Anyone know how to get current location elevation from the NAV?

 

Sorry, I don't have nav but if you do find out how to get the elevation I wouldn't put much faith in the numbers. The GPS units in everything but airplanes are pretty bad at resolving elevation. They're easily off by +/- 300-500ft.

 

Why is that? With enough satellites I would think that they would be dead on.

 

I don't know the physics of it but I know that when I was in death valley my Garmin 76 told me I was 412' below sea level and my friends Magellan told us that we were 150' below sea level. We were actually at -282'.

I've seen even larger discrepancies at higher altitudes.

 

GPS satellites are in low Earth orbit, which is 100-200 miles above ground. That means the vertical separation between the receiver and satellites is much smaller than the lateral separation (which could be tens of thousands of miles). Furthermore, all the satellites are roughly at the same altitude, in the same direction (up). Again, compared to the lateral difference, which could be in every direction and various distances. Since GPS uses triangulation for a position fix, the receiver has less range and separation to work with in the veritical (think of an inverted 3D pyramid that's not very high). As a result, there is more error in the vertical.

 

damn that was a good explanation

 

Hmm... You may be right about most of what you said - I'm not sure. What I can say with some degree of certainty is that GPS satellites are NOT low earth orbit. So they circle the earth thousands of miles above it, not a couple of hundred miles.

From http://www.gisillinois.org/gps/GPSDEF/sat.htm:

The 24 GPS satellites (21 active, 3 spare) are in orbit at 10,600 miles above the earth.

 

Thanks for pointing that out - they are much higher than I thought.

 

To answer the question: Just press the center circle part of your touchscreen when you have the map displayed. It will show you your current longitude, latitude, and elevation. Elevation only works when you have satallite coverage (your GPS text is green instead of white). I find it accurate within 10-20feet in elevation (when compared to signs on the roads about elevation).

 

Two ways. On the Map screen, touch the little "A" (that's suppose to be you in the TSX). Also, go to the diagnostics screen and hit the GPS info, although the info is in meters.....

 

Actually, the altitude error would be the same as the longitute and latitude errors, about +/- 25 feet typically..... much worse before we turned off S/A (selective availability) about 5 years ago.

 

When did you do this test? With a couple of good receivers, you shouldn't see more than 50 feet of difference, usually much less today. (It's not uncommon to have accuracies of +/- 15 feet) If this was over 5 years ago, then S/A would have made that much worse......

 

Thanks gpsiir! What's the best way to check elevation accuracy? I'm curious to see how good my iQue is....

 

This was only in the summer. I wonder if it was because we were in a valley or if it was because both of our gps are fairly old. My GPS 76 has to be 4yrs+. I replaced it with an iQue shortly after and I'm hoping it's going to be a little more accurate.

 

Best way is to find a survey crew that is using a differential GPS unit, those are typically accurate to +/- 1-2 inches. Are in or near a major city? There are differential transmitters in most major cities. BTW, "differential" is basically an additional signal that receivers can get that is transmitted from a ground based system that "soups up" the accuracy, a lot.

 

The valley shouldn't affect it unless you are in a narrow very high valley. I've never been to death valley, but the pictures I've seen show a wide open hellish sort of place. You want a clear view of the sky in all directions, at least 120 degrees of view for optimum performance.

 

I always thought that the reason you couldn't get accurate elevation through a consumer model GPS unit is because that data isn't released to consumers... were as the military model have access to the full range of data. I know that more and more information is released now versus a few years ago... but I thought elevation was still hidden from consumer models.

 

The military accuracy is the selective availabiity mentioned a few posts ago. Turned off 5 years ago. Consumer models have had elevation display for a long time - that's the discussion point of this thread.

 

Not quite right. "Civilian" GPS units can only access the unencrypted GPS navigation signal, but they have always been able to give users latitude, longitude, and altitude. When "Selective Availability" was enabled, the military deliberately degraded the civilian GPS navigation signal to the point that "normal" accuracy was +/- 150-250 feet. This was to make it less usable to US enemies and terrorists. In May 2000, by order of the president, S/A (selective availability) was turned off (and there was much rejoicing!). So, all civilian GPS receivers got much more accurate. Whew, okay, as I said, civilian GPS units can only get the unencrypted GPS, as you might guess, that implies that the US military (and its allies) get GPS units that also receive and decrypt another GPS navigation signal, one that makes military units much more accurate (the actual numbers are, of course, classified). Bottom line, both civilian and military GPS receivers give you the same information (lat., long., altitude, very very very very very accurate time), but the military units have better accuracy on the positioning coordinates.

Hope I didn't over-answer too much!