Jump to content
Sign in to follow this  
Followers 0
solohiker

Calibrating GPS with NGS bench mark

Recommended Posts

I went to the website below and did a search on GPS benchmarks. I found one at the local airport that has been certified horizontally and vertically. I compared the readings of my Garmin 12 with the coordinates of the bench mark. My reading was off by 4 meters.

 

http://www.ngs.noaa.gov/datasheet.html

Share this post


Link to post

solohiker -

 

Which measurement was off by 4 meters? Horizontal or Vertical?

 

What is the PID of the benchmark you used? I haven't seen one that is certified horizontally and vertically.

Share this post


Link to post

I feel that the Billion dollar system is more accurate than we give it credit....Most of the Coordinates are scaled.or adjusted to come close to what we might find out in the field.I actually use the readings that are given by the Trilateration Mathmatics of the System. I have done extensive research into these matters and find that you can go back to the exact spot with the GPSr system reading as the control for that point.We are always given that addage that we are not surveyors,but they use this same billion dollar system and do their adjustments as well,If you ask them for the data they are or have derived from a point they go out of their way to not let you see the data that they are adjusting..

did you lift the GPSr to put the reading in thats about 4 meters if your standing.Enter the mark on the spot,do not lift it this will affect your readings as well,they are very precise when used with the utmost of care when trying to calculate Elevation,that is why they have them set on a tripod so they do not move.There are only 2 signals that I am aware of Military and Enemy.If you look at the older data sheets they say within 6 seconds thats about 600 feet more or less?????

WHEN ALL ELSE FAILS *GEOTRYAGAIN* http://www.msnusers.com/MissouriTrails

 

[This message was edited by Trailblazer # 1 on March 12, 2003 at 09:20 AM.]

Share this post


Link to post

to be four meters. 4 meters is about 12-13 feet. He is also only checking lateral varience not vertical. Theoretically the lateral (lat and lon) measurements should not change by lifting a GPSr from the surface of a marker too several miles vertically over the marker. It should definatly not change so long as the marker is vertically under the GPS within the range that a person can hold it above the mark.

 

4 meters is acceptably close to a mark for a non waas enabled GPSr, with waas it should be within 3 meters, though with a benchmark established during a different datum, it would depend upon how well the transformations required to convert the datum information was done.

 

-Rusty

Share this post


Link to post

quote:
Originally posted by solohiker:

I went to the website below and did a search on GPS benchmarks. I found one at the local airport that has been certified horizontally and vertically. I compared the readings of my Garmin 12 with the coordinates of the bench mark. My reading was off by 4 meters.

 

http://www.ngs.noaa.gov/datasheet.html

 

SoloHiker, there's a lot of info on that page. Could you narrow it down a bit for those of us with short attention spans? icon_eek.gif

 

Thanks!

 

http://fp1.centurytel.net/Criminal_Page/

Share this post


Link to post

quote:
Originally posted by Criminal:

 

SoloHiker, there's a lot of info on that page. Could you narrow it down a bit for those of us with short attention spans? icon_eek.gif

 

Thanks!

 


 

The web page is a search engine for bench marks.

 

If you select search by PID and enter 'FG1961' you should be able to locate and download the data sheet that I used.

 

The coordinates are listed in UTM format as:

 

UTM 15 - 3,901,885.135 491,605.895

 

Map Datum NAD 83

 

I sent an email to the info address and asked about tolerance of horizontal data. The reply stated that the accuracy of GPS adjusted bench marks was 1 to 3 centimeters.

 

I am new to this too, but I think my data and assertions are accurate.

 

[This message was edited by solohiker on March 12, 2003 at 04:30 PM.]

Share this post


Link to post

Let me add that this benchmark is near a taxiway on a public Airport. Very flat with no obstructions. The State Highway department had a tripod over the bench mark with an electronic transponder on top. I took my reading one meter west of the tripod.

 

This is a small town airport and I guess I looked official enough when I asked to survey the geodesic disk. The highway department people had been in there everyday for a week.

 

There are only 2 GPS adjusted benchmarks in the County and they are both at this airport.

 

To find YOUR nearest GPS adjusted bench mark go to the website, search by County and select 'GPS sites only' in the Data Type Desired box.

 

http://www.ngs.noaa.gov/datasheet.html

Share this post


Link to post

... for hitting all of within my county. There is only 88 in the county I live in. That should be a good target for this summer. Unless I move first...

 

-Rusty

Share this post


Link to post

Thanks for the info on these adjusted benchmarks. Found one today and tested my Garmin Legend for Lat and Lon. Over the mark, the Garmin said it should be 10' SE and satellite page showed 15' "accuracy". With WAAS on, the Garmin said it should be 6' E with 8' "accuracy". (over 10 minutes for reading)

 

This mark, DY2499, is in a good open site, 165' elevation, near a steep cliff dropping to the ocean, with a good view of the sky and horizon.

 

Rig: Garmin Legend, firmware 2.50

Time: 11:30 AM Local, 16Mar03

WX: very clear and sunny after a storm blew through.

Sats: 8 received plus WAAS 47

Mark as listed: 33 44 29.32374N

118 24 05.66689W

Converted to Min: 33 44.488729N

118 24.094448W

Waypoint in Garmin: 33 44.489N

118 24.094W

 

The NGS lists the adjusted location of this mark to the one hundred thousandth of a second. The Garmin only allows one thousandth of a minute. A quick calculation shows the the rounded coords in the Garmin should have the waypoint about 2.8' NE of the benchmark instead of showing it to be 6'E. Yikes, even closer. I am surprised and pleased.

Share this post


Link to post

12 Sir,yes Sir,Acurracy (Military, .004% in the Center of the Bullseye)at 156,000 miles,By Kevin Kreigel,Commander of the Space Shuttle Columbia on the Laser shot on Feb. 14,2000 , the SRTM( Shuttle Radar Topography Mission). www.jpl.nasa.gov/srtm

 

THE MOST DANGEROUS ANIMAL IN THE FOREST DOES NOT EVEN LIVE THERE*********WHEN ALL ELSE FAILS*GEOTRYAGAIN **1803-2003 "LOUSIANA PURCHASE" 200TH ANNIVERSARY AND THE "LEWIS AND CLARK EXPADITION" http://lewisclark.goeg.missouri.edu/index.shtml http://www.lapurchase.org http://www.msnusers.com/MissouriTrails

Share this post


Link to post

quote:
Originally posted by shrdlu:

.... "The Billion Dollar System" has cost at least twelve billion.

...and it's one of the best military tools ever built. I wish every twelve billion spent by the government was so beneficial.


 

And every $$ worth it. WAAS icon_rolleyes.gif so far 6 billion and for what? Bugga all and only covers a fraction of the world that GPS covers.

 

.... and trailblazer# whatever don't talk so much crap

quote:
Most of the Coordinates are scaled.or adjusted to come close to what we might find out in the field

 

also changing the elevation won't affect lat&long, Cartesian yes but not Lat&long, never seen so much misconception and rubbish.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

Share this post


Link to post

CRAP...... I have the evidece on tape to prove what I say.I filmed the whole thing(SRTM) on NASA TV. I keep up very well with as much as possible.I also was talking to The crew,The ? I posed the crew was about the 57 degree angle of the Radian measurement system, via computer on the ground,which I also have the evidence thereunto,So unless you can prove that I am talking crap I would keep away from the snide comments,You have also tagged me several other times on this conspiracy thing...so if you can prove it do it if not please reserve you comments on these matters.

 

THE MOST DANGEROUS ANIMAL IN THE FOREST DOES NOT EVEN LIVE THERE*********WHEN ALL ELSE FAILS*GEOTRYAGAIN **1803-2003 "LOUSIANA PURCHASE" 200TH ANNIVERSARY AND THE "LEWIS AND CLARK EXPADITION" http://lewisclark.goeg.missouri.edu/index.shtml http://www.lapurchase.org http://www.msnusers.com/MissouriTrails

Share this post


Link to post

How did you come up with the 4 meters??

 

Did the receiver tell you this?

 

quote:
With the end of Selective Availability, you can expect horizontal accuracy of 20m to 35m (60' to 105') for older 8 channel units and 7m to 15m (21' to 45') for newer 12 parallel channel units in good reception conditions. For altitude you can expect accuracy to range from +/- 75m (225') for an 8 channel unit and +/- 35m (105') for 12 parallel channel units in good reception conditions. There are a number of environmental conditions that can affect GPS accuracy due to varying satellite signal reception conditions and can therefore lead to better or worse accuracy's than listed above. The satellite status page on most Garmin GPS units will provide a real-time estimate of the relative accuracy of the position reported by the GPS receiver.
quote:
Q. Can I post-process the data collected with my Garmin GPS unit to obtain greater accuracy?

 

A. Unfortunately not. Garmin GPS units and other handheld consumer-grade units do not internally store the raw pseudorange data from the satellites required to post-process differential corrections. This type of capability is only found in survey-grade GPS equipment.

 


 

[This message was edited by elcamino on July 11, 2003 at 04:47 AM.]

Share this post


Link to post

I work for a surveyor & I don't typically go out with a gps crew so I'm not an expert on this. The reason for the accuracy of surveying gpsr is timing and time. The longer you sit there & average a coordinate (typically 30+ minutes for our control points with an adjacent local base station for 8 or so hours aka rapid static) the better the accuracy, also we use software that compares & reprocceses that data to correlate to multiply D.O.T. GPS base stations. That I know of most civialian gpsr don't average coordinates, but that may be changing... There is also the problem that most gps units don't have atomic clocks in them, so the timing between measured signals and actual signals causes error. Using my own gpsr (Garmin eTrex, typically accuracy running 3-4 meters and a good 110 degree spread of 10 or so satellites, but rearly reading waas) I found that the coorinate never match exactly, but if you draw a circle around you with the radius being what your gps says accuracy is running, then the point will be in that circle, but the elevation is usually 2 to 3 times that error. If your coordinates are less that 2 meters off, then you just got lucky. I asked one of the field crew guys how accurate his GPS was & he said usually .03 to .1 feet.

 

As for Lat/Lon changing with elevation, that is possible because "straight above" to us usually means opposite of gravity, whereas gps coordinates are based on a mathmatical formula that isn't a perfect model of the earth. If you change the elevation of your gpsr you may actually shift Lat/Lon because the center of Lat/Lon may not always be the center of gravity. Shouldn't affect our gpsr though, their not near that accurate. So occasionally you'll find places where gps says water runs up hill, even if your watching it go down... icon_smile.gif

 

--Pyre Flos

when in doubt or danger... ATTACK!

Share this post


Link to post

That's right Trailblazer CRAP.

 

I tell you what about you get someone who is an authority to backup the crap and mindless talk you go on with.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

Share this post


Link to post

quote:
As for Lat/Lon changing with elevation, that is possible ....
in practice it might but on paper that simply doesn't happen as the underlying coordinate system is nothing that really makes much sense to many.

 

Take for example the points (in Earth Centred XYZ)

 

X= 1353755.2815 Y= -5052283.4915 Z= 3637924.2670

X= 1353946.0924 Y= -5052995.6076 Z= 3638440.4858

 

Those coordinates mean things to the underlying workings of a GPS but to most of us they may as well be on the moon.

 

What's the difference? purely only in the elevation.

 

Both are 35 00' 00.0000"N, 75 00' 00.0000"W, the same horizontal point but the first is 100m and the other is 900 metres directly above it.

 

In practice for sure there's some other influences but GPS coordinates are based on a formula that is a perfect model but the geoid creates some problems in determining orthometric heights from ellipsoid heights but that has little to do with being directly above or below some point.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

Share this post


Link to post

http://www.msnbc.com/news/937378.asp

http://www.ngs.noaa.gov/GEOID/geoid.html The Elevation Model is still under construction and the Dr.Dru Smith is the one I talked to at the Eros data center to get most of the information.

WHEN ALL ELSE FAILS*GEOTRYAGAIN **1803-2003 "LOUSIANA PURCHASE" "LEWIS AND CLARK EXPADITION" http://www.lapurchase.org http://arkansasstateparks.com/lapurchase/ http://www.msnusers.com/MissouriTrails

 

[This message was edited by GEO*Trailblazer # 1 on July 11, 2003 at 07:04 AM.]

Share this post


Link to post

Verifacation: Nasa, Question to Astronauts # 448719,The 57 degree angle of measure. IE Radian. 2/20,2000, NASA Jobs Homepage. http://spaceflight.nasa.gov/feedback/expert/ http://www.nasajobs.nasa.gov/ 2/20/2000 Best Wishes Kevin Kregel STS-99 Commander.

Do a little investigation and you will see that I have done all that I said I have whether you believe it or not is ok with me.I know some of what I am talking about as well as the rest of you do. We are all learning new things every day. It's the peace of mind I have in it all.I am not a know it all, but I wish I did.But no one is ever GREATER than the MASTER.

WHEN ALL ELSE FAILS*GEOTRYAGAIN **1803-2003 "LOUSIANA PURCHASE" "LEWIS AND CLARK EXPADITION" http://www.lapurchase.org http://arkansasstateparks.com/lapurchase/ http://www.msnusers.com/MissouriTrails

 

[This message was edited by GEO*Trailblazer # 1 on July 11, 2003 at 11:49 AM.]

Share this post


Link to post

quote:
Originally posted by GEO*Trailblazer # 1:

.... The Elevation Model is still under construction ....


 

Honestly you really need to check the difference between a "Website under construction" and the significance to any Geoid model.

 

If your not aware (and it's obvious your not) the latest Geoid model for the US is Geoid99, which is simply the latest in a string of previous Geoid models. As more data is collected then this will also be revised, just like it has done for the last umteen many years.

 

Heaven help us if the geoid was still under "Construction".

 

Every country has gone through revision of their Geoids and for the U.S you can find it HERE as GEOID99, uinder "construction" what crap.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

Share this post


Link to post

Thanks for the link I do have it. The Sites I linked to were in 2000,when all this was going on.I think one of these days we will have to sit down and brainstorm.I have nothing against you.But when (from Field Experince)mention things can change with movement in any direction.Just like in (Calibration)measurement of meters.The longer the stability the better,for a more accurate reading in any field of measurement.I am still a Certified Measurement, Tester.I could go into great detail about how the tools that I used had to be sent to the Bureau of Weights and Measures to be Calibrated by them,A error of .001 in. could mean $1,000,000.00 a day.I had to use micrometers to test every size.I like getting in the Theoretical discussions anyway, all that we know is still,just a theory.......I guess that is why it is called Surveying:Theory and Practice......its all just a guess,who is right????? The other links are valid,except now it's the STS-107 Crew then it was the STS-99 crew,and the # is still valid as well,You can do the rest of the homework if you like I alredy know that you will find what I have stated to be true....So 2 out of 3 ain't bad.

 

WHEN ALL ELSE FAILS*GEOTRYAGAIN **1803-2003 "LOUSIANA PURCHASE" "LEWIS AND CLARK EXPADITION" http://www.lapurchase.org http://arkansasstateparks.com/lapurchase/ http://www.msnusers.com/MissouriTrails

Share this post


Link to post

Your latitude and longitude does not change as your rise (or lower) a receiver, as long as it is in the normal to the ellipsoid, that is straight up or straight down. Geodetic latitude is defined as the line passing through a point, normal (at a right angle) to the ellipsoid and intersecting the plane of the equator. That being said, if you are doing post processing of the GPS carrier phase data to determine the 3-D vector between 2 points and you make an error in the height of one of both receivers, then you create an error in the vector and hence an error of the latitude, longitude and ellipsoid height of the new point. Also, please note that the geoid model GEOID99 (elevation model?) is not “under construction,” it has been the officially model for the U.S. for several years. Unfortunately, most countries haven’t addressed the issue of geoid models. Only the more geodetic advanced countries, (e.g., Canada, Japan, Australia, Western Europe, UK etc.) have really don’t anything significant in this area. Most countries must rely on the global geoid models developed by the U.S. National Imagery and Mapping Agency (NIMA), which has uncertainties in the 2-5 meter range, as compared with GEOID99’s 4-9 cm. This is a major problem that will require massive resources by observing among other things, absolute and relative gravity over the majority of the planet. The current GRACE gravity satellite mission will help provide some of the long wave length data, but the short wavelength info will be lacking for some time to come unfortunately. In addition, the last time I looked, which was about 9:30 this morning, Dr. Dru Smith still worked on the floor above me at the National Geodetic Survey headquarters in Silver Spring, Maryland not the EROS data center in South Dakota.

Share this post


Link to post

I am sorry if I got my places mixed up.I have spoken with so many different people over the years trying to get my questions answered about specific things. I talked to (Dr. Dan Roman)301-713-3202 xt. 161. aroun 11/5/1999, We talked about Control Points,a quantity,vector assumed to have 1 value throughout the particular discussion or investigation in full,The Center of the Earth ellipsoid,International ellipsoid.P.O.B Start Mean Sea Level,height system NAVD 88,orthometric elevation,absolute,the Geoid about geoid undulation, water has same potential,angle from center,& perameter Tranformation or conversion, the XYZ 3 dimension 3 axis, 7th scale.At the time of inquiry I did not know that it would all lead to this type discussions on the internet.So I will try and be more precise in who's name I use I am sorry, Dr. Dru Smith it was the other name listed next to Dr. Dan Roman at http://www.ngs.noaa.gov/GEOID/DEFLEC99/deflec.html 11/5/1999

 

WHEN ALL ELSE FAILS*GEOTRYAGAIN **1803-2003 "LOUSIANA PURCHASE" "LEWIS AND CLARK EXPADITION" http://www.lapurchase.org http://arkansasstateparks.com/lapurchase/ http://www.msnusers.com/MissouriTrails

 

[This message was edited by GEO*Trailblazer # 1 on July 11, 2003 at 07:11 PM.]

Share this post


Link to post

Todays Calibration Test.. (RTK) Real Time Kenetic GPS,Levica, against the E-Trex-RINO, WOW.....Theirs,DEG,MIN,SEC. 36-56-38.98853" 94-00-08.52483" elev. 1199.84' mine 36-56-39.0 94-00-08.5 elev. 1182.9',Accuracy (9')WASS Enabled, .01147" difference I'll let you do the math but that close enough for me.....

 

WHEN ALL ELSE FAILS*GEOTRYAGAIN **1803-2003 "LOUSIANA PURCHASE" "LEWIS AND CLARK EXPADITION" http://www.lapurchase.org http://arkansasstateparks.com/lapurchase/ http://www.msnusers.com/MissouriTrails

Share this post


Link to post

My virtual cache at a benchmark has been gathering accuracy data for almost two years. Over 90% of the geocaching finders have reported accuracy within 3-4 meters regardless of the GPS model.

 

Pretty amazing! icon_smile.gif

 

Alan

Share this post


Link to post

Clarification: that's a maximum of 3-4 meters! Many have been within 1-2 meters.

 

Alan

Share this post


Link to post

I did a study of the results over a 4 month period but haven;t updated it since.

 

The following GPS’s were reported used including my own Vista when I set up the cache. Readings were taken with 13 different GPSr’s on 12 different days between 11/25/01 and 3/24/02 at different times of the day. Weather conditions were not reported.

 

Garmin Vista (4)

Garmin etrex (2)

Garmin Summit

Eagle Expedition II

Magellan Map 330

Magellan 300

Unknown

“Cheap” Garmin

Garmin Street Pilot

 

Summary of results. Difference calculated from NGS coordinates. WGS84 coordinates in DD MM.MMM were converted to UTM using Mapsource program. Distance from benchmark calculated taking the square root of the sum of the squares of the difference between the UTM latitudes and UTM longitudes in meters and converting to feet.

 

12 readings under 12 feet

5 readings between 13-25 feet

1 reading at 40 feet (note that this was the Magellan 300 which is an older unit and reads to 2 decimal places only - about 50 feet accurate I’ve been told).

 

All were single readings except for Eagle (13 readings = 3.3 feet avg.) and one of the etrexs (7 readings = 25 feet avg.). There was one WAAS reading reported at 3.3 feet.

 

I think the results speak mountains as to the accuracy of the equipment we are using.

 

Alan

Share this post


Link to post

quote:
Originally posted by GEO*Trailblazer # 1:

Todays Calibration Test.. (RTK) Real Time Kenetic GPS,Levica, against the E-Trex-RINO, WOW.....Theirs,DEG,MIN,SEC. 36-56-38.98853" 94-00-08.52483" elev. 1199.84' mine 36-56-39.0 94-00-08.5 elev. 1182.9',Accuracy (9')WASS Enabled, .01147" difference I'll let you do the math but that close enough for me.....


 

One thing you need to do is at least get the terminology right RTK (Real Time Kinematic) and it's Leica not a Levica. Start referring to RTK as "Real Time Kenetic" and some will start to wonder.

 

As for the accuracy, so what does it really mean? Dos it really mean anything or simply a case of coincidence.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

Share this post


Link to post

Good catch Kerry thanks..I am still learning..

I just put one letter too many...VVVVVVV and MMMMAAAA This thingamajig for that whatyoumacallit http://www.Geocaching.com/mark/details.asp?PID=GG0052

WHEN ALL ELSE FAILS*GEOTRYAGAIN **1803-2003 "LOUSIANA PURCHASE" "LEWIS AND CLARK EXPADITION" http://www.lapurchase.org http://arkansasstateparks.com/lapurchase/ http://www.msnusers.com/MissouriTrails

 

[This message was edited by GEO*Trailblazer # 1 on July 19, 2003 at 07:24 PM.]

 

[This message was edited by GEO*Trailblazer # 1 on July 19, 2003 at 07:35 PM.]

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
Followers 0

×