[paulshikleejr]

Wow! I had no idea there was so much difference between the USGS DEM elevation data and SRTM elevation data.

Further, from the looks of it, SRTM data can be very off (given that USGS DEM data seems to better pass the "smell test" for the example hereafter).

This issue was thrust to the fore when I was doing my triplog for "Pat's Run 2012," the "all-in" version. Although the SRTM data looked weird for the run's route proper (the 4.2 mile course), it really seemed off for the all-in version.

To see what I mean, go to http://hikearizona.com/map.php?GPS=11909 and look at the profile in Route Manager. Then "ReSync SRTM" and look at the profile again. Quite different.

What tipped me off to this problem is that I don't think there's any way I did 921 feet of AEG that morning during the run or just wandering around before and after the run.

So I took the time to sync the track to USGS DEM data and got results that make much more sense.

Does the magnitude of this problem vary from area to area (with Tempe Town Lake being a particularly bad area) or is it like this pretty much all over?

Also, which dataset is really better for our purposes? Is it possible that I, in fact, did do an AEG of 921 feet that Saturday morning?

[joe bartels]

key points

SRTM - was acquired real fast via echos, not accurate in canyons or by any interference, references are 295 feet
USGS - standard is 1/3 arc which translates to reference points 30 feet apart
A more accurate 1/9 arc data set is underway but years from completion, which is 10 foot references.

Regardless of method or accuracy*, too much data compounds error rate. 50 to 100 points of data per mile is preferred imo.

* in terms of what is available, 100% is not

[Grasshopper]

Regardless of method or accuracy*, too much data compounds error rate. 50 to 100 points of data per mile is preferred imo.

I definitely agree with this statement and have confirmed it to be true on my last ~12 months, posted HAZ GPS Routes where I now prefer to have my GPS Receiver (an older technology Garmin 60CSx) lay down the maximum number of track points allowed (the units "most often" option).. which definitely equates to more than 50 to 100 points of data per mile. Even though there is a tradeoff between the AEG calculation (being incorrect) vs the extra number of track point per mile, I'm finding that I do prefer this "most often" points option due to the type of hikes I have been doing lately where there are more complicated off trail route finding scenarios with sometimes numerous off trail turns, ups/downs, etc.. (Right/correct or wrong/incorrect, I have been working under the conclusion that when later needing to follow a complicated route like some of these, that it is less complicated to correctly follow when having the additional track points).

[paulshikleejr]

The question of USGS DEM and SRTM Elevation data will soon be (if it's not already) moot.

Please see the following post for more info: http://hikearizona.com/dex2/viewtopic.php?f=6&t=5370&p=87427&hilit=USGS+DEM#p87427.