Field GPS Coordinates vs. Google Earth
Elk
Guys: Last year when I went to Kansas I had some GPS coordinates already plugged into my Garmin Rino 130 that I got from the pointer on Google Earth. I changed the time zone when I got there but I was consistently off about 300 yards. Any ideas to remedy so I don't have this problem on my New Mexico Elk Hunt? Any help appreciated. Thanks, Charlie
When I bought my Garmin Vista I transfered all of my waypoints from my old Garmin 12csx. When I got to some of the locations I was way off. Some of my waypoints from Lake Erie shore were in the water. Once I upgraded to Mapsource they were all where they were suppose to be. So it might be the base map that comes with the units that causes the problem.
Sounds like a datum problem, NAD27 vs. WGS84 (NAD83).
Go into your setup menu on your GPS and make sure the datum setting matches the datum of the map you are using.
Google Earth UsaPhotomaps Both right on the money.
yep. map datum for sure. You need to make sure you understand that stuff...or you'll never be where you think when transferring waypoints onto a map from gps or vice versa. Most maps I use are NAD27 CONUS and that is what I set my GPS for...but don't take it for granted.
Yup Datum issues been there done that have the t shirt
Thanks guys! I will give that some attention. Charlie
Does anyone know what Map Datum Google Earth uses?
God I love this site!
There is nowhere else you can get information like this so quickly. Anywhere else you might find the info on the right site, but it'd take you days to find the right site.
They don't make it very obvious, but Google Earth uses the WGS84 coordinate system.
In Charlies post he mentions changing the time zone in his gps. What would that have to do with a specific lat & lon? Isnt a lat and lon location a specific location on the globe no matter what time zone you are in? What does it matter if I preload them at home in Central and then go to Mountain and dont change the time zone short of the clock in my gps being off?
I always have my gps set to UTM. Much easier system to locate where you are on a map and datum doesn't matter.
Turns out my problem was my "Position Format" default setting on my Garmin did not match the "Position Format" that Google Earth uses. Mike the Cheesehead figured it out for me and now I am 100% accurate. Thanks to the other guys that offered help as well! C
Try Acme Mapper vs. your GPS on known points, mine Lowrance Hunt is spot on with what I'm plotting on Acme mapper here in OH, I hope it runs true in CO, as I've got a couple pre-loaded spots that I may set up camp if it reads true.
FYI, UTM is very good, but still requires a datum. You can get just as hosed with UTM referenced to the incorrect datum as you can with straight up geodetic coordinates.
Cazador, you are correct. I just looked at the bottom off my maps that I print out at home and sure enough WGS84 is the datum. Then I looked at my GPS. Thank god they are the same. But, thanks for pointing it out, now I will make sure they always match up. Here's another question for others (military point of view would be great). Is there better (i.e. more accurate datums) that I should use for different states? Thanks
Lips,
Unfortunately we're bound by the maps we're trying to reference. The good thing these days is most of the BLM, governments and such are updating to NAD 83 which for the average Joe is the same as WGS 84 which is a step in the right direction.
In short, we must always look at the map source, and look for the Datum referenced. From there, we can work in Geodetic (lat and longs) or a grid system. UTM, or State Plane. For most guys, getting the Datum, and then using UTM like yourself is the way to go.
Lips-Datum does matter with UTM if you are plotting to a map or inputting data to your GPS from a map.The UTM grid is directly related to the datum. You will be 200-300 yds off. Try locating a specific point on a map with the wrong datum in the dark.
If you are using a GPS to stay on public land vs. private or find specific places, make sure you know the datum of your maps and match your GPS to the map.
razor,
You are correct, time does not matter... unless you have a lunch date.
I have always heard that time does make a difference. It correlates where you are at a time of day in relationship to where the satellites are in their orbit. I don't have any scientific proof but it is an easy thing to change so I always change mine. Just got this from Garmin's website as to what causes location errors...... Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors...... If it's not important why is that feature there? I suppose it could just be for use as a clock. Hope this helps.
My GPS just died this sunday on a scouting trip. Going to get a rino because I hunt with a great guy who is in his late 50 but sends me in to the brush to push any deer. I don't mind but I think the rino's would be great to see where he is vs my position. Safety and we could work together to side hill and know where each other is. My question is do you need the $400 version or would the rino 130 at half the price work just fine. Thanks
Does the 130 have the 2 way radio?? I have the 530 and wouldn't think of owning something else IMHO...
Tracker,
The "clock errors" they are referring to are a matter of seconds or maybe fractions of a second. The clock in your receiver is just a cheap electronic clock and therefore it will think the time is very slightly different than what the actual (atomic) time is up on the satellites. We're talking seconds here... not hours or even minutes.
The satellite signal tells your receiver where it is above or on the surface of the earth and those very small clock errors or differences cause a very minor error in your coordinate position. The precise time (to the fraction of a second) is part of the mathematical formula that calculates your coordinate position. You can't change that formula by changing any settings on your GPS receiver.
The "clock errors" have nothing to do with what time zone you set your receiver to. You could have your time zone in your receiver off by 6 hours and your coordinate position will still be just as accurate as if you had the receiver set to the correct time zone. You just couldn't rely on your receiver's clock to tell you what time it is.
GPS receivers also suffer from atmospheric errors, multipath errors, etc.
I'm a Land Surveyor and we work with this stuff with $100,000's worth of precision GPS equipment on a daily basis and we have to understand these errors.
These errors are not usually a factor for the "average Joe" with a $200 handheld.
Mike, Thanks for the info.
Mike - Thanks for the clarification. I do change my time zone setting but didnt understand how that could make a difference. Now if they could just make them to default to the time zone of your current location like our cellphones do we wouldnt have to mess with it.
Lips/Fields: I have have the Garmin Rino 130 and for my needs it has it all. It has a 2 way radio, weather channel, point to point with a buddy and GPS. C
Mike,
What type of GPS do you guys use? Models?
cazador,
We use Trimble 4700's, 4800's and a GeoXT submeter.
