WGS 84 versys NAD 83

[+SeismicCat]

Hi,

 

I was wondering if I should be setting my GPS to NDA 83 when hunting for benchmarks. The reason I am asking is that I think I noticed a consistent bias between the GPS, set using WGS84, and location reported on the NGS data sheets.

[tosborn]

The Current Survey Control coordinates given on the NGS datasheets are NAD83. So you should set your GPS receiver to NAD83 if you are hand entering the coordinates. If you are transferring the coordinates to you receiver from a computer program, the program usually "converts" the coordinates to WGS84 behind the scenes and then does the transfer, since the base datum of most/all receivers is WGS84. How you choose to display the coordinates on your receiver after the transfer doesn't really matter that much....whether the coordinates for a benchmark are displayed as NAD27, WGS84, or NAD83, or UTM, they will all lead you to the same location.

 

I'm suprised that you think you notice a difference between NAD83 and WGS84. Everthing that I have seen and experienced says they are, for all practical purposes, the same.

[+Cardinal Red]

You didn't mention which GPS you are using. But if you have a consumer grade unit you can not tell any difference between WGS84 and NAD83. The average difference between them is around 1 or 2 meters. You have to spend a LOT to be able to determine the difference. Think roughly in the range of $6,000 for a "Mapping Grade" GPS with sub-meter accuracy to about $40,000 for a "Survey Grade" GPS with centimeter level accuracy. If you have a $500 unit, your question has no validity. But most users will set to NAD83 anyhow. We're human. We convince ourselvs that if they gave us two settings on our GPS, there must be a reason. The only reason is the users who would complain a choice was missing. It's easier to give us a placebo.

[+BuckBrooke]

SeismicCat,

 

Does you notice this difference for SCALED as well as ADJUSTED stations? For those (not necessarily yourself) who aren't aware of the difference, if you look at the benchmark page for a station, AC8346 for example, 2 lines under the coordinates, at the end of the text, it says " and location is SCALED". This means that the station is accurate in altitude, not X-Y coordinates, and could be off up to several hundred feet. You'll see the coordinates I got for that station were pretty good, not more than 20-30 feet off or so. If the coordinates were ADJUSTED, the accuracy would be better than your handheld unit, and you should get the same as the listed coordinates.

Edited August 13, 2006 by BuckBrooke

[Photobuff]

IMO, the difference is negligible for all of us impoverished hand held non survey type GPS units, unless...

 

your time is worth a nickel an hour, you've got $25, you've got a laptop computer, and the WAAS satellite gods are smiling in your area. Whacha do is download a copy of SA Watch, wire your GPSr to your laptop, and sit on the station for maybe 4-24 hours. After the program tosses the lower quality data, and averages the rest, you can get within a foot or so. The only trick is that your time window has to include a period where lots of satellites are available, and signal conditions are decent. There are many long periods where the quality of the fixes is quite poor, they tend to be off in the same direction, and no amount of averaging will help.

 

This worked for me last year, but with the repositioning of the WAAS satellites the quality of my fixes this year has been abysmal by comparison. Not sure how good I could do averaging without WAAS, though it might be surprising.

[+SeismicCat]

Thanks for all the good suggestions and education. I went out to two NGS benchmarks that were updated since th 1980 and sat on the locations for a few minutes until the Magellan Platinum could get a failry stable average. The locations came out within two meters of the true location. I am afraid that lmy initial comment had more to do with the fact that I was using local survey points.

 

What go me started checking the benchmarks was the need to be certain that the locations I was getting from the GPS were accurate within about 5 meters.

 

Again thanks,

[+Black Dog Trackers]

SeismicCat -

 

I don't know what your goal is or what "local survey points" are, but wanted to say that the GPS receivers we use are just gadgets and cannot be used for Real Life surveying concerning properties and the like.

 

As far as accuracy of NGS coordinates, to reiterate what BuckBrooke was saying, it all depends on whether the datasheet says "location is SCALED" or if it says "location is ADJUSTED". These represent 2 completely different classifications of survey mark. You can take these to mean: "location is SCALED" means +/- 600 feet and "location is ADJUSTED" means +/- 1 inch. An accuracy of +/- 5 meters is about right for a recreational use GPS handheld receiver.

[+GEO*Trailblazer 1]

The only difference I have noticed was from the original conversion of the data sheets.

Most of the Traiangulation Stations are very precise.

The other scaled marks may be off some.

 

But in the GPS I can find a very small differnce when finding the marks.

 

There are so many factors in determining what the original surveyors did,their inacuuracies,time of year...etc.

 

It is not worth the exhaustive effort to try and find out the why.

Unless you just have a lot of time to tinker.

[+Black Dog Trackers]

All of the triangulation stations are location ADJUSTED.

Other types, such as intersection stations, traverse stations, etc. are also location ADJUSTED.

 

Variations in time of year, measurement details, etc. don't matter much after the location of the station has been adjusted. If there are errors in measurement, the station will not survive the adjustement process and won't get into the NGS database. Conversions of data sheets from one datum to another don't matter either in looking for a mark with a handheld GPS receiver. The only thing that matters in how close your GPS receiver will get you to the mark in benchmark hunting is whether the mark is "location ADJUSTED" or "location SCALED".

[StripeMark]

All of the triangulation stations are location ADJUSTED.

Other types, such as intersection stations, traverse stations, etc. are also location ADJUSTED.

 

...well 99% of the triangulation stations are ADJUSTED. Note that triangulation station JE0669 is SCALED.

 

http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=JE0669

[Bill93]

Well, they may have used a tri station disk if that was all they had left in the truck, or maybe it is miscoded, which has been known to happen more than once.

 

But it is being used as an elevation mark only. Nothing else on the data sheet looks like a tri station other than the disk type. There is no box score, no mention of reference marks [which it was pointed out strangely have their own PIDs and elevation data], and no superceded horizontal control data.

 

Somebody needs to get a picture of it.

Edited August 15, 2006 by Bill93

[StripeMark]

yes, I noticed that there was no "boxed score" and I figured that was because it was SCALED.

 

There is a reference mark 1 (JE0670), reference mark 2 (JE0668), and a azimuth marker (JE0666).

 

I plan on getting out there this weekend.

Edited August 15, 2006 by StripeMark

[DaveD]

Station JE0669 is a triangulation station and does have RMs and an AZ MK. NGS, actually the Coast & Geodetic Survey (C&GS), set this and many other triangulation stations with similar alpha-numeric designations just like it across the plains states at missile silos, at the request of DoD. All the appropriate triangulation measurements were performed and the computations were completed and immediately classified and shipped off to what was then called the Army Map Service, later renamed the Defense Mapping Agency, then the National Imagery and Mapping Agency and currently the National Geospatial-Intelligence Agency. The original positions for these marks are still held as classified by NGA even though the silos have long since disappeared and many of the marks have been later reobserved with GPS and the current published values are better than the classified values. This particular mark was also leveled to by C&GS, but the vertical control was never considered classified.

[Team Fawlty]

You have to spend a LOT to be able to determine the difference. Think roughly in the range of $6,000 for a "Mapping Grade" GPS with sub-meter accuracy to about $40,000 for a "Survey Grade" GPS with centimeter level accuracy.

What is the difference? Assuming they all use the same satellite signal, what do you get for the extra money? Curiuos minds want to know.

 

Brendan

[Bill93]

They don't all use the same parts of the satellite signals. The GPS signals are very complex with L1 and L2 carriers and multiple modulations. A more complicated receiver can track finer features and get more accurate results. It also takes averaging time to get those accuracies.

[+Klemmer]

It also takes corrections done "after the fact" (post-processing) which you download the data for, back at the office, from the Gov't. These corrections take out many of the errors we live with. To do that, you have to record lots of data in the field (not just Lat/Long, but many hours of raw sat data....) So my daughter tells me, who uses the $$$ ones. I'm happy with my 3 meters or so on a good day....

[caseyb]

What is the difference? Assuming they all use the same satellite signal, what do you get for the extra money? Curiuos minds want to know.

 

Brendan

 

Consumer grade GPS receiver are single frequency receivers (only get L1 I believe). Survey grade receivers are dual frequency and receive both the L1 and L2 frequency. By getting both frequencies they are able to cancel out a lot of the error and get a much better position. In the very near future, there are going to be a number of new signals from new constalations (Galaleo and Glonass) as well as a new L2C frequency.

 

-Casey-

[+Cardinal Red]

Carrier-Phase GPS

 

This is a new version of GPS that can eliminate errors even better than other forms. Recall that a GPS receiver determines the travel time of a signal from a satellite by comparing the pseudo random code it's generating, with an identical code in the signal from the satellite. The receiver slides its code later and later in time until it syncs up with the satellite's code. The amount it has to slide the code is equal to the signal's travel time. The problem is that the bits (or cycles) of the pseudo random code are so wide that when the signals sync up there is room for error. Survey receivers are better as they start with the pseudo random code and then move on to measurements based on the carrier frequency for that code. This carrier frequency is much higher so its pulses are much closer together and therefore more accurate. At the speed of light the 1.57 GHz GPS signal has a wavelength of roughly twenty centimetres, so the carrier signal can act as a much more accurate reference than the pseudo random code by itself. And if it can get to within one percent of perfect phase like you expect with code-phase receivers you can (theoretically) obtain 3 or 4 millimetre accuracy.

 

In essence this method is counting the exact number of carrier cycles between the satellite and the receiver. The problem is that the carrier frequency is hard to count because it's so uniform. Every cycle looks like every other. The pseudo random code on the other hand is intentionally complex to make it easier to know which cycle you're looking at. But Carrier-phase GPS tackles this problem by using code-phase techniques to get close. If the code measurement can be made accurate to say, a meter, then we only have a few wavelengths of carrier to consider as we try to determine which cycle really marks the edge of our timing pulse. Resolving this carrier phase ambiguity for just a few cycles is a much more tractable problem and as the computers inside the receivers increase in processing power and functionality it's becoming possible to make this kind of measurement without all the steps that survey receivers go through.