. I recommend finding a spot and take the coords there over a period of days. See how the coords group, and use the coord that is recorded the most. When you go back to this one, you should be with about 2 to 10 feet.

For example, today I hiked a trail I have been doing for several years and I went to 4 waypoints that I have established with many readings over 2 years. I got within 1 to 9 feet of them all in my goto screen. This was with about 8 sats, and a WAAS sometimes being received.

[+DaveA]

My home WP was set while getting WAAS and after allowing the unit (Meridian) to settle for around 60 seconds. (It had been on for 5-10 minutes).

 

Repeating the above I am always within 7'. Note this assumes the unit is receiving WAAS corrections.

 

To better test the accuracy of your unit do this:

 

1. Turn unit on and leave it outside for 10 minutes. Set it in the spot you want for your waypoint and leave it alone for 60 seconds. Mark the waypoint.

 

2. Walk away and return.

 

3. After 30 seconds check your distance to waypoint.

 

If you are still 30-60ft away return the unit. It could be that your home has something that affects reception, but I would still return the unit and try a Meridian to compare if your accuracy in this test isn't within 20' at the outside.