I'm trying to figure out how to take my NAD83 project and transform it to NAD27 in TBC. I try to do this in settings, transformation, but the coords don't move to the coords calc'd out in Corpscon. They're somewhat close, but the real deal. Any one have some ideas?
You can change the projection to a NAD27 in TBC.
I had a project last year in central Utah hunting for section corners in a fire area. I don't ordinarily use NAD27 but the BLM keeps their GCDB in NAD27. They sent me the coordinates for the corners in NAD27 lat/long (4 townships). So I already had some control from OPUS in NAD83 (Utah SPC Central Zone). Wasn't sure how to convert but did a few NADCON and it was going to be a real pain doing a lot of coordinates.
So I tried a few things and found that TBC will convert it all very simply. It keeps its Global lat/long in NAD83 (actually WGS84). If you change your projection to a NAD27, in my case NAD27 SPC Utah Central zone. the Local lat/longs will be in NAD27 and the Global will still be in NAD83. You just need to change back and forth between the two projections settings (a few clicks). To enter a coordinate you need to be in the projection that your coordinates are from. It's just automatic.
It seems to me the the conversion was about right on to NADCON. I don't think there is a pure conversion, even NADCON is sort of an inexact model.
Anyway, it worked well for me in the hunt for PLSS corners.
Got it to finally transform horizontally correctly, but can't move them down in elevation for some reason. Seems like it would be fairly easy to do. Anyone know how to move TBC points down in el?
Datum transformations for classical horizontal triangulation coordinates are ellipsoidal-based. The third dimension is dealt with in terms of ellipsoidal height, not in terms of elevations which are related to the geoid.
It's apples and oranges.
NAD83 can't be transformed to NAD27. There are programs that do an approximation, but they have errors in the transformation ranging (in my experience) from .5' to 6' and I've been told by other surveyors of up to 15'. If you want actual NAD27 numbers it is proper to occupy NAD27 control monuments and do an adjustment to them in the area. If you just want mapping/GIS quality then the guesstamation programs will get you in the ballpark. TBC will do this. Just import NAD83 numbers in a file set to NAD27 and you will have a set of good NAD83 lat/longs and a guesstamated set of NAD27 lat/longs and coordinates. How close they are depends on how well the guesstamation file is for your area.