newbie - how to calibrate GPS?

[jallred]

So we got a Garmin eTrex Venture HC for Christmas today. We spent a few hours playing around with it (walking around the neighborhood, trying to find 2 nearby geocaches, etc.) and it seems to be about 50-100 feet off. I'm trying to figure out if this is a user error problem or a hardware problem (perhaps the unit needs to be calibrated?)

 

On the satellite acquisition screen it shows that I should be within +- 15 feet, but as I'm walking down the street it shows I'm walking about 50 feet to the west of the street on the map. As I approach a cross street the map shows I'm already about 50-75 feet south of it.

 

We went looking for a geocache and when we got to the location we were in the middle of the street about 30 feet from the entrance to the park where the geocache was supposed to be located. We found the same sort of thing when we went looking for the other cache. It seems that it will be impossible to find caches if we are always 50-75 feet away from it!

 

The map shows a dotted line for where we have walked. Should that dotted line be anywhere close to the actual street, or is it common for it to look like we are far to the west and south of where we actually are?

 

I'm hoping this is just a simple calibration issue (for instance I stand in the middle of an intersection, put the pointer on that intersection and tell the unit - this is where I am right now). Or maybe not.

 

Thanks for any help for a newbie!

[+Boots...]

All I can guess is that maybe the Datum is not set wo WGS84. All caches listed in Geocaching.com are in teh WGS84 datum. I think that is default on the Garmin units, so you SHOULD be all set, but I can't give you any other suggestions?

[+Prime Suspect]

Unless you purchased and loaded separate MapSource maps, you only have the base maps on your unit. Their accuracy is not very good. Don't base the unit's accuracy on the base maps.

 

There is no calibration for consumer GPS units. However, it may not have a complete almanac, if you were anxious to get out and use it. Turn in on and let it have a good unobstructed view of the sky for about a half hour. Then try marking a point, and navigating back to it. See how close you get.

[+Hrethgir]

Also, make sure you have WAAS turned on, that could help. And also remember that the cache coordinates are only as good as the hiders GPSr. If the hider had an older unit that wasn't as accurate, say +/-30ft, and your accuracy is at +/-15ft, that means you could be up to 45 feet away even though you are showing as right on it. But don't put too much stock in your pointer on the map versus where you actually are, the map could just be a little off.

[SiliconFiend]

If you're using the Topo 2008 map, then the street accuracy of it is known to be bad. (50-150 feet off as you describe) Only City Navigator and its friends will have both accurate street locations and street names. (Topo 3.0 has reasonably accurate street locations but no names)

[+StarBrand]

The units base maps (and as noted the TOPO maps) are not super accurate when it comes to streets and roads.

 

A few tips though:

- turn off the battery save and turn on WAAS.

- make certain you are using fresh batteries.

- let the unit get a good clear view of the sky for 30 minutes if it has been off for a long time or moved a great distance. Do that before moving around with it.

- Don't expect the unit get you right on top of very many caches. In general these units have an accuracy in the 20 - 25 foot range. Add your error with the hiders possible error and you could easily have to search an area up to 50 feet from where your unit says Ground zero is at. Very often it is much closer but you should be prepared for a wider search.

- Don't put a lot of stock into the error reading. That is what is known as an EPE. Estimated Postional Error. Read it something like: "I think the current reading is within xx feet of the actual spot defined by these coordinates - and further I think that estimate is correct about 60% of the time. At least under the current signal and satellitte configuration". The actual error is sometimes less and sometimes quite a bit more.

[onetimeoneplace]

By default on the Venture HC, WAAS is turned off. Turn it on in the Setup menu to get better accuracy.

[+lil'hobo]

Maybe a little of topic, however, I too received a Venture Hc and I discovered how to turn on the WAAS, but where is the battery save off feature found? And what does turning it off do, such as does it more accurate in that function? (my coords are currently right on with some known spots)

 

Thanks