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Accuracy circle Vista HCX?


wv hick
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The manual says that the circle around the location pointer is an indication of the accuracy of the location. In other words, the GPSr is reasonably sure one is located within that circle. This does not seem right to me because for example if I view the accuracy data field, it may say 15 foot, while the radius of the circle may measure 400 feet! Can anyone explain this (at least better than the manual)? Thanks!

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The manual says that the circle around the location pointer is an indication of the accuracy of the location. In other words, the GPSr is reasonably sure one is located within that circle. This does not seem right to me because for example if I view the accuracy data field, it may say 15 foot, while the radius of the circle may measure 400 feet! Can anyone explain this (at least better than the manual)? Thanks!

I've come across the same thing. On my old Venture you'd rarely even see the accuracy circle unless you were zoomed right in close, but on my new Vista HCx it's huge all the time. Even when I have a +/- 3m fix, it's still (comparing it with the scale on the bottom left hand corner) about 300m in diameter, i.e. about 10x what it should be.

 

I asked the same question on Usenet and someone claimed that the size of the circle also reflected the accuracy of the underlying map (whatever that might mean) which in my case (since I'm in Ireland where there are effectively no maps available) is the basemap. Sounds like bullsh!t to me, TBH.

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The manual says that the circle around the location pointer is an indication of the accuracy of the location. In other words, the GPSr is reasonably sure one is located within that circle. This does not seem right to me because for example if I view the accuracy data field, it may say 15 foot, while the radius of the circle may measure 400 feet! Can anyone explain this (at least better than the manual)? Thanks!

I've come across the same thing. On my old Venture you'd rarely even see the accuracy circle unless you were zoomed right in close, but on my new Vista HCx it's huge all the time. Even when I have a +/- 3m fix, it's still (comparing it with the scale on the bottom left hand corner) about 300m in diameter, i.e. about 10x what it should be.

 

I asked the same question on Usenet and someone claimed that the size of the circle also reflected the accuracy of the underlying map (whatever that might mean) which in my case (since I'm in Ireland where there are effectively no maps available) is the basemap. Sounds like bullsh!t to me, TBH.

It is indeed a combination of the accuracy of the receiver combined with the accuracy of the map. As found in the Garmin FAQ:

Question: What is the circle around my position on the map?

 

Answer:

This is the "Accuracy Circle". This circle represents the approximate accuracy of the GPS position based on the Estimated Position Error (EPE) and the map data quality. The position of the GPS device on the map should be located somewhere within this circle. This feature can be turned on or off on the GPS device. Please refer to the owner's manual for instructions in doing this

It makes sense, since if you have a low quality (accuracy) map, the position of the GPS, however accurate, may not reflect the position on the map. For instance If you had a position +/- 3m, say at a road intersection, if the map's position of the road intersection were out by a lot due to inaccuracy of the map, the GPS would not show you on that intersection. However, the "accuracy circle" should be big enough to encompass the intersection. If you had a more accurate map, the position shown by the GPS on the map would be closer to the intersection, and the "accuracy circle" would be smaller.

 

With respect to the basemap, yes it is inaccurate, and your "accuracy circle" will be large. In another FAQ from Garmin:

Question: Why is my position on the map screen not accurate?

 

Answer:

The basemap in your unit has been drawn from a map with a scale of 1:1,000,000. You can expect an accuracy of .25 miles (1,320 feet) from a map of this scale.

 

Additionally, when you view your position on the GPS map page, you will notice that a circle displays around your current position. This circle shows the area you can expect your actual position to fall within. The less accuracy the unit has at the time, the larger the circle will be. The diameter of this circle considers both your GPS accuracy and the accuracy of the map to which you are trying to plot your position.

 

City Navigator, which is a more detailed mapping program, has an accuracy of about 40 feet. Therefore, you would see a smaller circle around your actual position (assuming you have a good GPS position fix), than you would using a basemap.

I don't understand why you say there are effectively no maps available to you in Ireland. According to the coverage area, both City Navigator Europe and Metro Guide Europe indicate that they have full, detailed coverage in Ireland. Maybe I'm missing something?

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I don't understand why you say there are effectively no maps available to you in Ireland. According to the coverage area, both City Navigator Europe and Metro Guide Europe indicate that they have full, detailed coverage in Ireland. Maybe I'm missing something?

I use my GPS for hiking only, (I have a nuvi for the car) CityNavigator or MetroGuide wouldn't be an awful lot of use for that :) There are no topo maps available for Ireland , and in any case even if there were, if the quality was anything like what I've seen on the latest UK topo maps, I wouldn't bother.

 

I do have some 'home made' contour maps that were created by the SMC (Scottish Mountaineering Council) based on processing of Shuttle SRTM data that covers the whole of the British Isles which are better than nothing.

 

Anyway returning to the subject, I kind of understand the logic in what you said about the accuracy of the underlying map, but for the application I use it for, i.e. hiking, the only map elements that mean anything in that context are waypoints I've put in there myself. I personally couldn't care less what the positional accuracy of a road intersection 10km away is.

 

I'd at least like the opportunity to turn this 'feature' off, and have the circle just represent the current EPE. As it is it's just an annoying distraction.

Edited by AlunS
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Replying to my own post here :)

 

Anyway, after some extensive Googling, it would appear that on some Garmin GPS's (including the 60 series and the GPSmap 76 series) it's possible to completely disable the basemap which might be the answer to my problem. Does anyone have any idea if this is possible on the Vista HCx or it's siblings?

 

Another option might be to edit the basemap somehow to fake a higher accuracy if I could get at it somehow.

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Replying to my own post here :)

 

Anyway, after some extensive Googling, it would appear that on some Garmin GPS's (including the 60 series and the GPSmap 76 series) it's possible to completely disable the basemap which might be the answer to my problem. Does anyone have any idea if this is possible on the Vista HCx or it's siblings?

 

Another option might be to edit the basemap somehow to fake a higher accuracy if I could get at it somehow.

If you turn on "lock on roads" it will go away; there is no way of turning it off independently of the lock on roads setting. (This is the case for the 60's and 76's; should be the same for other recent Garmins as well.)

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If you turn on "lock on roads" it will go away; there is no way of turning it off independently of the lock on roads setting. (This is the case for the 60's and 76's; should be the same for other recent Garmins as well.)

Doesn't seem to make any difference on my Vista HCx .. the circle is still there.

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Which is because "lock on road" requires a map like City Navigator or Metroguide to lock on to. Besides, if it doesn't find any road to lock on, or your speed is too slow, it will still show the circle.

Some navigators do allow you to turn the circle off independently. A unit like the iQue 3600a, where nearly everything imaginable have a specific setting, allows that.

 

In your case, just disregard it. I haven't checked what happens if you disable the basemap, but effectively that would make the circle even larger, since in relation to the screen, you could be anywhere.

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Some navigators do allow you to turn the circle off independently. A unit like the iQue 3600a, where nearly everything imaginable have a specific setting, allows that.

That would be great, and a much better use of software resources than useless stuff like games :grin:

In your case, just disregard it. I haven't checked what happens if you disable the basemap, but effectively that would make the circle even larger, since in relation to the screen, you could be anywhere.

I've seen references on the net that suggest that on units that support the disabling of the basemap the circle then reverts to representing the real EPE.

 

I only bought the Vista HCx because of the high sensitivity receiver, and the fact that I can set it up so that it automatically records track logs to the memory card. The limited mapping that is available is of no use to me really, and if I could have got a non-mapping unit with those features, I would have done. Now what i'd really be interested in would be something that I could upload my own scanned in and calibrated raster maps to!

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Replying to my own post here :grin:

 

Anyway, after some extensive Googling, it would appear that on some Garmin GPS's (including the 60 series and the GPSmap 76 series) it's possible to completely disable the basemap which might be the answer to my problem. Does anyone have any idea if this is possible on the Vista HCx or it's siblings?

 

Another option might be to edit the basemap somehow to fake a higher accuracy if I could get at it somehow.

To disable the map, go to the map page, select menu>setup map, move the cursor over to information, press the menu button again and select either "show none" or "hide basemap". However, the "accuracy circle" still shows - about 20m radius when my GPS is showing an EPE of +/- 6m (30m at +/- 11m). It wouldn't surprise me if the "accuracy circle" is based on 95% probability, while the EPE is one-sigma, or some such thing. However, I can't yet find how to turn off the "accuracy circle' completely.

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Replying to my own post here :grin:

 

Anyway, after some extensive Googling, it would appear that on some Garmin GPS's (including the 60 series and the GPSmap 76 series) it's possible to completely disable the basemap which might be the answer to my problem. Does anyone have any idea if this is possible on the Vista HCx or it's siblings?

 

Another option might be to edit the basemap somehow to fake a higher accuracy if I could get at it somehow.

To disable the map, go to the map page, select menu>setup map, move the cursor over to information, press the menu button again and select either "show none" or "hide basemap". However, the "accuracy circle" still shows - about 20m radius when my GPS is showing an EPE of +/- 6m (30m at +/- 11m). It wouldn't surprise me if the "accuracy circle" is based on 95% probability, while the EPE is one-sigma, or some such thing. However, I can't yet find how to turn off the "accuracy circle' completely.

You're a star! I hadn't thought to hit the menu button again once I'd selected the information tab. It is very briefly mentioned in the manual, now I read it more carefully, but it only says you can do it, no mention of what options are available, otherwise I might have twigged earlier. I'm normally quite good at discovering every single option in kit like this, so I'm a nit surprised I missed that one!

 

My 'home made' contour maps I mentioned still display OK, although the whole map is covered with cross-hatching which is a little irritating though.

 

EDIT: Apparently this is because the maps are defined as being transparent, and when there's no underlying basemap that's what it shows for some reason or another. There's a link on the page where these maps are stored that points to instructions for making them non-transparent, so it looks like I'm saved.

Edited by AlunS
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The cross hatching is there to tell you you are outside the map. If you have a map enabled, but reach the rim of the map, just having what's outside white could confuse you into believing that there are just no features, but still mapped land. Now it's easy to see that there's no map at all.

 

When you then disable everything, you are effectively outside the map wherever you go, so everything is cross hatched.

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Just to say that I've just gone through my home made contour maps with MapEdit and cgsMapper (which is a real PITA!) and converted them all to non-transparent and now the cross-hatching has disappeared. I've disabled the basemap which was next to useless anyway, and the error circle has gone down to the same order of magnitude I was used to on my old non-mapping etrex Venture! It still isn't the same diameter as the displayed EPE, but at least it's less of a distraction than it used to be.

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