I think this is a great GPS device and I have enjoyed using it...I have two questions:
First, can it display your altitude (my old Magellan Roadmate could do that), and if so, how does one get to that screen?
Second, is there a way to erase previous destinations? If the unit is stolen I would not want someone to have a list of where I have been...
Thank you!
You can get altitude from the satellite info screen. Click on the satellite reception icon in the upper left of the main menu. You can clear previous entries by going to 'Where to ?' -> Recently Found -> 'Clear'.
I've been looking at the elevation readings lately and first, it seems that you can only get elevations for the place you're at. Is that correct? Second, I find that it is often off by as much as 100 ft even in Ohio where elevation changes are not abrupt. In one case it seemed to be keying on a location that was 5-10 miles away. Does anyone know how that works?
Correct, on the Nuvi you can only see the elevation for where you are currently at. On the Nuvi altitude is calculated just from the GPS signals. Altitude readings are typically not as accurate as lat/long readings. You could try turning on WAAS (if it isn't) and see if that eventually makes more accurate altitude readings for you.
I'll try that because the WAAS was not turned on. Based on your description, it should be quite accurate so I'll double check. I assume there's a database that translates long/lat info into elevation.
Joined: 24 Mar 2007 Posts: 54 Location: Ontario, Canada
Sun Sep 16, 2007 1:27 pm
rmellon wrote:
I'll try that because the WAAS was not turned on. Based on your description, it should be quite accurate so I'll double check.
I didn't read Tim's comment to indicate that it would be "quite accurate" . Even with WAAS, I wouldn't expect great altitude accuracy from an auto navigation unit using GPS, it's just not a priority for this unit so I would expect little effort to have gone into that part of the design. There are some Garmin handhelds (eTrex Vista series) that feature an atmospheric pressure-based altimeter and I suspect avionic units are designed for altitude accuracy, but not nuvis or streetpilots). Point is, don't get your expectations too high.
rmellon wrote:
I assume there's a database that translates long/lat info into elevation.
Tim pointed out that current position altitude is derived from GPS signals, rather than a position-to-altitude translation database. That's accurate. However, Garmin sells Topo maps for their units which do map lat/lon with contour (topographic) altitude, so that is a kind of database if that's what you meant. I've recently played a little with Canada Topo maps on the nuvi and they work, but don't really add very much value for an automotive unit.
Even with WAAS, I wouldn't expect great altitude accuracy from an auto navigation unit using GPS
If WAAS is turned on, and you are in a location that can receive a WAAS signal, and the WAAS data has been received and applied to the signal being received from the satellites, then the altitude reading from the Nuvi should be pretty much the same and just about as accurate as the altitude reading I get from the GPS in the airplane I fly.
Further diving into the "lat/long/altitude/database" conversation, what GPS devices don't do is say "I've calculated your latitude and longitude as these coordinates and I know that point on earth is at this elevation". They don't clamp you to the ground.
Your latitude, longitude, and altitude are all derived from the satellites. Because the satellites are "looking down at you" that is the angle from which triangulation becomes the least accurate, and thus producing less accurate calculations for altitude.
Since altitude is important to pilots, and GPS was seen as a potential good tool for pilots, the FAA developed the WAAS system to enhance the altitude accuracy for GPS. With WAAS, pilots can legally fly approaches to airports down to a couple hundred feet in some locations using GPS.
Note too the subtle difference between "altitude", your height with respect to sea-level, and "elevation" which refers to the altitude of the ground at a particular location. So you could say that GPS devices display altitude. If there is a topo map that map might show elevations, but the GPS doesn't care about, nor calculate elevation.
Great stuff guys. Thank you. I've turned on my WAAS and the "altitude" (about 3 ft above the "elevation" when I hold it at waist height) is right on. It does fluctuate some even when the NUVI is stationary. I'm getting a stated tolerance of around 15 ft for the spot location here.
Is there any reason to turn off the WAAS except to save battery power?
Joined: 24 Mar 2007 Posts: 54 Location: Ontario, Canada
Sun Sep 16, 2007 3:19 pm
Yap, great stuff Tim. I've definitely learned things here I didn't have precisely formulated in my mind. Thanks for the altitude vs. elevation distinction as well, quite an important one in this conversation.
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. Even with WAAS, I wouldn't expect great altitude accuracy from an auto navigation unit using GPS, it's just not a priority for this unit so I would expect little effort to have gone into that part of the design. There are some Garmin handhelds (eTrex Vista series) that feature an atmospheric pressure-based altimeter and I suspect avionic units are designed for altitude accuracy, but not nuvis or streetpilots). Point is, don't get your expectations too high.