Is there an Altitude readout on GPS?

[deutschra]

Does anyone know how to find the Altitude reading anywhere on the GSP display. I have looked in every corner of the display and cannot find it nor is it in the SYNC manual.

[Buster1]

Does anyone know how to find the Altitude reading anywhere on the GSP display. I have looked in every corner of the display and cannot find it nor is it in the SYNC manual.

 

I am uncertain, as I don't have the built in GPS. But altitude is uncommon in Vehicle GPS's. Some Garmin/Magellan/etc vehicle GPSs however, have that feature. But not all.

[VonoreTn]

Does anyone know how to find the Altitude reading anywhere on the GSP display. I have looked in every corner of the display and cannot find it nor is it in the SYNC manual.

You hit the middle button on the top row, the one labeled "i", for information. Under the first screen "where am I", it is a little red number to the right below the city and state, giving feet above sea level. I wish it was bigger, because I like to monitor it as I drive. It gives a new dimension to driving.

 

Also on the same line to the left is the exact GPS longitude and latitude coordinates, something useful to emergency if you are ever in an accident. A helicopter could find you within feet with that information, even in the middle of a wilderness.

[Tazzboo]

From what I have gathered. It was a change that they did on the fly to the new models/builds/jobs. I have a earlier build and mine does not have the altitude on it. However Oldschool did and his was a little newer than mine. If you find it it will be on the information tab under there. But I could be wrong on that to since I don't have it on mine. Oldschool were is it again?

 

Tazzboo :shift:

[rpoffen]

From what I have gathered. It was a change that they did on the fly to the new models/builds/jobs. I have a earlier build and mine does not have the altitude on it. However Oldschool did and his was a little newer than mine. If you find it it will be on the information tab under there. But I could be wrong on that to since I don't have it on mine. Oldschool were is it again?

 

Tazzboo :shift:

 

Keep in mind that the altitude display from GPS is not very accurate, it could be a few hundred feet off. The more satellites it can "see" improves the accuracy some.

[Buster1]

Keep in mind that the altitude display from GPS is not very accurate, it could be a few hundred feet off. The more satellites it can "see" improves the accuracy some.

 

True, and if you want to get super geeky, the GPS satellite constelation is a sphere, and the earth is not a perfect sphere.

[twiggy144]

True, and if you want to get super geeky, the GPS satellite constelation is a sphere, and the earth is not a perfect sphere.

 

 

My portable Garmin GPS has a precision of +/- 4 meters. I believe this precision is in all 3 dimensions, obviously including altitude. I cant see why built in GPS's would not have the same precision.

Edited August 12, 2010 by twiggy144

[lolder]

My portable Garmin GPS has a precision of +/- 4 meters. I believe this precision is in all 3 dimensions, obviously including altitude. I cant see why built in GPS's would not have the same precision.

They should but many don't.

[Buster1]

All GPS's have a horizontal position for +- 3 meters or so. But all have a vertical error of 'hundreds' of feet. We deal with this daily in the pilot world/military application...I'll see if I can dig up some info on that phenomenon.

[Waldo]

Pretty sure the altitude was part of the Job2 update. If you have map version 2.0 from the factory, you should have altitude. Also included in that update are nearby weather, the scale "bar" and a couple other minor updates.

[twiggy144]

All GPS's have a horizontal position for +- 3 meters or so. But all have a vertical error of 'hundreds' of feet. We deal with this daily in the pilot world/military application...I'll see if I can dig up some info on that phenomenon.

 

Strange. The last couple of road trips when I was on a landmark or a scenic road with a sign stating the altitude, my GPS was almost dead on.

[Buster1]

Here's some technical stuff...but the Bottom line is that GPS altitude/elevation readouts CAN be very far off.

 

Buster

 

GPS, because of the math, works from the smooth WGS84 ellipsoid directly. The height it offers, unless it tells you otherwise, will be an ellipsoid height (distance from the Earth's centroid of mass to the surface of the ellipsoid) as determined by the functions of trilateration and free-space ranging. However, I think that most user grade receivers have a built in geoid model and perform the adjustment. You may be able to tell by looking at the $GPGGA NMEA sentence -- see NMEA protocol information for more information. This NMEA sentence (sometimes) reports the difference between the geoid and the ellipsoid at the current location.

If, on the other hand, one were to use the multi-frequency, geodetic quality GPS systems, one only uses the autonomous measurements to get into the ballpark for timing and position, then uses a network between multiple receivers located on well-surveyed sites where the cartesian coordinates are well-known (at least for the purposed of a given survey). Then, once again, one calculates the cartesian coordinates of all receivers in the survey, including the unknown point, creates a fairly rigid geometric network among the various points, and performs a least-squares adjustment to determine the parameters of x, y, and z for the point of interest. Beyond that, one generally transforms the coordinates from cartesian to one of the more conventional forms: geographic, state plane, transverse mercator, etc., for textual and graphical dissemination.

 

GPS determination of height using a commercial, consumer grade GPS is problematical for several reasons.

 

3d positions are calculated using the code-phase method, where one uses the pseudorange between satellite(s) and receiver at a given epoch to determine a position. If 4 satellites are in view, the procedure is straightforward. If more than 4 are in view, then most receivers "over-determine" the position using all combinations of 4 satellites from those in view to establish positions, and then perform yet-another-least-squares-solution to attempt to determine which satellite combination is best. This combination is tested periodically to make things better, but usually not tested each and every solution.

 

Using code-phase, however, allows for a larger error budget which contributes to the overall error of autonomous positioning. In general, to get good 3d positioning, one would want a satellite directly over head and 3 others below the horizon in a constellation similar to the structure of a statically-depicted carbon molecule. All 4 hydrogen bonds are at ~120 deg to each adjacent bond. Since GPS signals don't traverse dirt too well, that's impractical for surface-based receivers.

 

Since most satellites used in conventional consumer-grade hardware are selected for being somewhat above the horizon, accuracy of vertical determination suffers.

 

We've consistently seen horizontal accuracies of 6m or better ever since Selective Availability was switched off in 2000. However vertical accuracies of 10-20 meters are not uncommon because of A) the problem cited about constellation selection being inconsistent for vertical determinations... and B) the problem of using the increased error-budget laden code-phase solutions.

[lolder]

...... but then maybe not.

[Scott A320]

Many of the A-320 Airbus variants I fly are GPS equipped, but the data is never used to establish altitude. Rather, the signal is integrated with the triple inertial reference units and VOR navigation station distances to define aircraft position for navigation and terrain avoidance. Consequently, the aircraft can fly GPS instrument approaches in poor weather conditions to airports wholly lacking navigation facilities. The only method to determine GPS altitude is by accessing several "pages" deep into the flight management computer display. In doing so I've noticed it significantly differs from the barometric altimeters, which are extremely accurate. We can now fly above FL290 (29,000') with a mere 1000' vertical separation from other aircraft.

 

My FFH built in 5/2009 lacks the altitude display in the "information" window.

However, it can be accessed by the following: Ignition switch on, shifter in "park", press the 3 and 6 radio buttons simultaneously.

A speaker test-mode appears in the NAV display. Press the cancel button, and a complex test menu appears.

GPS is at the bottom of the first column. Press the GPS button to display the number of satellites observed, lat/long, and altitude!

Have fun playing with the other test features! BTW, it irritates me to no end mfgrs overlook these details in the manuals.

Edited August 15, 2010 by Scott A320