ClubTread Community
Register | Active Topics | Top 10 | Search | Guidelines | Report Spam
Username:
Password:
  Login   Donate
Support ClubTread
  Trail Wiki
Save Password
Forgot your Password?

 All Forums
 Group Discussion
 Maps, GPS, Orienteering
 negative elevation? (GPS question)
Bookmark and Share     Reply to Topic
Author Topic  

canuckgurl
Senior Member


surrey, bc
Canada

1389 Posts

 Posted - 05/23/2008 :  09:47 AM  Show Profile  Reply to this posting
hi.. just got a new gps.. was at the beach and reclibrated it.. right at the oceans edge in tswn it read -4 elevation.. whats with that? i also tried it at 2nd beach.. can't remember what it read there, but it was also in the negative..

once i get it all figured out, i'm sure it's gonna be so cool! ;)
ClubTread Supporter

OK Jack
Advanced Member

Fungi Filmin', Wine Drinkin', 'Shroom Eatin', Early Risin', Deer Whisperin', Curry Cookin', Macro Maniac

Chilliwack + Osoyoos
3657 Posts

 Posted - 05/23/2008 :  10:01 AM  Show Profile  Reply with Quote
Did it get wet ???

Were you underwater ???

Are you trying to trick us ???

canuckgurl
Senior Member


surrey, bc
Canada

1389 Posts

 Posted - 05/23/2008 :  10:03 AM  Show Profile  Reply with Quote
hehe! no.. not at all.. trust me, i'm confused.. its a Garmin Vista HCX.. there must be a reason..???

magnetite
Senior Member


North Vancouver, BC
Canada

1577 Posts

 Posted - 05/23/2008 :  10:16 AM  Show Profile  Reply with Quote
A four metre difference is really nothing to be concerned about. In fact, it's quite low and tells me that your GPS is doing a fine job.

GPS units can measure elevation in two different ways. Method one involves calculating the elevation based on the satellite positions. This method can result in a 10 metre error or more, so it's no surprise that the elevation reading was -4.
The second, and good old fashion method involves an internal barometer which calculates the elevation from atmospheric pressure.

Both methods have their advantages. The elevation calculated in method one is not subject to change due to a change in the weather, but it will tend to bounce around by as much as 15 to 20 metres within a few mnutes. The elevation calculated in method two will not tend to bounce around much, but it can change significantly in the same place over time due to weather changes, by many tens of metres. Better GPS units use satellites to automatically calibrate an internal barometre, based on an average elevation value.

Another thing to consider is that "sea level" does not actually refer to the water level. I know, that sounds strange. Sea level is a theoretical number derived from a calculated shape of the earth, and is fixed for a given datum. Think of it this way - the water level changes continuosly due to tides, but the elevation above sea level for any point on the earth does not change. So, you could be standing right at the water line at low tide, and your elevation would be below sea level. Make sense? In some minds it does.

Edited by - magnetite on 05/23/2008 10:17 AM
ClubTread Supporter

Dru
Mountain Grammar Police

Sardonic sandbagging scoundrel, Cascade Climbers lobotomized spraymeister, space blanket flyer, new millennium vulgarian betaboy and friend to all squids

Climbing, a mountain
Canada

∞ Posts

 Posted - 05/23/2008 :  10:28 AM  Show Profile  Reply with Quote
a glitch in the matrix

canuckgurl
Senior Member


surrey, bc
Canada

1389 Posts

 Posted - 05/23/2008 :  10:39 AM  Show Profile  Reply with Quote
THANK YOU! i knew there had to be some sorta logical reason.. it does kinda make sense.. it did ask me if i knew the correct elevation and pressure and if i wanted to use the current gps altitude ..

another thing i found weird..

i turned it on cuz i wanted to see how much elevation i did when going up and down some stairs and just to test 'er out..

i recorded the Total Ascents when i reached the top, as follows:



36 m
69 m
102 m
124 m
156 m
190 m

it was pretty consistent but i shut it off just to see what happened and sure enuf, the total ascent was less (in between 102 and 124)..

so, i was wondering.. do people keep their gps's on at all times?

magnetite
Senior Member


North Vancouver, BC
Canada

1577 Posts

 Posted - 05/23/2008 :  10:48 AM  Show Profile  Reply with Quote
You need to leave the GPS on if you want to map your hike as a GPS track or if you want to record your elevation profile over the duration of the hike. If you just want to find out where you are, you only need to turn the unit on at that point. Get some good recahrgeable NiMH batteries. Otherwise, you'll be spending $5/day on non-rechargeables.

noeldodd
Junior Member


Calgary, AB
Canada

269 Posts

 Posted - 05/23/2008 :  11:11 AM  Show Profile  Reply with Quote
quote:
Originally posted by canuckgurl


so, i was wondering.. do people keep their gps's on at all times? [/blue]



On, for grabbing a track log of new treks, getting peak altitudes, etc. I'm a stats hound.

Usually on, if I'm testing a new 'shortcut' on a familiar route. It's usually not really shorter, I find.

But for trips I've done before it's usually off. Sometimes I'll load the old tracklog just for comparison sake, but usually I'll leave it off until I ask questions like 'did I take the left or right fork last time?'... then I leave it on for a bit.

It's tough to get lost if you have distinct trails and visible landmarks, but that's assuming an ideal trip. Throw in a busted ankle, bush, and darkness, and having a GPS might make for some better decisions, or show you some options. So when battery life is a concern, I leave mine off.

Lastly, my wife seems to think it's a crutch, and prefers a paper map. But she doesn't quibble about knowing exactly how far away the car is when we are heading home, so it's usually on for the last few km's!
ClubTread Supporter

OK Jack
Advanced Member

Fungi Filmin', Wine Drinkin', 'Shroom Eatin', Early Risin', Deer Whisperin', Curry Cookin', Macro Maniac

Chilliwack + Osoyoos
3657 Posts

 Posted - 05/23/2008 :  12:04 PM  Show Profile  Reply with Quote
Yup... Leave it "on", Root... (I use rechargeable batteries)...

I used mine yesterday while running around my new 10 acre place in Osoyoos, and by following the "trail" it leaves on the screen, I could find my way back to exactly the same rock I found eariler...

With it set at NADS83 datum, it matches exactly the survey plans and the Google Earth info...

Rick...

guntis
Senior Member


Smurf Village, BC
Canada

1770 Posts

 Posted - 05/23/2008 :  12:10 PM  Show Profile  Reply with Quote
OK Jack, is NADS83 the current standard of choice? Personally, I find it easier to wrap my head around the old fashioned Latitude-Longitude format, but I can see how it can be cumbersome.

...oh, just found this thread...
http://www.clubtread.com/sforum/topic.asp?TOPIC_ID=26066

Edited by - guntis on 05/23/2008 12:12 PM

swebster
Senior Member


Vancouver, BC
Canada

1357 Posts

 Posted - 05/23/2008 :  1:07 PM  Show Profile  Reply with Quote
quote:
Originally posted by guntis

OK Jack, is NADS83 the current standard of choice? Personally, I find it easier to wrap my head around the old fashioned Latitude-Longitude format, but I can see how it can be cumbersome.

...oh, just found this thread...
http://www.clubtread.com/sforum/topic.asp?TOPIC_ID=26066



NAD83 is the datum, not the coordinate system. You could specify a location in lat/long (NAD83) or UTM (NAD83).

ShadowChaser
GPS Geek

Trail cuttin, GPS packin bushwhacker, wiki hike compilin, who is now Hope-less


2567 Posts

 Posted - 05/23/2008 :  1:21 PM  Show Profile  Reply with Quote
quote:
Originally posted by OK Jack
With it set at NADS83 datum, it matches exactly the survey plans and the Google Earth info...



Google Earth uses WGS 84, not NAD 83.

quote:

As Google Earth currently uses WGS84 datum projection, GIS files using NAD83 projection are not supported by Google Earth

magnetite
Senior Member


North Vancouver, BC
Canada

1577 Posts

 Posted - 05/23/2008 :  1:41 PM  Show Profile  Reply with Quote
quote:
Originally posted by ShadowChaser

quote:
Originally posted by OK Jack
With it set at NADS83 datum, it matches exactly the survey plans and the Google Earth info...



Google Earth uses WGS 84, not NAD 83.

quote:

As Google Earth currently uses WGS84 datum projection, GIS files using NAD83 projection are not supported by Google Earth




Wrong, on several points. There is no such thing as a "datum projection". Datum and projection are separate things. Google Earth uses a cylindrical projection, and WGS84 datum. There is no such thing as a WGS84 projection.
The difference between NAD83 and WGS84 is approximately 50 centimetres. Considering that GPS error is typically 5 metres or higher, you can consider NAD83 and WGS84 to be the same. So, OK Jack is correct; setting your GPS to NAD83 will result in a match with Google Earth. MapSource will upload data into Google earth in WGS84 regardless of the datum you set your GPS to. In fact, regardless of the datum you used, it's always stored in WGS84 anyway.

Edited by - magnetite on 05/23/2008 1:45 PM
ClubTread Supporter

OK Jack
Advanced Member

Fungi Filmin', Wine Drinkin', 'Shroom Eatin', Early Risin', Deer Whisperin', Curry Cookin', Macro Maniac

Chilliwack + Osoyoos
3657 Posts

 Posted - 05/23/2008 :  1:41 PM  Show Profile  Reply with Quote
OK... let me rephrase...

My GPS (an older Garmin 12) is set at NAD83... (as advised by Dr. Dru and Mr. Magnetite a while back, when I was trekking out Tamihi way and finding that the 49 th parallel ain't on the 49th !!! Now it is !!!)...

I like to set the "units" at Deg, Mins and Sec (decimilized), however, at the flip of a switch, I can get it in UTP etc, etc...
So yesterday, I took a bunch of (obvious) readings out at my place, like the corner posts, centre of roadway etc... And when I go to the RDOS map site (where the property survey plans are, and who use UTP coordinates) the readings are EXACTLY what they should be... Then when I go to GE, and "float" the mouse over the exact same corner post, I see it as the same, within the thickness of a mouse hair... LOL !!!

So I was probably wrong re GE's datum basis, but I'm happy that my GPS readings are within a millimeter (on my 19" screen) of where they should be...

Sorry about any confoosion...

OK'Jack...

canuckgurl
Senior Member


surrey, bc
Canada

1389 Posts

 Posted - 05/23/2008 :  2:12 PM  Show Profile  Reply with Quote
okay.. and i thought i was confused earlier, now i'm really really confused.. haha! but its okay.. i'll just do what i can, i'm just havin' fun with the basics right now.. heck, it even has games! ;)

thanks!

pdops
New Member


Hinton, Alberta
74 Posts

 Posted - 05/29/2008 :  08:24 AM  Show Profile  Reply with Quote
More on datums at http://www.gpsinformation.org/dale/measure.htm#datums

quote:
Translations among the various datums can be a cause of some errors. Garmin used the the Molodensky transform parameters for those datums and performs transformations as needed. This is a simplified model and can result in errors on the order of 10 meters in some cases. With this transform the WGS-84 and NAD-83 numbers are always the same while a better translation will show a slight difference in these two datums. For most of the US this difference is less than a meter rising to about 2 meters on the west coast. Still less difference than the accuracy of the unit in most cases. Modeling something as complicated as the earth does not lend itself to mathematical transformations so to achieve accurate results some information is contained in tables with model information used to access the entry in the table which is then interpolated to get the answer. It is not known where Garmin depends on models alone and where they supplement with tables.
  Topic  
 All Forums > Group Discussion > Maps, GPS, Orienteering Bookmark and Share     Reply to Topic

Register | Active Topics | Top 10 | Search | Guidelines | Report Spam