Different Gps Readings At The Same Place

[bone04]

i brought my boat home today and turned on the gps, standard horizon cp160(with WAAS), and set my two hand helds by the antenna, garmin etrex and magellan platiumn. the W numbers on all three were the same, the N numbers were off .612 for the etrex, .616 for the magellan, and .608 for the standard. on a web site i found, the difference between .616 and .608 is 48ft. those are decimals in minutes. funny how the W numbers were all the same. also the N numbers moved around just sitting there. well, i guess finding a cache with one gps as compared to who put it there and their gps, theres always a small error involved. also i think sitting in one spot to let the gps settle down might do some good also.

 

me and my wife found 6 caches today. a few of them were searched pretty hard for. it seemed that after we searched long enough the gps finally led us in the right direction. guessing it took a few minutes to get right. dont know. had fun. and just sharing this with you guys to see if any of you have tried this expeirement. cool

 

bone04

[+TresOkies]

"A man with one GPS knows exactly where he is. A man with two GPSes can never be sure."

 

I didn't make that up. It's someone's signature (sorry can't remember who).

 

I have 4 different Garmin units. Each of them gives a slightly different reading. When I'm hunting Tupperware, I let the GPS guide me within about 50 feet and then I start thinking "OK, where would I put it". So, accuracy isn't that necessary. When you have a group of cachers, no two of them are ever standing in the same spot, although the folks with Garmins tend to be in the same areas and the folks with Magellans tend to be together in different areas. Kind of like a cache event.

 

-E

[State Street Hunter]

Remember, the stated accuracy for any unit is plus or minus. Thus the adifference between two units could be twice the stated accuracy. This would not likely happen, but it is possible.

 

Your example is a good reminder that no single reading should be taken as an absolute position. Only redundent readings coupled with checks on known points should be accepted as anything near true values, and then within the limitations of your GPS unit.

[bone04]

any input on why the W numbers were right on on all three units?? also, state street hunter, where can i find a point that has numbers that are correct. just wondering, would be neat to see how the gps's compare. thanks for the responses.

 

bone04

[+Still Searching]

Someone please correct me if I'm wrong but isn't hand held GPS accuracy better while moving? I am aware of the "calming down" time theory. I have used this myself and it does seem to work. On the other hand I work with a couple of different systems that will guide ag equipment and the accuracy is about twice as good while moving. These systems wor on WAAS also.

[State Street Hunter]

In response to bone 04, go to this link http://www.ngs.noaa.gov/cgi-bin/datasheet.prl This is the NGS Datasheet retreival page. Use your prefered search method. The county version is very simple. Follow instructions and search for GPS sites only. This will give a list of current list of control in your area. Also you may want to contact yor local Highway Dept., local Public Works Director, County Engineer or Surveyor. Most likely they have knowledge or are responsible for other Control Points in the area. Just remember that even though this is usually public information, the employees in these offices are used to dealing with people who use this data on a daily basis and may seem impatient with those who don't. An honest explanation of why you want the information will go a long way to disarming these folk.

 

Be observent of the Coordinate System and Map Datum used for the values given and set your GPS unit to match. Note that for consumer grade units WGS 84 and NAD83 are essentialy the same. Also note that with the same the finest resolution typicaly is .005' or roughly 5 feet unless set to UTM or some other local grid and then the display will be to the foot. State Plane systems tend to be preferred by Survey and Mapping Professionals, hence these values being listed on the NGS data sheets. To convert between some of these systems use the program available here. http://crunch.tec.army.mil/software/corpscon/corpscon.html

 

When taking readings on these sites, be sure to leave your unit on the mark long enough to get a nice averaged reading. Also, take observations on different days at different times to take advantage of different satelite geometry. Use WAAS if you have it. It does improve accuracy. Disregard any vertical error. Vertical accuracy on consumer GPSrs is a joke. Ignore it. It doesn't matter.

 

In the end you will find you fall within the specs for yor unit. Plus or Minus. It is performing as designed.

[+Rosco Bookbinder]

As I have posted and mentioned in some of the other discussions about this subject, check out the results of our "test" on this http://www.geocaching.com/seek/cache_detai...03-e6d73de04fcc. We really weren't surprised as to the results. Everyone in any kind of weather was very close. The original coordinates were taken with a brand new Garmin (can't remember the model) and a Magellan about 6 months old. We both had the same numbers....