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GPS Elevation Readings

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jimlc(at)att.net
Guest

Posted: Wed Mar 15, 2006 10:11 am Post subject: GPS Elevation Readings

I have a Garmin that is WAAS enabled.  I may not be spelling that 

correctly.  It is remarkably accurate.  It will consistently show my 

sail boat in my slip.  However, it will also show me to be about 50' 

above sea level while on the sea in Hawaii.  The tides are very small 

in Hawaii, so that is not the cause.  It must be the earth 

model.  Also, I remove it and use it in my motor home on the 

mainland, and whenever I cross a pass where an elevation sign is 

provided, I will be right on.  That 50' elevation error in Hawaii is 

the greatest error I ever remember noting.  This is over a three year period.



Jim Crowder

At 10:25 AM 3/15/2006, you wrote:

 	  Quote: 	
		  Michel is right about GPS altitude errors, even with selective

availability turned off.  GPS altitude is based on a computer model

of the earth's shape, not your actual distance from the dirt, and can

be off considerably. Long-term testing of a GPS unit at a particular

location will not reveal errors in the earth-model.



Although GPS is a 3-dimensional positioning system, the altitude

component does not have the same accuracy that lateral position has.



Mike G.

N728KF

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Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Wed Mar 15, 2006 10:37 am Post subject: GPS Elevation Readings

On Mar 15, 2006, at 7:10 PM, Jim Crowder wrote:



 	  Quote: 	
		   That 50' elevation error in Hawaii is the greatest error I ever 

 remember noting.


	


That is an error in the geoidal height (a.k.a geoidal separation), Jim, 

i.e. the difference between the ellipsoid the calculation is made on, 

and the correction for the actual elevation of the MSL. Compared to the 

ellipsoid, the sea level varies all around the world. Your GPS has a 

worldwide correction look-up table that is coarse to accommodate the 

memory chips it is stored on. Say that the table gives a correction by 

one by one degree of latitude and longitude, within these 60 NM wide 

tiles, the geoidal height may vary as much as the 50 feet you are 

talking about.



Cheers,

Michel

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pwmac(at)sisna.com
Guest

Posted: Wed Mar 15, 2006 4:15 pm Post subject: GPS Elevation Readings

Michel & Jim,

Here is what I understand about GPS accuracy. Please correct me if I have 

it muddled.

What Michel said is correct, You get different altitude accuracy depending 

on where on earth you are located. But take note of the impact on the 

difference in satellite location. For example for an altitude calculation 

if one is in a location where the horizon is not visible - say in a canyon 

- then the angle between the satellites is less so the accuracy is not as 

good as it would be if the visible satellites would be on opposite 

horizons. However, with regard to X-Y calculations overhead satellites give 

adequate accuracy. In boats and planes the horizon is  visible (with regard 

to the RF transmission) so this is much less a factor and the accuracy is 

mostly dependent on the distance of the satellite above the horizon and 

whether WAAS satellites are visible. This explains why the altitude varies 

while you have not changed your physical altitude - like when you are 

parked or driving along slowly on a flat road. BTW I get only a 10'-20' 

elevation error on the northern part of the sea of Cortez.

  WAAS (Wide Area Augmentation System) involves additional satellites 

funded by the US government and is used to enhance coordinates including 

elevation. This enhancement as always depends on where these specialized 

satellites are located in the sky. I get WAAS accuracy even here in Baja. 

Of course WAAS  is not complete so the availability is intermittent. Your 

GPS should have a setting to give you an alarm when WAAS is not available.

One more thing in case you want to compare GPS readings with a map is that 

you must set your GPS to have the correct map datum which should be "NAD27 

CONUS" for the US & Mexico to match our topos.The default for my two 

different brands of GPS is "WGS84"="NAD83".  Want to see how much of a 

change is there between the two datum just switch back and forth and see 

the result. I noticed that one has to wait longer to get accurate altitude 

readings than to get X-Y readings - so be patient. This is especially 

noticeable when sitting on a known bench mark on top of a mountain. Some 

times I have waited 10-15 minutes before I got the GPS to agree with the 

real elevation. Once more I am sure is has to do with the location of the 

satellites as well as a slower calculation algorithm.

Note for accurate altitude the GPS antenna has to be able to see both 

horizons. So in a boat, plane or auto does the GPS have to look thru 

structure or humans? Is your antenna above all things that can affect the 

horizontal view? Does your GPS even have a remote antenna to allow good or 

better placement?

What all this means to a pilot is you had better plan on at least a 50' 

error for altitude.

Jim, if you don't like the altitude error where you are located, just put a 

correction into your GPS. Works just fine until you get to a different look 

up location as Michel explained. That GPS feature is designed for just that 

purpose.

         Regards, Paul

==================

At 10:34 AM 3/15/2006, you wrote:

 	  Quote: 	

On Mar 15, 2006, at 7:10 PM, Jim Crowder wrote:

 > That 50' elevation error in Hawaii is the greatest error I ever

 > remember noting.

That is an error in the geoidal height (a.k.a geoidal separation), Jim,

i.e. the difference between the ellipsoid the calculation is made on,

and the correction for the actual elevation of the MSL. Compared to the

ellipsoid, the sea level varies all around the world. Your GPS has a

worldwide correction look-up table that is coarse to accommodate the

memory chips it is stored on. Say that the table gives a correction by

one by one degree of latitude and longitude, within these 60 NM wide

tiles, the geoidal height may vary as much as the 50 feet you are

talking about.

Cheers,

Michel

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jimlc(at)att.net
Guest

Posted: Wed Mar 15, 2006 6:18 pm Post subject: GPS Elevation Readings

The error is no big deal as on the boat I know what my elevation 

is.  When I use it in my motor home, I have never detected a 

noticeable error, not that I don't think there has not been 

one.  It's better than any altimeter I have ever used.  My previous 

GPSs were not that accurate.  The Garmin I am talking about updates 

very quickly when it can see the sky.  I updates so quickly, as I 

turn the motor home the course up screen rotates with the turn.



Jim Crowder



At 05:13 PM 3/15/2006, you wrote:

 	  Quote: 	
		  Jim, if you don't like the altitude error where you are located, just put a

correction into your GPS. Works just fine until you get to a different look

up location as Michel explained. That GPS feature is designed for just that

purpose.

          Regards, Paul

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MichaelGibbs(at)cox.net
Guest

Posted: Thu Mar 16, 2006 8:54 am Post subject: GPS Elevation Readings

Paul sez:



 	  Quote: 	
		  WAAS (Wide Area Augmentation System) involves additional satellites 

funded by the US government and is used to enhance coordinates 

including elevation. This enhancement as always depends on where 

these specialized satellites are located in the sky.


	


These "additional satellites" are actually on the ground!  The 

transmission picked up by your GPS receiver does come from the 

satellites, of course.  From the Garmin web site:



 	  Quote: 	
		  How it Works



WAAS consists of approximately 25 ground reference stations 

positioned across the United States that monitor GPS satellite data. 

Two master stations, located on either coast, collect data from the 

reference stations and create a GPS correction message. This 

correction accounts for GPS satellite orbit and clock drift plus 

signal delays caused by the atmosphere and ionosphere. The corrected 

differential message is then broadcast through one of two 

geostationary satellites, or satellites with a fixed position over 

the equator. The information is compatible with the basic GPS signal 

structure, which means any WAAS-enabled GPS receiver can read the 

signal.



Who benefits from WAAS?



Currently, WAAS satellite coverage is only available in North 

America. There are no ground reference stations in South America, so 

even though GPS users there can receive WAAS, the signal has not 

been corrected and thus would not improve the accuracy of their 

unit. For some users in the U.S., the position of the satellites 

over the equator makes it difficult to receive the signals when 

trees or mountains obstruct the view of the horizon. WAAS signal 

reception is ideal for open land and marine applications. WAAS 

provides extended coverage both inland and offshore compared to the 

land-based DGPS (differential GPS) system. Another benefit of WAAS 

is that it does not require additional receiving equipment, while 

DGPS does.

Other governments are developing similar satellite-based 

differential systems. In Asia, it's the Japanese Multi-Functional 

Satellite Augmentation System (MSAS), while Europe has the Euro 

Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS 

users around the world will have access to precise position data 

using these and other compatible systems.


	


Mike G.

N728KF

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JeffFowler(at)aol.com
Guest

Posted: Thu Mar 16, 2006 11:09 am Post subject: GPS Elevation Readings

Hello folks,

 

  I've been reading this GPS string with interest since it started and  

there's an error question that has come up. I'd like to inform you of a couple  of 

things I've viewed on TV about it.

 

1. They are measuring the shift of the Earth's plates with it for  earthquake 

studies. This is measuring much less change than 10 meters, and that  is also 

in altitude as the earth's plates rise and fall at their edges and this  is a 

factor they look at.

 

2. Did you know they make a remote control for R/C airplanes that can be  

programed using GPS to fly a path? this thing uses altitude too.

 

Just some things I happened on for you to think about in regard to this  

thread.

 

 

Jeff Fowler

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temco(at)telusplanet.net
Guest

Posted: Thu Mar 16, 2006 12:26 pm Post subject: GPS Elevation Readings

Mike 



Mike your statement -- (These "additional satellite" are actually on the

ground!)  -- is probably confusing for some. A better clarification may be

that, the GPS system is monitored by 25 ground systems. The information

gleamed on the ground is then forwarded to two synchronous satellites which

in turn with the ground monitor stations create a correction message. Your

WAAS enabled GPS then receives and applies this correction factor to the

data it receives from the orbiting constellation of GPS satellites. Ergo,

now you have very precise nav info in areas where there are ground monitors

and coverage by the two WAAS synchronous satellites.



Ted Palamarek

Edmonton, Ab

                ________________SNIP----------------------



These "additional satellites" are actually on the ground!  The 

transmission picked up by your GPS receiver does come from the 

satellites, of course.  From the Garmin web site:



 	  Quote: 	
		  How it Works



WAAS consists of approximately 25 ground reference stations 

positioned across the United States that monitor GPS satellite data. 

Two master stations, located on either coast, collect data from the 

reference stations and create a GPS correction message. This 

correction accounts for GPS satellite orbit and clock drift plus 

signal delays caused by the atmosphere and ionosphere. The corrected 

differential message is then broadcast through one of two 

geostationary satellites, or satellites with a fixed position over 

the equator. The information is compatible with the basic GPS signal 

structure, which means any WAAS-enabled GPS receiver can read the 

signal.

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