I've just downloaded an NGS Data Sheet from www.ngs.noaa.gov
Located very close to this coord:
N335639
W0930855
I've got the HARN Sheet for PID DL0431

I've tied it with Locus, and have a survey about 2 miles SW of this station. I want to get a good elevation on my survey site.
I've got about a 3 hr observation on both my survey, and this HARN Sta.

What do I do?

Do I use the Lat/Long, and the elev. directly below "CURRENT SURVEY CONTROL"?

Do I use the Orthometric elev. in Locus?

Help, Randy

-- Thanks, Nate

Nate,

We just got done with our phone conversation so you know most of what you wanted to already.

For the benefit of the rest of our gentle readers I would like to suggest spending some time with Practical GPS Surveying that you can download from the Ashtech ftp server.

Also spend some time on the National Geodetic Survey web page where you can find lots of information about the Geoid99 model and the relationship between GPS derived heights and differential levels.

Getting good elevations with GPS that are within the error budget limits of many survey projects is something you CAN do with proper procedures.

Test your ability to determine elevations on a bench mark. The very best way to know that you have learned to do something properly is to try it where you already know the answer.

Nathan,

Been traveling this week and just now getting back to the message board.

I'm trusting Phil gave you the info needed.

If not call me or email direct.



I have added informtion I recalled from the GEOID 99 Readme file,

Deriving Orthometric Heights From GPS
-------------------------------------
One key problem is deciding which orthometric height datum to use.
NGVD 29 is not a sea-level datum, and the heights are not true orthometric
heights. The datum of NAVD 88 is selected to maintain reasonable conformance
with existing height datums, and its Helmert heights are good approximations
of true orthometric heights. And, while differential ellipsoidal heights
obtained from GPS are precise, they are often expressed in the NAD 83 datum,
which is not exactly geocentric. In addition, GEOID99 rests upon an underlying
EGM96 global geopotential model, and EGM96 does possess some error of
commission.

This leads to a warning:
Do not expect the difference of a GPS ellipsoidal height at a point and
the associated GEOID99 height to exactly match the vertical datum you need.
The results will be close when converting NAD 83 GPS ellipsoidal heights into
NAVD 88 elevations; but, maybe not accurate enough for your requirement.

However, one can combine the precision of differential carrier phase GPS with
the precision of GEOID99 height differences, to approach that of leveling.

Include at least one existing bench mark in your GPS survey (preferably many
bench marks). The difference between the published elevation(s) and the
height obtained from differencing your adopted GPS ellipsoidal height and
the GEOID99 model, could be considered a "local orthometric height datum
correction." If you are surveying an extensive area (100+ km), and you
occupy a lot of bench marks, then you might detect a trend in the corrections
up to a one part-per-million level. This may be error in the GEOID99 model.

We do not currently consider geoid-corrected GPS orthometric heights as a
substitute for geodetic leveling in meeting the Federal Geodetic Control
Subcommittee(FGCS) standards for vertical control networks. Studies are
underway, and many less stringent requirements can be satisfied by geoid
modeling. Widespread success has been achieved with the preceeding models,
GEOID96, GEOID93 and GEOID90.

Modified By Randy Black on 9/28/2001 at 2:04 PM