I GPSed elevations to my site for a LOMA. One benchmark was a second order class 0 BM that is also the azimuth mark for a second order horizontal station. I was dismayed that my OPUS-RS position was 41.4 meters from where NGS said it should be. I rechecked the data sheet and the horizontal position was scaled from a topo map despite the fact that a higher level position for it exists. To top it off the W 075 09 03 as reported is wrong. I scaled W 075 09 05 on the topo map. OPUS-RS said W 075 09 04.71225.
My question are GEOID09 values to be used for FEMA forms?
KV1620 *CURRENT SURVEY CONTROL
KV1620 ___________________________________________________________________
KV1620* NAD 83(1986)- 40 49 23. (N) 075 09 03. (W) SCALED
KV1620* NAVD 88 - 205.038 (meters) 672.70 (feet) ADJUSTED
KV1620 ___________________________________________________________________
KV1620 GEOID HEIGHT- -33.66 (meters) GEOID09
KV1620 DYNAMIC HT - 204.943 (meters) 672.38 (feet) COMP
KV1620 MODELED GRAV- 980,156.3 (mgal) NAVD 88
KV1620
Which is the GEOID09 height 672.70 or 672.38, and what is the other value?
Paul in PA
Howdy,
There are a minimum of three heights in the Current Survey Control section of data sheets for benchmarks. They are NAVD 88, geoid and dynamic heights. Note as well that the Modeled Gravity entry also refers to NAVD88. Note that the NAVD 88 and DYNAMIC HT values differ. They differ because the NAVD 88 height is determined with respect to gravity at the location of the benchmark; dynamic heights are determined with respect to the reference latitude of N 45 degrees. See: http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=DSDATA.TXT
In the pre-GPS world it was not possible to determine good coordinates for benchmarks. They were plotted on the best map available and scaled. The accuracy of the scaling was sufficient given the gravity data base data positional accuracy. Updating the coordinates for benchmarks and making GPS observations on them (ideally to OPUS-DB standards) is a good thing.
The GEOID 09 model (and all preceding hybrid geoid models) is based on a gravimetric model that was then manipulated with GPS observations on benchmarks to best fit the benchmark data set. This point does not have GPS observations associated with it. Therefore the values at this location did not participate in the creation of the GEOID 09 model. In other words, the model predicts the GEOID 09 height at this point without reference to the other values associated with it.
The GEOID 09 height is -33.66 meters (you can get millimeters if you use the tool). This is the distance from the NAVD 88 height of the point to the NAD 83 ellipsoid (remember h - H - N = 0).
Did I over explain again?
Cheers,
DMM
41.4 meters is only slightly worse than expected for SCALED positions. Note that the position is truncated to whole seconds, so there is up to 70 or 100 ft of error in each coordinate right there.
>"despite the fact that a higher level position for it exists"
What other position information exists? As an Az mark the bearing from the tri station would be pretty good, but no distance would have been recorded.
I also observed an L1 only elevation to the next benchmark down the road, KV1621. It was not GOSable so I have an offset point to reference in.
Paul in PA
NGS Data Sheet Answer ?
OPUS-RS finally did my GEOID09 solutions beginning at 8:30AM today. I had requested the GEOID03 first.
From my ultra-rapid OPUS-RS on KV1620:
EL HGT: 171.375(m) 0.020(m) 170.120(m) 0.020(m)
ORTHO HGT: 205.030(m) 0.023(m) [NAVD88 (Computed using GEOID09)]
Difference is 33.655 (m)
From the Data Sheet:
KV1620 ___________________________________________________________________
KV1620* NAD 83(1986)- 40 49 23. (N) 075 09 03. (W) SCALED
KV1620* NAVD 88 - 205.038 (meters) 672.70 (feet) ADJUSTED
KV1620 ___________________________________________________________________
KV1620 GEOID HEIGHT- -33.66 (meters) GEOID09
KV1620 DYNAMIC HT - 204.943 (meters) 672.38 (feet) COMP
From my software holding NJWC CORS fixed H&V:
Ellip 171.369 m
Ortho 205.012 m (GEOID03)
After lunch I will resubmit my OPUS-S and 3 OPUS-RS files for GEOID09, get the Rapid Orbit for my software and switch my software to Geoid09, holding NJWC CORS for H and BM KV1620 for V and reprocess and compare.
NJWC is 5 miles ESE of my project and KV1620 is 1 mile E. I may or may not bring PAMS and LUMT into my project as further checks, but this project is already tight.
I had 3 GPS points on my project and compared with my traverse, verticals were within 0.01'.
Life is good, I am out to do a level loop to tie in my second benchmark with the offset GPS. I lost time yesterday with a probable gas freezeup in my remote gas filter, the engine quit while I was driving. One hour parked with that side of the vehicle serendipitously facing the sun, cleared up the problem by the time AAA showed up. Signed for the service call and drove home. I have gas line antifreeze in there now. I dressed for the cold yesterday and fotgot about prepping the Jeep.
Paul in PA
NGS Data Sheet Answer ?
"I am out to do a level loop to tie in my second benchmark...'
This is the FIRST thing I would have done.
Jim, I Had An Available Baby Sitter For GPS Yesterday.
I was set to do it after breakdown of GPS by myself. Had to defer that to get my vehicle to the mechanic.
It makes no difference before or after, since it doesn't move. I now have the elevation and still have not submitted for my rapid orbit OPUS solutions.
Paul in PA
