Is My Garmin Accurate

[rebel6392000]

i did a little test yesterday i went to this website (http://geocoder.us/) and put in an address to get the latitude & longitude and then set out to see if my gps got me to this location. it did but was off 2/10 of a mile so is my gpsr off or is the latitude & longitude they gave me off? 2nd question I have is at garmins website ther is a patch for the topo map that say Basemap patch now available. WARNING: This software will not work unless you already own a MapSource product. i dont really understand what this is telling me im still a newbie please help if you can

[+ctgreybeard]

i did a little test yesterday i went to this website (http://geocoder.us/) and put in an address to get the latitude & longitude and then set out to see if my gps got me to this location. it did but was off 2/10 of a mile so is my gpsr off or is the latitude & longitude they gave me off? 2nd question I have is at garmins website ther is a patch for the topo map that say Basemap patch now available. WARNING: This software will not work unless you already own a MapSource product. i dont really understand what this is telling me im still a newbie please help if you can

I tried my own home address on the site and the map had it pegged exactly but the coordinates were a little North (.001 deg) and West (.004 deg) of where my VistaC says. But the web site shows the spot in the street and I am in the middle of my lot, a quick check shows that the web site is correct if I was standing in the street. Good thing it's Saturday and the traffic is light!

 

Bill W

[rebel6392000]

i did a little test yesterday i went to this website (http://geocoder.us/) and put in an address to get the latitude & longitude and then set out to see if my gps got me to this location. it did but was off 2/10 of a mile so is my gpsr off or is the latitude & longitude they gave me off? 2nd question I have is at garmins website ther is a patch for the topo map that say Basemap patch now available. WARNING: This software will not work unless you already own a MapSource product. i dont really understand what this is telling me im still a newbie please help if you can

I tried my own home address on the site and the map had it pegged exactly but the coordinates were a little North (.001 deg) and West (.004 deg) of where my VistaC says. But the web site shows the spot in the street and I am in the middle of my lot, a quick check shows that the web site is correct if I was standing in the street. Good thing it's Saturday and the traffic is light!

 

Bill W

lol yep its a good thing ..my gpsr said it was accurate to within 20 feet so im not sure why it was off maybe I will check it again today with my home addy as well and see what I get

[+Team Trail Walker]

Make sure you GPSr is set to the correct Datum. The geocoder.us web page says they use NAD83.

[rebel6392000]

Make sure you GPSr is set to the correct Datum. The geocoder.us web page says they use NAD83.

oh ok well i will have to find a place that uses wgs84

 

thanks for the info

[+Team Trail Walker]

You should be able to change your GPSr to use any number of datums including NAD83.

[+ctgreybeard]

You should be able to change your GPSr to use any number of datums including NAD83.

I switched back and forth and didn't find much difference between the two for my location.

[Neo_Geo]

2nd question I have is at garmins website ther is a patch for the topo map that say Basemap patch now available. WARNING: This software will not work unless you already own a MapSource product. i dont really understand what this is telling me im still a newbie please help if you can

There are 2 different downloads on that page.

 

"WARNING: This software will not work unless you already own a MapSource product," refers to the MapSource program update. What they're saying is that you have to have the MapSource program installed on your computer for the update to work. I guess some people thought that downloading the update gives you a free version of the MapSource program - but it doesn't.

 

"Download MapSource United States TOPO Basemap Patch" is another separate update specifically for the US Topo maps in MapSource. If you have the Topo maps, you should install this update too.

[peter]

NAD83 and WGS84 are almost the same and most consumer GPS receivers that include both actually treat them identically. They're within about a meter of each other so it makes no difference within the accuracy of our devices.

 

I'm not surprised that the location of a specific address from that website is off a bit. Map databases don't include each address separately but rather just the locations of the start and end of a whole block of addresses. So they might record the locations of #1000 and #1400. Then if you specify #1200 it'll give you the location half way in between those points. That usually works reasonably well but sometimes the addresses aren't evenly spaced or the numbering pattern isn't what the map database has recorded.

 

So I'd be almost certain that the discrepancy is due to the website giving you an inaccurate location rather than an error in your receiver. To test it for sure you could look for some GPS-measured USGS benchmarks.

[rebel6392000]

ok thanks great info from all

[+Sputnik 57]

I live in the 4200 block. Actually, the last house on that block. My address is 423x, and geocoding places me about 1/3 of the way down the block (assuming, I suppose, that the last house on the block must be 429x). As Peter pointed out, geocoding is not an exact science (yet). Your GPSr is far more accurate.

[GeoBobC]

"NAD83 and WGS84 are almost the same and most consumer GPS receivers that include both actually treat them identically. They're within about a meter of each other so it makes no difference within the accuracy of our devices."

 

I am not following that. If I create a waypoint in Mapsource (I live in the Seattle area), note the coordinates, change map datum from WGS84 to NAD83 CONUS, and then create a second waypoint with the identical coordinates of the first (which have now been transformed), the two waypoints are 308 feet apart.

[Grasscatcher]

I am not following that. If I create a waypoint in Mapsource (I live in the Seattle area), note the coordinates, change map datum from WGS84 to NAD83 CONUS, and then create a second waypoint with the identical coordinates of the first (which have now been transformed), the two waypoints are 308 feet apart.

 

If the difference in coordinates between WGS 84 and NAD 83 in the Seattle area is 308 ft.....these definitely are terrible times we live in !!!!!!

 

In Mapsource , with Mapsource set on NAD83, make a waypoint. Click on the waypoint tab and expand the list out where you can see the coordinates for that point. Then go to Edit>Preferences>Position...and change from NAD83 to WGS84 and note how much the coordinates for that point "DON'T" change between WGS84 and NAD83. If you do the same thing except change to NAD27Conus, THAT is where you probably came up with 308 ft.

[GeoBobC]

Duh. You're correct; I was referring to NAD27 CONUS not NAD83.

[+Sputnik 57]

NAD27 was developed as a model for use in the US in, well, 1927. Many old USGS topo maps still use this datum, and it is handy to have if you happen to have a paper topo map and are trying to match GPS coordinates to it.

 

WGS84 is a world-wide datum based in part (IIRC) on the 1983 model of the North American Datum. They should be pretty darn close.

[+dhbaird]

i did a little test yesterday i went to this website (http://geocoder.us/) and put in an address to get the latitude & longitude and then set out to see if my gps got me to this location. it did but was off 2/10 of a mile so is my gpsr off or is the latitude & longitude they gave me off? 2nd question I have is at garmins website ther is a patch for the topo map that say Basemap patch now available. WARNING: This software will not work unless you already own a MapSource product. i dont really understand what this is telling me im still a newbie please help if you can

 

I did a test of the same sort of idea the other day, but with a reference point twist. There are 6 different Benchmarks listed on the geocache.com site that are within walking distance of my home. I copied down each of them, including elevation from the information page on the site. I input them as waypoints in my GPSMAP 76cs and then went for a walk.

 

I was 6 feet off from the first BM with an error of +/- 12 feet indicated on the GPS... I went over to the first Bench Mark, laid the GPS down on it and did an AVG for 100 different logs and it came up with about the same. I also calibrated the altimeter and then walked to the next Bench mark and did the check all over again.

 

Over the course of the test I averaged 6-10 feet away from the BM with accuracy indicators staying about the same +/- 12-15 feet. The altimeter was within 1-2 feet at each point after the calibration at the first BM.

 

I covered about 1.5 miles in my walk and came home feeling reasonably secure that despite chaging conditions/locations/tree cover etc that my accuracy seemed pretty constant.

 

Does that help?

[Bikezilla]

geocoder is off.

 

I put in my address and it shows it down the block, around the corner and 2/3rds the way down the next block (all the same street) This is a difference of almost a dozen houses, each with a decent lot size.