scroggin wrote:Edit - does the GPS require an altimeter to generate an elevation profile or does it do it based on the contours? Is it just a feature of the GPS/maps you use?
The GPS receiver takes the elevation from GPS, based on the reference ellipsoid model of the sea level (not the actual sea level), not from the map contours as such.
For example, this is the graph showing the geoid vs the ellipsoid model at the equator. The blue line is the Geoid elevation, and the green line is the Ellipsoid model elevation.
(https://www.unavco.org/education/resour ... ivers.html). What this means is that the GPS receiver is calculating the elevation from data received from a minimum of four satellites and the reference ellipsoid, using the reference table built in to the receiver, and not the actual sea level.
So if you have a model without a barometric altimeter, that is how it calculates the elevation. If you had one of these GPS devices, and a map with contours, and you are standing at a contour line as shown on the GPS, the maps contour line can be more accurate elevation than the displayed elevation calcuated by the GPS receiver.
The barometric altimeter, once calibrated can give a more precise elevation when compared to a GPS only derived elevation - Garmin's own website states that the GPS satellite heights can be off from map elevations by +/-400 feet, however with units with a barometric altimeter (with proper calibration) the accuracy is down to 10 feet. The tricky thing is that it will need proper calibration, which can be frequent if the weather is changing as you ascend/descend. It will be important to manually calibrate the elevation with known elevation points from maps or trail markers. The models with baro-altimeters do have an auto calibrate function, but I've noticed that it can take up to 2 hours to adjust after inputting an incorrect elevation point.
So even if you don't have a map installed on the device, the GPS will still show you an elevation calculated, and will record an elevation profile based on that.
