GPS Status Most Accurate?

[fcoliver]

In GPS Status the coordinates shown are usually, but not always, the same as the coordinates shown on Route Summary and Browse Map. Usually, but not always, they're the same as the coordinates shown after clicking the GPS button.

Do you think I can safely assume the coordinates shown on the GPS Status screen will always be the most accurate?

And how much of a change is an acceptable amount? For example, I took note of a number of readings over a 30 minute period while sitting in my easy chair, and they ranged from N36.18821-36.1880 and W81.71892-81.71930.

Does anyone know offhand the distance represented by one degree, by 0.1 degree, by 0.01 degree?

Also, does anyone know what is represented by the number (in degrees) just below the coordinates in GPS Status. I'm assuming it represents the direction of last movement/travel.

Any other thoughts?

 

[zerimar]

I'm not a guru on gps' but as long as you are indoors and not close to a window, the gps accuracy is not that high. In order for the device to to try to pin-point your current position, it tries searching for satelites, thus giving the variation in Lat and Long.

 

[Boyd]

From everything I've read, the SiRF chips favor quantity over quality. So where some other chips might not lock on to the satellites, the SiRF chips get a signal but the fix may be less accurate.

My guess is that you're just seeing "noise". The TomToms are optimized for auto use and won't move the map pointer very often unless you're at driving speeds. But on my Garmin GPSMap 60CSX, if I zoom all the way in (at 20 ft scale I believe) it can be pretty wild to see how much scatter there is to the readings that come in - they look like a shotgun blast. However, if you average them all together it's really pretty accurate.

Somewhere on another site I've seen some analysis of this which pretty much conlcuded the same thing.

 

[fcoliver]

Thank you both for your thoughts.

As I understand this, signals from just two satellites can give a fix. The third satellite adds elevation. It seems that reading 4 or 6 (or even 12) satellites would greatly improve accuracy (reproducibility?)

So I'm sitting here at my desk (which IS beside a window) and reading 12 satellites. No, make that 8 satellites. Wait, now it's 11. In my easy chair, next to a window and a sliding glass door, I usually receive 4-8 satellites, and sometimes 12.

It seems that getting signals from that many locations would greatly reduce the scatter/error.

Does anybody know the significance of the blue bars representing each satellite, and whether they need to be above the line for an accurate read? (And why is this not even mentioned in the manual or online help?)

Is there an explanation for the discrepancy (at times) between the coordinates on the "GPS Status" screen, the "Route Summary" screen, and the coordinates listed on "Browse Map"?

 

[fcoliver]

Other Answers...

I think I've answered the other two questions:

On the "GPS Status" screen the number in degrees below the coordinates appears to represent the direction of travel.

Also, it appears that each degree of longitude represents 69 miles at the equator, or 34.5 miles at 60 degrees latitude. So at my latitude of N36 it's probably about 50 miles. Right?

I still haven't gotten my head around the concept of how these numbers relate to distance, e.g. if the longitude goes from N36.18800 to N36.18860, what does that mean in feet (or yards, or miles)?

 

[lec510]

the SiRF chips get a signal but the fix may be less accurate.

This is not true.
Both Garmin and TomTom may use the same Sirf-III chipset, but they implement it differently.
Garmin go after WAAS which get better accuracy, while TomTom implement QuickGPS, for quicker signal fixes.

 

[GOer910]

Thank you both for your thoughts.

As I understand this, signals from just two satellites can give a fix. The third satellite adds elevation. It seems that reading 4 or 6 (or even 12) satellites would greatly improve accuracy (reproducibility?)

So I'm sitting here at my desk (which IS beside a window) and reading 12 satellites. No, make that 8 satellites. Wait, now it's 11. In my easy chair, next to a window and a sliding glass door, I usually receive 4-8 satellites, and sometimes 12.

It seems that getting signals from that many locations would greatly reduce the scatter/error.

Does anybody know the significance of the blue bars representing each satellite, and whether they need to be above the line for an accurate read? (And why is this not even mentioned in the manual or online help?)

Is there an explanation for the discrepancy (at times) between the coordinates on the "GPS Status" screen, the "Route Summary" screen, and the coordinates listed on "Browse Map"?

Actually, correction, you need a minimum of 3 satellites to obtain what is known as a 2D GPS position (Lat / Long), but the receiver has to have reasonable knowledge of the current altitude; this also called altitude fixed mode. In order to resolve the 3D positioning (Lat, Long, and Elevation)you need 4 satellites or more. The other factor that will also determine your GPS 2D or 3D calculated position accuracy is the geometry of the satellites in the sky being tracked by your GPS receiver. The bars in the TomTom satellite window represent the CNo (carrier to noise ratio) which you can consider as quality of the signal received for each individual satellite being tracked, the stronger the signal the taller the bar is, and it turns blue when the measurements for satellite are actually used to calculate the final GPS position displayed on the map. Hope this helps a bit.....

 

[fcoliver]

Thanks. That's sorta' what I suspected: Once the satellite bar turns blue, that signal is being used in the processing.

Think there's any significance to the horizontal line running across those bars (about 1/5 from the top)?

Can anyone explain (or has anyone else noticed) the discrepancy in the coordinate numbers between "GPS Status", "Browse Map" and the "Route Summary" screens?

 

[fcoliver]

Got one answer, but still puzzled

TT CSR (via email) advises that coordinates listed on the "GPS Status" screen will be more accurate than those listed on "Browse Map" or "Route Summary."

Which still leaves me wondering why there would be different coordinates on the three screens.

Also a related question: Why does my longitude change with the zoom level? (This was posted in another thread, but didn't generate much response, so I'll try again.)

My longitude changes by almost 21 degrees going from max to minimum zoom. TomTom says this is normal.

Has anyone else noticed this, and does anyone think it is normal?

 

[jimj]

One degree of latitude is equal to 60 miles, one degree of longitude is equal to varying distances depending on what latitude you are at if I remember correctly. I could be wrong because the recall factor is slowing.

 

[drledger]

have never really given it much thought but 21 degrees of longitude variation seems like an enormous amount unless you are standing near the north or south pole.

 

[fcoliver]

have never really given it much thought but 21 degrees of longitude variation seems like an enormous amount unless you are standing near the north or south pole.

My point exactly, drleger. TomTom CSR tells me this is normal, and others on this thread say their longitude does not change with zooming.

How can that be?

 

[Boyd]

I just tried on my 920T. Went to Browse Map, then tapped the icon in the upper right corner and YOU ARE HERE appeared. Now I zoomed all the way in, tried several other zoom levels, and zoomed all the way out until the whole US filled less than half the screen.

The coordinates never changed. While I was zoomed out to the max I touched a random point on the screen and the coordinates changed accordingly. Now I zoomed all the way in. The point I touched no longer showed because it was off the screen at full zoom, but the coordinates didn't change.

Next I touched the icon again so the screen showed YOU ARE HERE and made note of the coordinates. Then I went to the GPS status screen and the coordinates matched exactly. However it wouldn't surprise me if they changed a bit over time since there's a margin of error for a GPS fix.

I suspect this doesn't generate a lot of response simply because most people don't care a lot about coordinates and their relationship between the different screens. But if you think your GPS is defective you should see if you can get it replaced.

 

[drledger]

I fly with a Lowrance Airmap 2000 and any deviation is not a good thing.
I know that I only get altitude with four satellites received but the fix is a bit iffy. It gets much better as more satellites are received and included. WAAS makes it so accurate that I am within feet. So a 21 degree deviation being normal makes no sense. I will contact support and see if I get a different answer.

 

[Boyd]

fcoliver, if this is a repeatable problem why don't you post screenshots at different zoom levels to document it? I'm curious as to exactly what you're seeing and why it seems to be different from what others report when trying the same thing. And I'd think it would also be helpful to have a record when contacting TomTom support.

 

[fcoliver]

Screenshots

Sure, Boyd. The same phenomenon occurs on virtual TomTom, which I guess would be the source of the screenshots.

Somewhere on this forum I've seen a "how to" on including screenshots. (Unless someone wants to point me to it. )

 

[fcoliver]

Got the screenshots, but....

Now I need to figure out (or have someone tell me) how to get them into this forum.

I've tried to copy/paste the .bmp from MS Picture Manager and from MS File Explorer, and I've tried to drag-and-drop from both locations into the forum reply, which shuts down the forum page.

How do you do it?

 

[Boyd]

You can capture the actual GPS screen this way: https://www.tomtomforums.com/showpost.php?p=4490&postcount=7

AFAIK you will have to host the screenshots on another site, then use the following BBcode to imbed it in your post

[ img ] http://whatever.com/screenshot.jpg [ /img ]

Leave the whitespace out when actually doing this so the system will parse the link. This is how I've been embedding pictures in my posts, but maybe a moderator has another solution for hosting the images on this site?

Another option would be attaching it to you post. Use the Manage Attachments button below the post editing window for that.

 

[fcoliver]

Longitude changes w/ zoom

Thanks, Boyd. I captured the GPS screens per your instructions, then I tried attaching, but that didn't work - files too large.

Didn't really understand anything about the other method.

Maybe I'll be presumptive and attach them to an email....

Well, I see that won't work, either. What's next?

 

[Boyd]

You need to have your own website somewhere to upload the photos. For example, most internet providers (Verizon, Comcast, etc) give you some free webspace where you can upload files. Look in the support/help section of your providers website for info on how to use this feature. Another alternative would be free photo sites like flickr.

Upload your screenshots to one of these sites, then bring it up in your browser and copy the URL (web address). Now use the [ img ] tags in my example above to include them in your post. Here's an example where I uploaded a screenshot to a website that I have:

To see exactly how I did this, click the "quote" button and it will include the code which was used in a reply. Also, you might want to convert the screenshots to .jpg files, which will be smaller. You should be able to attach one of them to a post - I did it before and it worked. To convert the file you could use Photoshop, or if you don't have it then someone else can probably recommend a shareware version which runs on the PC (assuming you use Windows - I'm on the Mac).