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Etrex Vista Cx Accuracy Off


R-Men
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My eTrex Vista Cx seems to be consistently off by 0.6 seconds (latitude only). I was out caching this morning and found my brand new eTrex to be off by 0.6 seconds at each cache ~50ft. I'm new to gps so I was not sure if this is a normal varience or not. I have the newest update 2.50 firmware, have run a WAAS burn in (I let the system sit for 20minutes and then all the satelites began to have D's on them) I'm not sure what all the D's mean but I got them today. I also have WAAS on and power saver off and am using WGS 84 datum (is this right for the southwest?).

 

This Geocaching is fun we found six today including NC-7 which I believe is some kind of army positioning thing. My father-in-law was with me and we found most of the caches we looked for, he was using a GPSMAP 60cs and we were out in the AZ desert (so no tree cover). He was with in feet of the cache which is how I came about the variance of 0.6 sec. we stood side by side and compared at each cache.

 

Thanks in advance for all the help.

 

Prof. R

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My eTrex Vista Cx seems to be consistently off by 0.6 seconds (latitude only). I was out caching this morning and found my brand new eTrex to be off by 0.6 seconds at each cache ~50ft. I'm new to gps so I was not sure if this is a normal varience or not.

No, fortunately consistent systematic errors of that magnitude seem to be rather rare for consumer GPS receivers and when they are observed can usually be traced to some incorrect setting. Your use of WGS84 as the datum is correct if you're getting the coordinates from the geocaching.com site, but I'm wondering what location format you're using. Geocaching.com uses degrees & minutes (HDDD MM.mmm') and therefore doesn't use seconds of arc at all.

 

BTW, the 'Ds' indicate that your receiver is getting differential correction data for that satellite from the WAAS signals and that should usually make your positions more accurate.

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WGS84 is the correct Datum for anywhere in the world if you got the coordinates off of a Geocaching.com Cache Page, or downloaded GC coords. The "D"'s mean you are obtaining DIFFERENTIAL correction from the second receiver (WAAS) built into your GPSr. The amount of error caused by the atmosphere is determined by the WAAS system. When you see the "D"'s, it means you are enjoying the benefit of that error correction information. And power saver off is your best bet.

 

You want to be careful about holding two GPSr receivers too close together for comparison purposes. There is the potential they could interfere with each other. Holding them too close to your body would not be a good idea either.

 

You have a good grasp of the distance .6 seconds of Latitude represents. It's actually about 60 feet. But I have to wonder why you are even referencing seconds. The Geocachhing standard is HDDMM.mmm

H = Hemisphere (North, South, East West)

D = Degrees

M = Minutes

m = decimal minutes

 

For a 60 foot Latitude error I would have expected you to have called it .01 minutes. Not .6 seconds. Even though they both indicate exactly the same information. Did you and your father-in-law enter exactly the same coords in exactly the same way? Did you convert the Geocaching standard of HDDMM.mmm to the HDDMMSS.s it sounds like you are using? Did you then use the GOTO feature to find the Caches? Or did you just look at the raw Lat / Long data and follow that to the Cache? Normally a new GPSr will NOT be set to that format (HDDMMSS.s) unless someone changed it.

 

A 60 foot "error" out in the open with a WAAS corrected fix does seem bit high. But these things do not have surgical precision by any means. I have no experience with that new Garmin "x" chip. But I would expet the accuracy should be just as good as any Garmin "cs".

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I converted the unit to read in HDDMMSS.ss (?) so both units would read the same way to check coordinates. The thing is I was consistantly off the cache by ~50ft. I am using GPSBabel to convert the .loc points to .gpx for use on my Garmin. Can the Garmin read .loc files? The other thing, I guess before I go to far, is that we both uploaded the coordinates different ways but both recievers were reading the same coordinates.

 

Is it common for a unit to read only a latitude wrong or is it usually off all the way around?

 

I guess I'm trying to make sure the unit isn't squirrelly(?) or something that I need to contact Garmin about replacement/repair.

 

Thanks again for the help.

 

I am really enjoying this.

 

BoB

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I converted the unit to read in HDDMMSS.ss (?) so both units would read the same way to check coordinates. The thing is I was consistantly off the cache by ~50ft. I am using GPSBabel to convert the .loc points to .gpx for use on my Garmin. Can the Garmin read .loc files? The other thing, I guess before I go to far, is that we both uploaded the coordinates different ways but both recievers were reading the same coordinates.

 

Is it common for a unit to read only a latitude wrong or is it usually off all the way around?

 

I guess I'm trying to make sure the unit isn't squirrelly(?) or something that I need to contact Garmin about replacement/repair.

 

Thanks again for the help.

 

I am really enjoying this.

 

BoB

 

I too have experienced significant errors when using the Garmin Legend Cx. Recent use in forested or canyon locations gave errors of 20-40 feet compaired with a Garmin GPS III+. I suspect the patch antenna is low in gain or contributes to significant multipath error. Although possible for GPS units to interact with one another, it is highly unlikely for the LO's to cause a problem...unless one of the units does not meet FCC reradiation specs. I noted that moving the Cx about 5 feet could cause the location to change by 15 ftte or more and returning to the same spot does not always give a repeated reading.

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One way to get a feel for the real world accuracy of your GPSr is to look for a horizontal control station Benchmark. A vertical control station will not give you as accurate a result.

 

Go to Datasheet->BY POINT RADIUS

 

Remember to check the Datasheet for the Datum they are using.

For this purpose essentially NAD83 = WGS84

 

If you have any trouble navigating to an actual Datasheet PM/email me for help.

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I have finally got a chance to check out a horizontal benchmark from the page you gave me to look-up. When I got to the marker I was off by .002 w (min?). or about ~7 ft. My question now is , Is this normal? Also is it odd that it is only off on longitude? Should I contact Garmin or is this an OK variance.

 

I was using the NAD83 adjustment to match the marker, and I had an accuracy of + 16 ft. accuracy. I have installed the 2.50 update and have allowed the unit a 20minute burn-in.

 

Thanks, for all the help.

 

R

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Off by 7 feet at a known marker ...... heck thats great! :ph34r: +/- 30 feet is the rule that we teach new geocachers. Koikeeper and I have identical GPSs and they seldom display the same exact distances. You are also at the mercy of the cache hider .... who says their coordinates are exact. :ph34r: ImpalaBob

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This thread topic is well timed :)

 

I'm trying out a Vista Cx that I just purchased today. So far all I've done was pop in some new batteries & was watching its performance as I was driving home. I haven't yet bought any enhanced maps for it, so am only watching the basemap...

 

Well, I'm travelling North along a road that happens to be on the basemap and, just like the OP was mentioning as well, my plot trail is showing me travelling along at 60' west of the road for the whole duration of the trip (about 30 miles). Umm,, should I be nervous about this?

 

I'm going to try some more tests before getting really upset about it, but am a bit disillusioned about the performance of Garmin's flagship eTrex... :)

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... add some info to my previous post...

 

Now that I've made a couple round trips to/from work, I notice something interesting. While the Vista Cx doesn't seem to stay anywhere near the basemap roads at all (from 60' away & up to 300' off!) I notice that it follows it's OWN tracks extremely well.

 

Meaning, going back against a plotted trail and the Vista Cx lines up with it's own track perfectly, just not the basemap road. Makes me wonder.. are the basemap roads that badly orientated? I am actually eager to get the City Navigator V8 software for this GPSr because I am mostly very impressed with it. BUT, I hesitate shelling out another $150 (Canadian) for the software because of how poorly the thing works against the basemaps... maybe (hopefully!) the Vista tracks the enhanced roads with more precision. Anyone have any insight on this? :wub:

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It could be the basemap. There is a "snap to road" (or something like that) setting that you can turn on if you know you are on a road.

 

As far as following its own track, one of the sources of error is atmospheric conditions, which can be relatively consistent (that's why WAAS works) so it's quite likely that it will be consistently inaccurate. If you were in your car it's also possible that the signal was distorted by the windshield/roof.

 

FWIW, I've never seen an error that large between my Vista Cx and the roads in my copy of CNNA7. I've never checked the accuracy of the base map though.

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I'm not sure . . . but it is usually true. Someone more knowledgeable about the maps will have to explain it.

 

GPS satellites don't know or care whether you are near an intersection. The only time the maps would matter at all if if you have the option to stick you to a road turned on, and corners wouldn't make any difference there.

 

I suppose in a car you are more likely to be moving slower near an intersection, but I don't know if your speed affects the accuracy or not.

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This thread topic is well timed :o

 

I'm trying out a Vista Cx that I just purchased today. So far all I've done was pop in some new batteries & was watching its performance as I was driving home. I haven't yet bought any enhanced maps for it, so am only watching the basemap...

 

Well, I'm travelling North along a road that happens to be on the basemap and, just like the OP was mentioning as well, my plot trail is showing me travelling along at 60' west of the road for the whole duration of the trip (about 30 miles). Umm,, should I be nervous about this?

 

I'm going to try some more tests before getting really upset about it, but am a bit disillusioned about the performance of Garmin's flagship eTrex... :unsure:

 

 

Relax...the basemaps are terrible.....basemaps use very few vector points...Imagine the shape of Florida made with only three points...you're looking at a "point angle" instead of a rounded tip. It's impossible for your track to stay on this "abrreviated" map because using so few points it can only approximate the contours of the real highway.City Navigator uses many more points in creating the contour of roads. You won't have a problem with more detaild maps loaded. In your case the track is correct and the map is "wrong" (or poorly detailed). Looking on your gps, if your familiar with your local roads, you can easily see that the basemap depiction doesn't come close to approximating the real roads. I was horrified when I trid to navigate with the basemap....getting City Select made me very happy.

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I'm not sure . . . but it is usually true. Someone more knowledgeable about the maps will have to explain it.

 

You might logically think that but that is not the case. I have found the "corners" on the base map to be off even more than "straight line" sections. One would think in a long arcing curve that a point would be used that is tangent to somewhere along the curve but it doesn't work that way. I can see the same thing in maps that I make myself...cutting down on the amount of points used in defining a road leaves you with all sorts of weird shapes with no apparentlogic behind it. Bottom line it's nothing to fret over, just understand the basemaps do not have enough vector points to create an accurate profile of the roads you see.

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Thanks stevesisti that helps relax my concerns quite a bit. Yes the roads & river/lake contours depicted in the basemap are quite crude & "blocky" in their shape, I suppose they're just ment for general reference more than an accurate track, and GOOD TO HEAR the enhance maps are more detailed.

 

Garmin really should make a bigger effort to tell new customers that the basemap details are only general representations and not an indication of what the GPSr is capable of. I'd wonder if more than a few new customers got scared off (like I almost did!) when eagerly fireing up their new GPSr toy & see how "wrong" it's ability to track a road appears to be. Maybe they did write that warning somewhere in the manual but I just missed it? Anyway thanks. Guess I'll go buy City Navigator now like I planned to do!

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I have to chime in and say that the basemaps on my garmin legendC/60cx are the worst of all the gps I have owned, the detail maps are WAY better. most people say they have problems using the lock on road feature but I can say that with the most current detail maps loaded it works really well. The only time I have noticed this not working quite right was when there were nearby angle roads/signal was poor due to tall buildings or tree cover.

 

oh and when I say detail maps I am not talking about the topo, the roads on there are also horrid.(great topo though)

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Garmin really should make a bigger effort to tell new customers that the basemap details are only general representations and not an indication of what the GPSr is capable of. I'd wonder if more than a few new customers got scared off (like I almost did!) when eagerly fireing up their new GPSr toy & see how "wrong" it's ability to track a road appears to be. Maybe they did write that warning somewhere in the manual but I just missed it? Anyway thanks. Guess I'll go buy City Navigator now like I planned to do!

 

I agree. They need to make it clearer that it comes with essentially no useful maps. Their website could also be a little clearer as far as what map packages are available and what the exact differences between those are.

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