Altitude

[Jospee]

Updated to 1.5 (very nice) but wiped my settings back to default...

So, not being able to remember how they were i'm try to reset them, but for the life of me can't recall which is best for Altitude, from GPS or from Map???

Can anyone help?, used for walking (if that makes a difference)

Cheers, J.

[Bosun]

I think it depends on who you ask.

I have mine set to "Maps" as I presumed the altitude data from the OS was pretty good.Then I read somewhere that GPS was best.

Left it at "Maps" though.

Ian

[LostMike]

I have mine set to GPS as this gives the calculated height where you are rather than the average height for the tile you are in. If you want to see the difference all you have to do is move the joystick and it tells you what the map height of the cursor point is.

So for me having it set to GPS seems to give me the best of both worlds.

I remain hesitant about the calculations of total height ascended but it is much better than it was before 1.5.
_________________
LostMike
Satmap A10. Platform 21
Software version 1.5.9193
Satsync 1.525

[Bosun]

[Jospee]

GPS it is. Thanks guys

[FrequentFlyer]

[LostMike]

FrequentFlyer asked

[lucevans]

I used to use "map" as the source of height data on the assumption that this would give a more accurate cumulative record of ascent and descent over the course of a day's walk. My rationale for this was: the GPS-derived height is subject to error, which accumulates the longer you record it while changing your altitude when walking in the hills, but in theory the map-derived height is not subject to this error, because it is based on fixed survey data.
However, when using the "map" setting, at the end of a day's walk the total ascent and descent figure on the Satmap was often quite obviously wrong. I tried switching to "GPS" as the source of height data, and the figures that resulted looked more like what I had actually done that day, despite the unavoidable imprecision of data gathered by that method.
The only explanation I could come up with was that the horizontal resolution of the height data embedded in the maps is quite low (is it 100m squares?) and for a low-speed, long-duration activity like hill walking, this would result in a rather crude altitude profile.
To be honest, I'm rather puzzled by the whole thing!

[ChrisJakarta]

Perhaps of interest that the latest manual (p 31) notes:

"GPS elevation data is also available and is accurate to ± 50m."

Sounds like map-based elevation should be more accurate. That said, having used the unit set to GPS elevation here in Indonesia where there are no maps, on round trips I don't see huge differences in total ascent and total descent.

Chris

[davidor]

I'm surprised they quote a GPS accuracy of + or - 50m as a gps, whilst not particularly accurate on heights, should be a lot better than that. I think the map vs GPS depends a lot on the terrain. If the ground is gently rolling then the map is probably quite good, but if the terrain varies greatly, i.e. cliffs, ravines, steep inclines etc, you can be walking along the bottom of say a cliff yet pick up the height from the map at the top. If the spaicing of the height points is 100m, the actual height can vary a lot in that distance. I've walked a couple of miles on the flat before now and it has told me I've ascended over 2000ft!

[FrequentFlyer]

[lucevans]

FrequentFlyer,

I think Garmins use GPS altitude (I'm pretty sure my old 60CSx does) but Garmin seem to have a much better algorithm for smoothing out the "bumps" in GPS height data collection. I'm not sure how that would work, but maybe something along the lines of "if horizontal speed is less than 5mph and change in altitude is greater than 10 metres/second then do not add change of altitude to ascent/descent total -because it's not a realistic change in height - i.e. it must be a GPS error"

[djhpk]

I have been over this issue of using map v gps for altitude information many times.

Despite what it has said in various issues of the user guide, altitude from map is the average of the altitudes at the corners of a 200x200 metre grid square.

If you are in terrain where the altitude does not change much over over that sort of distance, altitude from map can be quite accurate and so can the calculated ascent and descent.

However, if you are in steeper terrain the picture can be quite different and I have seen major errors when using altitude from map and so generally use altitude from gps.

Near to my house, there is a summit with 360 degree views and a spot height of 370 metres. Altitude from map gives this as 339 metres whereas on many visits, the error using altitude from gps has never exceeded 5 metres!

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