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-   -   negative elevation? (GPS question) (https://forums.clubtread.com/33-maps-gps-orienteering/25352-negative-elevation-gps-question.html)

canuckgurl 05-23-2008 10:47 AM

negative elevation? (GPS question)
 
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! ;)

OK Jack 05-23-2008 11:01 AM

Did it get wet ???

Were you underwater ???

Are you trying to trick us ???

canuckgurl 05-23-2008 11:03 AM

hehe! no.. not at all.. trust me, i'm confused.. its a Garmin Vista HCX.. there must be a reason..???

magnetite 05-23-2008 11:16 AM

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.

Dru 05-23-2008 11:28 AM

a glitch in the matrix ;)

canuckgurl 05-23-2008 11:39 AM

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 05-23-2008 11:48 AM

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 05-23-2008 12:11 PM

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. [:I]

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! [:p]

OK Jack 05-23-2008 01:04 PM

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 05-23-2008 01:10 PM

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...
https://www.clubtread.com/sforum/topi...TOPIC_ID=26066

swebster 05-23-2008 02:07 PM

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...
https://www.clubtread.com/sforum/topi...TOPIC_ID=26066
NAD83 is the datum, not the coordinate system. You could specify a location in lat/long (NAD83) or UTM (NAD83).

ShadowChaser 05-23-2008 02:21 PM

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:

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

magnetite 05-23-2008 02:41 PM

Quote:

quote:Originally posted by ShadowChaser

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:

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.

OK Jack 05-23-2008 02:41 PM

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 05-23-2008 03:12 PM

[xx(][xx(] 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! :)


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