Accuracy Of Benmarks

[+kevin917z]

i am just wondering is the benchmark elevation very accurate like to a few centimeters or so? i just want to know if i can calibrate my barometric altimeter in my gps, i have been getting 2-5 meters higher than the benchmarks say, they are all scaled at least the ones ive seen

[+2oldfarts (the rockhounders)]

kevin,

 

E 472 is a mark that the elevation is adjusted & the location is scaled.

 

Whereas Dry Park Lookout is a mark that the elevation is scaled & the location is adjusted.

 

If the benchmark designation is a 'number' as in E 472, then the elevation is adjusted (usually) and it will be very accurate (less than an inch). If it has a 'name' as in Dry Park Lookout, the location is adjusted (usually).

 

So, you could just pull up the benchmarks in your area with your zip code and go to a 'number' mark and set your altimeter. That sounds like the perfect way to calibrate an altimeter.

 

We did not know that any GPSr had one included. We have a barometer for the weather only. Our GPSr gives the elevation but it is not adjustable and we have found it not to be very accurate at all.

 

Shirley~

[+kevin917z]

ok, i think im understanding this then, so adjusted is super accurate but scaled is an estimate and not very accurate, from what i read im not sure about vertcon though, im thinking its fairly accurate but not very good?

[holograph]

We did not know that any GPSr had one included. We have a barometer for the weather only. Our GPSr gives the elevation but it is not adjustable and we have found it not to be very accurate at all.

Yes, some of them have barometric altimeters in them. They can be calibrated either by manually setting it from an accurately known elevation, or by letting the GPSr calculate the elevation from the satellite signals. The GPS-based calibration is not very accurate.

 

I've set mine from benchmarks a few times just for kicks, but I have no reason to care much about altitude. On a couple of occasions I calibrated it at one benchmark, then compared it at other relatively nearby benchmarks on the same afternoon. It was usually OK to within a few meters, but no more accurate than that.

 

edit:

ok, i think im understanding this then, so adjusted is super accurate but scaled is an estimate and not very accurate, from what i read im not sure about vertcon though, im thinking its fairly accurate but not very good?

VERTCON is a datum conversion program that converts elevations from the older datum (NGVD 29) to the current datum (NAVD 88). When many benchmarks were set, their elevation was accurately determined using the NGVD 29 datum. VERTCON converts that elevation into the the NAVD 88 datum. There is a small uncertainty introduced because of the mathematical model used, but VERTCON introduces only about 2 cm of uncertainty.

 

As someone is sure to point out, the real measure of accuracy is the "order" of the vertical data. However, the meaning of the various vertical orders is somewhat complicated, so it is easier to just look to see if the elevation is ADJUSTED (accurate) vs SCALED (estimated).

Edited August 31, 2005 by holograph

[DaveD]

holograph is correct about the orders of accuracy. The difference between a 1st-Order and a 3rd-Order height is important to surveyors but it immaterial for someone with a hand-held receiver.

 

Anyone interested the details of the horizontal and vertical orders of accuracy can review the Federal Geodetic Control Committee “Standards and Specifications for Geodetic Networks” http://www.ngs.noaa.gov/FGCS/tech_pub/1984...ol-networks.htm, and the procedures for geodetic leveling can be found in NOAA Technical Report NOS 73 NGS 8 “Control Leveling” -- http://www.ngs.noaa.gov/PUBS_LIB/TRNOS73NGS8.pdf

[+Gonzo-YT]

Once calibrated, I find that my Vista C is just about bang on. It's always within a few meters. Of course, weather conditions can have an effect.

[+kevin917z]

well i calibrated it today and used it to mark 12 points, when i got home i loaded them into the computer and found that i was within 1 meter on about 8 of the 12 points, the other 4was 1-3 m accuracy which is pretty good since i wasnt standing on the contour line at each point so it looks like im getting 1-2 meter accuracy,

 

im still not sure what the auto calibrate on my rino 130 means, i am assuming that it is adjusting the satellite determined elevation by using the barometric altimeter

the elevation on the satelite page is usually 10 meters off what the other pages say as the satellite page only shows satellite determined elevation

Edited August 31, 2005 by kevin917z

[+Gonzo-YT]

The pressure altimiter is far more accurate than GPS elevation, even with a good sattelite fix. However, the downside of the pressure altimiter is it needs to be calibrated regularly, as changes in atmospheric pressure due to weather will change the readings over time.

 

Auto calibrate means that it will calibrate the pressure altimiter using the GPS reading -- not terribly precise, in other words. However, it is still a good feature, as although your exact elevation might not be spot-on, your elevation gains, losses and plot over time will be very accurate using the altimiter.

 

If you have a known elevation, you should always calibarate manually from that. That's where your closest benchmark comes in very handy.

 

Regards,

Anthony

[+BuckBrooke]

It's probably always best to check the NGS datasheet elevation, as the elevation on the disc may be out of date. I guess I'm personally missing the niftyness of using my handheld for altitude calculation. I've never really worried about elevation, and gone more for latitude and longitude, especially due to the inherent 1-multi meter error with the handheld.

[+Gonzo-YT]

I actually have found my altimiter useful for finding benchmarks. The ones I have been looking for have scaled coordinates, and if they even have a description at all, too often all of the reference points are missing. If I am lucky I have one landmark like an old roadbed or something.

 

The one thing I know I can count on is an accurate elevation, so my altimiter can give me clues about where to search. I've also been able to find at least one by walking a contour line on my GPS map screen.

 

Regards,

Anthony

[+GEO*Trailblazer 1]

I use all the bells and whistles myself.

I have found that the accuracy is better than reported,the disclaimer is for protection.

 

I have kept track of almost every mark that I have recovered.

My RTK test was within .001 in the X,Y and Z.

 

I am sure that when the diffrence between the datums are calculated it would even be much more precise.

 

But I am not a Geodisist and that type of accuracy will never play a part in this game.

 

Now if I wanted to do a better survey,I would use the High dollar equiptment.

 

But for now,It is close enough for me.