GPS Status Most Accurate?

[Larry5120]

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.

Using google earth and the numbers you entered above, that equates to about a 75 foot north/south variance, and a 120 foot east/west variance.

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

At least for longitude variance, that depends on you current latitude. The closer to the equator, the greater the mileage between longitude degrees, the closer the pole, less. For instance, using the latitude of 36.188 degrees, a one degree longitude difference = about 55.8 miles.

To get the specific answers yourself, just install google earth if you don't already have it and play around, or find some lat/lon calculators via google and plug your numbers in to see what you get.

Larry

 

[drledger]

Here is my posted question to TT:
My longitude changes by almost 21 degrees going from max to minimum zoom. TomTom says this is normal." Depending on where you are on the north/south meridian this can be thousands of miles. Does TT really believe this is an acceptable deviation?

]
Quote from TT Support:
Please rest assured that it is not normal for the location to change when zooming in or out on the screen. If this occurs, we recommend changing the configuration of your system. To do so, please take the following steps:

- Tap the satellite signal bars on the bottom right of the primary navigation screen.

- Tap the satellite signal bars again on this screen.

- Select Minutes or Seconds.

This should correct the issue.

The answer does not seem to answer the question of why such a variance.

 

[tomlouie]

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?

Are you sure that its your longitude DEGREES that are changing, not the longitude SECONDS? Because I punched in the lat/lon you gave, and the only time I see any longitude change at all during zooming is when I have lat/long in DD MM SS format. Only then, do I see a momentary change in the longitude SS during zooming (when I have the zoom slider in motion). But, as soon the zooming completes (I release the zoom slider), the longitude returns to the correct value.

I think what we're seeing is some mild rounding errors in the displayed longitude SS during the zoom operation (when the slider is moving).

Tom

 

[[email protected]]

Degree Min. Sec. measurements

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?

The earth is approximately 24,000 miles in circumference, which means it is 12,000 miles from the North Pole to the South Pole. And the difference in latitude from the North Pole (+90 degrees) to the South Pole (-90 degrees) is 180 degrees. So 1 degree of latitude is 12,000/180 miles which is 67 miles. Let's round that to 60 miles to make the next computation simpler. There are 60 minutes to a degree, so 1 minute of latitude is 1 mile. There are 60 seconds to a minute, so 1 second of latitude is 1/60 mile or 5280/60 feet. That comes to about 90 feet.
The distance corresponding to a second of longitude will depend on where you are. If you are at the equator, a circle of longitude is still the same 24,000 miles, so 1 second is again about 90 feet. If you are at one of the poles, the circle of longitude reduces to a point, so 1 second is 0 feet. OK, we did some approximating in the preceding paragraphs. To get a more exact value, we should have used 24,901 miles as the circumference and not done any rounding. That would have given us 101 instead of 90 feet.

 

[jimj]

The earth is approximately 24,000 miles in circumference, which means it is 12,000 miles from the North Pole to the South Pole. And the difference in latitude from the North Pole (+90 degrees) to the South Pole (-90 degrees) is 180 degrees. So 1 degree of latitude is 12,000/180 miles which is 67 miles. Let's round that to 60 miles to make the next computation simpler. There are 60 minutes to a degree, so 1 minute of latitude is 1 mile. There are 60 seconds to a minute, so 1 second of latitude is 1/60 mile or 5280/60 feet. That comes to about 90 feet.
The distance corresponding to a second of longitude will depend on where you are. If you are at the equator, a circle of longitude is still the same 24,000 miles, so 1 second is again about 90 feet. If you are at one of the poles, the circle of longitude reduces to a point, so 1 second is 0 feet. OK, we did some approximating in the preceding paragraphs. To get a more exact value, we should have used 24,901 miles as the circumference and not done any rounding. That would have given us 101 instead of 90 feet.

I;m not sure but I believe that in aviation we used nautical miles which were 6000 feet and would change your calculations slightly.

 

[fcoliver]

Thank you, DrLedger, for checking on this w/ TT CSR.

They gave you a totally different answer from the one I got:

12/26/07 from TT support via email:
We are always happy to help. The fact that the latitude changes when you zoom in or out on the browse map is normal. This usually happens because of the shift of the map. Currently there is not any way that we would be able to stop this from happening. We regret any frustration that this may have caused you.

I've tried all three possible GPS configurations (DD-MM-SS, degrees-decimal minutes, etc) and still get almost 21 degrees variation in the longitude between min and max zoom. (The latitude does not change.)

The longitude does not change while moving the slider button but changes as soon as I release it. If I just tap the top or bottom of the zoom slider the longitude changes, and returning to previous zoom level returns to the previous longitude.

Coordinates shown at max zoom are closest to (but not the same as) the coordinates shown in the "GPS status" screen.

This does not make sense to me, and apparently no one else is experiencing this phenomenon.

Regardless of what TT customer support told me, I do not think it "normal" for the longitude to change with zoom level, certainly not 21 degrees.

I captured some screen shots but haven't been able to put them on the forum. If anyone is interested, I can attach them to an email.

 

[Boyd]

Fred: since you seem to be having a hard time with this, send me a PM and I'll give you my e-mail address so you can send the screenshots, which I will post here.

The reason I asked for the screenshots is that I'm having a hard time believing your interpretation of the coordinates. As tomlouie and tomhead have said above, I think it's either fractional degrees or seconds which are changing since an error of 21 degrees would be huge.

If we can all look at the same screens maybe we can make sense of this.

 

[Boyd]

Here are the screenshots which fcoliver sent me. I am numbering them for reference:

#1

#2

#3

#4

#5

#6

#7

 

[Boyd]

My only comment on this is.... what possible value is there to zooming out to the 5000 mile range? I didn't even realize you could zoom out that far!

So whatever coordinates are being shown, I don't think they are particularly useful since that isn't a setting which you would normally use for anything (please correct me if you think there's a need to view the map at that scale). At that zoom scale, each screen pixel is hundreds of miles wide, so I'd guess there's some rounding error involved in the coordinate display.

 

[fcoliver]

Thank you, Boyd.

I very much appreciate your assistance in getting the screen shots into the Forum.

And I totally agree with you: there's no value or purpose in the map at minimum zoom. I took the screen shots at max-min-max zoom because that demonstrates the maximum variance in the change in the longitude coordinate (almost 21 degrees variance, most of the time.)

The amount of deviation is proportional to the amount of zoom, so at max zoom, the coordinates are closest to the actual coordinates of a given location (assuming the coordinates given in "GPS Status" are the 'actual' coordinates.)

So here's what I find puzzling/irritating:

1) TomTom Customer service told me (via emai):

We are always happy to help. The fact that the latitude changes when you zoom in or out on the browse map is normal. This usually happens because of the shift of the map. Currently there is not any way that we would be able to stop this from happening. We regret any frustration that this may have caused you.

And their response to drledger on the same question:

Please rest assured that it is not normal for the location to change when zooming in or out on the screen.

I've never experienced changes in location/coordinates when zooming in Google Earth, Google Maps, Mapquest, or other mapping programs.

2) Why is my unit exhibiting an action that is apparently not reproducible on the units of other Forum members?

 

[tomlouie]

I've never experienced changes in location/coordinates when zooming in Google Earth, Google Maps, Mapquest, or other mapping programs.

All the other programs you mentioned run on bigger faster clients (your computer/browser) AND bigger faster backend servers (google.com, mapquest.com, etc). The tiny little TomTom has to react, compute, retrieve and render everything (and listen for GPS satellite signals!) all by itself on much smaller slower limited hardware.

I can understand TomTom's need for the rounding, but this particular roundoff error feels like a bug instead of a deliberate design feature. Unfortunately, I bet other issues/bugs would get priority over this one.

Why is my unit exhibiting an action that is apparently not reproducible on the units of other Forum members?

Tell us the exact steps your taking from numbered screenshot to screenshot, and I bet we can reproduce it.

 

[Boyd]

The amount of deviation is proportional to the amount of zoom
________________
1) TomTom Customer service told me (via emai):

We are always happy to help. The fact that the latitude changes when you zoom in or out on the browse map is normal. This usually happens because of the shift of the map.

Well this may tell you more about the knowledge of TomTom's support staff than the GPS itself. But I think the response quoted above is probably accurate, and is consistent you your observation that the coordinates change proportionally with the zoom level.

This implies that the software chooses a location to center the view based on some virtual screen space, and when you zoom out to astronomical levels it causes rounding. Sure, it could be either a "bug" or a "feature" and without studying source code we can't really know.

But when I tested this on my GPS I certainly didn't zoom out that far. But I did zoom far enough for the US to fill the width of my screen. That seems to be as far as I can ever imagine needed to zoom, and the coordinates didn't change within that range. I'm guessing that the other TomTom support person never imagined that you had zoomed out anywhere near that far.

So I know this just bothers you for some reason, but it seems like the equivalent of worrying that your car speedometer is off by 21 miles per hour if you drive at 5,000 mph. Driving down the highway at 70 mph that would translate into an error of 0.3 mph which isn't really going to change anything

I don't think you're going to get anywhere with this myself, and I'm not really sure what you're looking for or how it would help you to know why this happens.

 

[fcoliver]

Larry5210, thanks for responding...

I found some lat/long info on a Google search, along with some lat/long calculators, and I had figured at my lattitude that a degree of longitude was around 55 miles.

As to the other question, is a 120-foot variance a normal amount for the TomTom?

I've heard about super-accurate GPS units with reproducibility measured in centimeters, although I recall they used land-based transmission towers along with the satellite signals. And I'm guessing they don't cost a couple hundred bucks.

 

[fcoliver]

tomlouie... the steps from screenshot to screenshot:

For #1 I touched the bottom of the zoom slider (max zoom) then touched the upper left arrow (current GPS position.) For #2 I touched the top of the slider bar, noting the latitude stayed the same with a longitude change of almost 21 DEGREES. For #3, I touched the top of the slider bar, and the coordinates returned to the original numbers.

On screenshots 5-7 I just alternated tapping the top and bottom of the slider bar.

If you tell me your TomTom does the same thing, I'll sit down and shut up.

 

[tomlouie]

If you tell me your TomTom does the same thing, I'll sit down and shut up.

I sat with my TT1V3 and tried what you described to the letter, even starting with the lat/lons shown in the screenshot, I couldn't get the lon changing behavior you're seeing.

... until I noticed that you had your unit set to Left Handed. Once I used the Prefs to set my unit to Left Handed, I see the exact same bug you've been describing.

It's a bug. Nothing to do with round off error. TomTom must be calculating the displayed lat/lon in Browse mode from the left edge of the visible screen, which is offset if the zoom scroll bar is on the left as it is when set to Left Handed mode.

So, there you go. You can either continue to use it in Left Handed mode and just ignore this bug. Or set it to right handed mode.

Mystery solved!

Tom

 

[Boyd]

Mystery solved!

Tom

Brilliant! - Nice job. I had just been thinking that something was different about those screenshots and realized it had the lefthand option set, but it never would have occured to me that it would be a factor in the calculations!

 

[tomlouie]

I found some lat/long info on a Google search, along with some lat/long calculators, and I had figured at my lattitude that a degree of longitude was around 55 miles.

As to the other question, is a 120-foot variance a normal amount for the TomTom?

I've heard about super-accurate GPS units with reproducibility measured in centimeters, although I recall they used land-based transmission towers along with the satellite signals. And I'm guessing they don't cost a couple hundred bucks.

120ft isn't unusual for most GPS units under certain conditions. It's not a flaw in the unit, just part of the vagaries of affordable GPS in real-world situations. The super accurate stuff comes from using differential-GPS signals from local DGPS broadcasting stations, which most consumer GPS units won't have.

Tom

 

[tomlouie]

Brilliant! - Nice job. I had just been thinking that something was different about those screenshots and realized it had the lefthand option set, but it never would have occured to me that it would be a factor in the calculations!

Funny thing is that I normally run my TomTom (Nav6 on Palm) in Left Handed mode, but the TT1V3 that I'm trying out now is my wife's, and she has it in Right Handed mode.

So when I saw the LeftHanded mode screenshots, they looked "normal" to me.

Tom

 

[Boyd]

Well here's another good one. I have the 920T which came with the remote control. When you use the remote to zoom in and out in Browse Mode, the zoom bar and cursor menu go away since the remote itself controls these functions.

With left-hand mode set, I went to browse the map by tapping the screen and noted the coordinates. Then I zoomed in ONE CLICK with the remote and lo and behold... the coordinates changed when the zoom bar vanished. This doesn't happen when the screen is set to right-hand mode.

Heh, just further proof that the left hand is the hand of the devil...

I wonder what other bugs left handed mode may expose you to?