GPS Techies Needed!

[GaryKD]

Hi Everyone,
I recently upgraded my GPS. The unit has the capability to find a destination via latitude/longitude. Being new to this type of finding a location, I was very surprised when entering an address (on the Internet not the GPS unit) I get 4 different sets of latitude/longitude for a single address. Anybody know why this is the case. I've tried a couple dozen addresses and the same is true for all entries. An example is:

The address:
1012 West Recreation Way
North Salt Lake, Utah 84054

The following format is
degrees minutes seconds
Google returns:
Lat = 40 36 55 95
Lon = -111 49 7 79

USC returns:
Lat = 40 50 25 36
Lon = -111 54 54 03

Yahoo returns:
Lat = 40 49 49 14
Lon = -111 56 14 43

Terraserver returns:
Lat = 40 49 48 50
Lon = -111 56 14 15

When the GPS unit can not find the address, I thought using Lat/Lon would be spot on. Now I seem to have too many spots to be right on.

[Midniteoyl]

Hmm... i just looked up the addy in Mapquest and Google.. It shows the Pony Express RV Park.

Looked up the addy in iTouch, and, well, I dont think its the RV Park

Latitude and Longitude of a Point

To me, that is your problem

Why? Dont know... lotta error built into these maps.

[GaryKD]

The location is the Pony Express RV Park.

[AZgl1500]

The lat/long info provided by those websites uses info provided from a lot of different sources.

When I want accurate GPS coordinates, I use Google Earth on the desktop and watch the cross hair. Position that where you want it. That will produce you a very good coordinate, and even that depends on how close you zoom into your point.

Just moving the GPS unit from the street curb at the driveway and then walking to either lot boundary will give vastly different readings.

I don't depend on coordinates unless I know the source.

Also, the same GPS unit you have in your hand can offer up vastly different readings depending on how long it has been ON, how many satellites it has in view. It takes 6 or more satellites to give accuracy readings down below 10-30 feet.

My Garmin 2730 GPS has the option to make a fixed location over an extended time period which will give you the mean average off all the readings that it calculates during that time span.

Try this little test. Zoom in your GPS as tight as it can go. Don't touch it, just watch the map for a half hour. You will soon see how more/less satellites can effect the results. Your position will move around the point that you are at. If your GPS has tracking dot capability, you will see the dot crumbs zig zag all over the place. You can at times see those crumbs move more than 300 feet away from where you are!

Just play with it, and learn how it behaves.

Another item to be very aware of is the air density. Heavy clouds? = poor reception. The higher those bars are, the better the accuracy.

6 satellites or more with all of the bars just barely showing will produce a highly inaccurate reading. 4 satellites showing with the signal strength bars nearly pegged out with a couple of lessor strength satellites can produce accuracies down to 10 feet or less. It really depends on the quality of the instrument you paid the $$ BUX for.

A $99 GPS will get you within the same block as your intended destination, it might even at times get you within 30-50 feet. But, it cannot compete with an expensive unit that has a super sensitive receiver with the latest computing algorithms.

GPS tracking requires a lot of mathematics, and very good receiving conditions for good accuracy.

[OLYLEN]

It all depends on where when how the Lat Long was derived. If it was read from a GPS receiver it will most likely be within a few feet. If it was derived from a map,who knows. Addresses are not on a grid system like Lat Long but at time just an arbitrary point on the earth only the post office may know. Add to this the data base the Lat Long comes from may have been input by hand and garbage in garbage out. As to the accuracy of a GPS receiver, even the older ones are most likely more accurate than the maps you are looking at and this all depends on the position of the constellation of sats. you have in the sky at any one given time. 4 sats spread in all 4 quarters in the sky are better than six low on the horizon or in 2 quarters of the sky. So The accuracy of a Lat long is dependent on, how many sats and where in the sky to give the gps itself a good Lat Long and where the Lat Long came from that you are comparing the GPS to.

LEN

[Midniteoyl]

Older GPS's are more accurate if they included WAAS, which is no longer used in consumer units (costs).

Another thing is car GPS's use a function called 'snap to road'. This basically means that even though it thinks you are driving through the buildings to left, it 'knows better' and will show you on the road. If the coordinates where gotten through using a GPS (like Google), the actual point could be 100-200 feet away.

Doesn't explain why a service like iTouch shows the coordinates as be over 10 miles away though..