What did I do wrong when I logged this with the NGS?

[TillaMurphs]

The end of our description in the NGS datasheet (for RD0910) says this:

 RD0910' 
RD0910'GPS (HH2) COORDINATES FOR THE MARK ARE  N 45 16 30.5, W 123 00 44.3
RD0910' 

And that is how we entered it in the submission form.

 

However, at the top of the datasheet it now says this:

 RD0910						 *CURRENT SURVEY CONTROL
RD0910  ___________________________________________________________________
RD0910* NAD 83(1986)-  45 16 30.27   (N)	123 00 44.28   (W)	 HD_HELD1  
RD0910* NAVD 88	 -		58.825  (meters)	 193.00   (feet)  ADJUSTED  
RD0910  ___________________________________________________________________

 

1) We put down HH2 and it ended up as HH1 at the top of the datasheet?

 

2) The decimal portion of the seconds for the latitude does not match what we put down? And, somehow they ended up with more significant figures in the longitude than we provided?

 

What did I do wrong?

 

(By the way, we included the HH2 coordinates on this submission because the previous scaled coordinates were off by about 180 feet.)

[+PFF]

This appears on the data sheet, although I don't know how to interpret it:

 

"The horizontal coordinates were established by differentially corrected

hand held GPS obs and have an estimated accuracy of +/- 3 meters."

 

By the way, you did a great job with the new description!

 

-Paul-

[Bill93]

It looks like somebody else submitted coordinates from a better grade GPS, without any other information.

[wister6813]

I have noticed this as well. I submit HH2 coordinates in a GPX file, and when the database is updated some are listed as HH1 coordinates on the data sheet.

 

Frequently this is occurs with the reference marks. I assume once I have confirmed the mark exists the NGS is calculating the position based on the information in the box score and giving it the HH1 classification.

But others are plain-jane, garden variety vertical control marks that do not have visible link to other information. An example is KC0512 which was part of the update that occured a couple of days ago.

 

Bob

[+jwahl]

They may have transformed the WAAS values to NAD83. As I recall WAAS corrections yield ITRF00 values which differ by a meter or so from NAD83.

 

Not sure what epoch to use... My try below doesn't match for longitude very well, but suffice to say, the two values are shown with differing datums or epochs at least.

 

-jlw

 

HTDP (version 3.0) OUTPUT

 

TRANSFORMING POSITIONS FROM ITRF2000 (EPOCH = 05-01-2010)

TO NAD_83(CORS96) (EPOCH = 05-01-2010)

 

INPUT COORDINATES OUTPUT COORDINATES INPUT VELOCITY

 

test

LATITUDE 45 16 30.50000 N 45 16 30.48682 N 0.00 mm/yr north

LONGITUDE 123 00 44.30000 W 123 00 44.23858 W 0.00 mm/yr east

ELLIP. HT. 55.000 55.367 m 0.00 mm/yr up

X -2449483.046 -2449482.222 m 0.00 mm/yr

Y -3770100.191 -3770101.379 m 0.00 mm/yr

Z 4508956.985 4508956.959 m 0.00 mm/yr

 

-- or trying again using 1986 works a little better

 

HTDP (version 3.0) OUTPUT

 

TRANSFORMING POSITIONS FROM ITRF2000 (EPOCH = 05-01-1986)

TO NAD_83(CORS96) (EPOCH = 05-01-1986)

 

INPUT COORDINATES OUTPUT COORDINATES INPUT VELOCITY

 

test2

LATITUDE 45 16 30.50000 N 45 16 30.47544 N 0.00 mm/yr north

LONGITUDE 123 00 44.30000 W 123 00 44.25529 W 0.00 mm/yr east

ELLIP. HT. 0.000 0.382 m 0.00 mm/yr up

X -2449461.960 -2449461.583 m 0.00 mm/yr

Y -3770067.736 -3770068.944 m 0.00 mm/yr

Z 4508917.908 4508917.646 m 0.00 mm/yr

Edited May 10, 2010 by jwahl

[Bill93]

This still doesn't explain the upgrade from HH2 to HH1 quality.

 

The data was taken as ITRF2000 (Jan 31, 2010), and NAD83 (1986) is reported on the data sheet. This results in HTDP output

NAD83(1986) N45 16 30.48 W123 00 44.24

 

The difference between the submitted coordinates and the data sheet values is 7.11 meters. For any epochs the difference between WGS84 or ITRF and NAD83 in the middle of the US is no more than 1 and a fraction meters.

 

Because none of these transformations are anywhere close to the latitude, and the is tag HH1, I have to conclude the data came from someone else's measurement.

Edited May 10, 2010 by Bill93

[TillaMurphs]

So is there anything wrong with the way I reported the coordinates in my submission? I only ask, because I have a few recoveries I am waiting to submit – just wanted to make sure I am describing my HH coordinates as required before I send them.

 

It does seem coincidental that no one at the NGS has done anything with this mark for 76 years and now they get my submission and someone else’s coordinates at the same time (*scratches head*).

 

Also, Paul, thanks for the nice feedback!

[+jwahl]

Looks like a question for NGS.

 

Are you the source of the KMB description?

 

My mistake was interpreting the input coordinate as 45 16 30.3 instead of 30.5 in the HTDP transformation.

 

-jlw

[TillaMurphs]

Are you the source of the KMB description?

 

-jlw

 

Yes.

[+Klemmer]

Folks:

 

I'm not sure about the geodetic / database issue here (I'm as puzzled as the rest of you), but I do know:

 

WAAS is not a datum. As many of you know, they (or often it) are a set transponders on geosynchronous satellites that (with help from ground stations) can improve the accuracy of your GPS receiver. What Lat / Long coordinates your consumer handheld units displays to you is also heavily influenced by the datum you set into the unit. In fact you can set in NAD27 as a datum, and get good valid coordinates (using WAAS) to cross over to older USGS 1:25K Topo maps. I've done that several times.

 

So - for NGS to assume the coords are HH1 and are input with an ITRF2000 datum,and then translating them to HH1 NAD83 just does not seem right, as several folks above have said.

 

Can anyone give DaveD a heads-up to check this out? I've also got 20+ scaled finds (some way off) I'm getting ready to log, including HH2 coords.

 

BTW: TillaMurphs - I agree with Paul - keep up the good work!

 

Klemmer

[+jwahl]

I already admitted that the datasheet coordinate diff and HH1/HH2 designations are probably not due to any datum transformation.

 

But as a side topic. WAAS corrections while not a datum are with reference to base stations that were positioned to (WBS84 now ITRF00) coordinates, thus the value you get on your receiver will be corrected based on that and be OFF of NAD83 by a small amount. Once you are getting WAAS corrections and if you are set to NAD83 you are actually getting ITRF00 positions.

 

You can verify this by doing a very long average on a NAD83 point if you have a way of reading the coordinate out of your unit to sufficient precision.

 

Again now probably a side issue since the most plausible explanation is someone else reported the station with a HH2 capable device.

Edited May 12, 2010 by jwahl

[Bill93]

My understanding of the typical HH unit, after considerable experimentation, is that it always keeps its internal data in WGS84, with or without WAAS being used to tweak that data as it is acquired. I'm not up to date on WGS84 vs ITRF00. Is that the latest in the series of WGS84(G1150) etc?

 

The display of point coordinates can be transformed to (and manually entered waypoints from) NAD83 or any of the datums in its list but that doesn't change what is stored in its memory.

 

The transformation table used in all recreational grade units I've tried uses no offset between WGS84 and NAD83. Probably their whole transformation table comes from that US military document that listed transformations and was published before any data was available at a precision that would show the difference between WGS84 and NAD83.

 

I worked a lot at trying to figure out what was stored in my unit to better precision than ddd.ddddd display. The purpose would be for averaging data taken in many sessions without the rounding error which is a slight corruption for long term averages. I think I missed the easiest way--some of the alternate file formats in MapSource are text readable and contain more digits. I have not done any work to check whether those least digits remain constant when transferring to and from the handheld unit, or whether rounding and recomputation will mess them up. I expect they will be as good as the signal data allows.

Edited May 12, 2010 by Bill93

[+Black Dog Trackers]

FWIW, I have evidence that the HH1 coordinates listed at the top of the datasheet are faulty.

 

The description that TillaMurphs include the statement:

"32 FEET NORTH-NORTHEAST FROM THE CENTER OF 10TH STREET"

Using Google Earth, I used its ruler function to measure perpendicularly from the center of 10th street to TillaMurphs' HH2 coordinates and got 32 feet.

The distance from the center of 10th street to the HH1 coordinates is only 14 feet.

 

I believe that TillaMurphs' measured coordinates are better.

[DaveD]

Don't get too deep into the weeds on this. In the specific case of the station questioned by TillaMurphs, my records indicate that we had a position in the NGS archieves from the U.S. Geological Survey where they had run a geodetic traverse over the station. I took their NAD 27 coordinates and transformed them to NAD 83 and that's what we show on the datasheet. In the order of preference I use NGS observations to Reference Marks from the box score, then any USGS data that may be available and finally hand-held positions that I harvest from this site. In the case of data from this site, if there have been more than one recovery where the bench mark hunter provided coordinates then I will average the values from the various submissions as this provides a bit of redundancy. Remember the intention of HH1 and HH2 is only to improve the values from scaled so that the mark is easier to find for the next person. The difference between NAD 83 and WGS 84 is about 1 m in the horizontal so this falls well within the tolerance for either value and the time required to make any difference is not really worth the effort. If someone wants to submit a survey-grade position for publication to NGS they can use OPUS-DB (database). Thanks to all who put a coordinate in their post. I am often frustrated by some who indicate that the scaled bench mark position is off by several hundred feet and then never provide the coordinates they got.