I have an NGS Data Sheet that shows the vertical accuracy as "Vert Order Third ?".
Don't think I've ever seen the accuracy listed as "Third ?" Very odd.
I have long hours GPS occupation of the point for 4 days this week. The OPUS reports are fairly consistent. The elevation range on our OPUS points is .15 feet. But, that range differs from the published elevation (Third ?) by right at 1 foot.
I called NGS and they told me that it wasn't unreasonable for my data to vary ~ 1 foot for a "Third ?" order elevation.
That brings up two questions.
One, is a difference of 1 foot really reasonable?
Two, when I publish my final data how much should I emphasize the difference between my answer and the existing NGS elevation?
Thoughts?
Larry P
Datum difference? VERTCON error? Subsidence/uplift?
NGVD vs, NAVD? that's about the difference we're finding around here...
Am I mistaken, or wouldn't that mean that the elevation on the point was determined based on a 3rd-order level run? Wouldn't that mean it has nothing to do with the datum or that sort of factor? I must be wrong if you are talking about being +/-1 foot.
3rd order is 0.05' per root mile. This should not be the cause of your headache.
You can convert the metric if you want:
From Benchmark Reset Procedures Manual, Curtis L. Smith, NGS, May, 2007.
Good luck, Thadd
I don't think most NGS data sheets give ellipsoidal heights, which is what GPS inherently measures. There is a geoid model involved in getting from ellipsoidal to orthometric. How good is the geoid model in your area?
The order of the point is defined as others have mentioned by relative accuracy to other nearby benchmarks, and not by accuracy relative to ellipsoidal and geoid. So it's apples and pomegranates.
Howdy,
It would be helpful if you had provided the PID. Nonetheless, the answer to questions about all elements on a data sheet are found in the file: dsdata.txt
It can be viewed/downloaded at: http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=DSDATA.TXT
The relevant section is copied below:
ORDER AND CLASS: ORTHOMETRIC VERTICAL
*************************************
Vertical station order and class for first-, second-, and
third-order stations are defined in the Federal Geodetic Control
Committee publication "Standards and Specifications for Geodetic
Control Networks". In addition:
Normal bench marks with unknown order will display a '?'.
Vertical control which were determined only for the purpose of
supplying a height for Horizontal Distance Reductions are
assigned an order of 'THIRD'. If these types of heights do
not have supporting observations then the Order is displayed
as 'THIRD ?'.
Obviously these heights are not intended for use as benchmarks, i.e. not the origin of height projects nor as checks on benchmarks.
Cheers,
DMM
I never knew that; thanks for the explanation.
