We have Topcon GPS equipment. In the data collector (FC250), in the Coordinate System Configuration, there are two options:
1. NAD83
2. NAD83_NO_TRANS
Option 2 seems to produce NAVD88 elevations when using our local CORS station as a base (using CDMA capabilities). Option 1 produces elevations a few feet different.
We are having difficulty grasping what is going on. Any opinions?
I am not a Topcon user but I would guess that "NAD83" is applying transformation parameters to a supposedly WGS84 (ITRF) position. However, if the original position is already NAD83, and then it applies transformation parameters, then the resulting position is no longer NAD83. Then, when the geoid model is applied, it is applied to an incorrect ellipsoidal height. On the other hand the NAD83_NO-TRANS is not doing anything to the position, and therefore it is applying the geoid model (assumed to be GEOID12A) to the correct NAD83 ellipsoidal height.
Most (if not all) RTN networks in the US would be broadcasting corrections for NAD83 (2011), and therefore no transformation should be applied.
From the TopSurv 8 manual:
Use NAD83_no trans with VRS.
> Use NAD83_no trans with VRS.
That's correct for use on a VRS because the correction is already being broadcast in NAD83 (thus the _NO_TRANS). The best way to have confidence in the selection is to go to a known NGS control point and make a quick observation. Coordinates using NAD83 instead of NAD83_NO_TRANS should produce coordinates that are off by about 3'S & 1'E.
I am not buying the write-up from Topcon. There were numerous realizations of NAD83 prior to the release of Magnet. They are stuck in the 90's assuming any Lat Long and Height is relative to WGS-84 datum. If you are working in the lower 48 on a VRS it's a 99% bet you must use NAD83 NOTRANS.
This is only one issue I have with Magnet. Do NOT use the office software to compute a CAF. I have had it spit results of 0.965 on sites where I know it's nearly 1.000.. It is apparent the software is written from a dogmatic entry level 1990's view.
Rant off...
