Converting NAD27 to NAD83(2003)

I work for an electric utlity that is building a GIS and trying to come out with some "Spatial Standards".  I have learned that they are using City GIS in NAD27 and converting the data to NAD83 using NADCON.  NADCON gives NAD83 with 1986 values.  The CORS staions in our area give it in EPOCH 2003.  There is a couple of feet floating around.

Is there a better routine for doing this conversion?

Thxs.

David

David You are looking a couple of issues. First is the city GIS using NAD 27. You would have to look at what they held in their initial adjustment but I doubt that you will find out much info from the city. They could have all sorts of distortions depending upon what they held for their initial base map conversions. As a utility company then I would base my work on either the HARN adjustment or OPUS solutions to do my adjustments. Let the city move this to their datum if they wish but keep you internal GIS maps consistent and less error prone. I'm guessing though that you need to fit their maps so instead of using the 86' values then use the utilities on the NGS site to move from NAD-27 directly to the HARN values. Again, if the NAD27 values are hosed then your HARN values will be hosed also. You say building a GIS. Do you have good quality survey data on all your poles and lines. I would likely use OPUS on some select control points and move your entire system using this and all at one time. You probably have a bunch of legacy data though with good survey data but no georeference. Time to head to the field and do some OPUS sessions to tie them all to the NSRS. Deral

What were they thinking?  GIS in NAD27 seems a little like an EDM that reads in cubits or a calculator that operates in Roman numerals.

Sadly Bill a great many places still use NAD27 and have distortions out the wazoo in all their data. I did many control surveys well back in the day holding the NAD-27 values as fixed and likely messing up our really good survey data between them.  it was what we had then though and until GPS we really did not see the huge and often glaring problems. 

I am certainly not dissing the NGS and the huge effort that it took to do a network across the entire CONUS but it just is not up to today's snuff.

NADCON never was intended to solve any and all problems related to converting between NAD27 an NAD83.  It's merely one approach that works well when applied properly, but I personally think it is overused probably because it is so well known, has the cachet of the NGS and users quite often don't know of any other approaches or don't want to take the trouble to use other approaches.  Setting up multiple 7-parameter Helmert transformations can work quite well and be refined to as small an area as needed for accuracy demands of users.  Running coordinates through Corpscon/Nadcon is much easier so it usually what folks do.  Unfortunately it simply doesn't do the trick sometimes and you wind up with "a couple of feet floating around" depending on where on is in the transformational area.  That's the way it is in our area.  It would really help if geospatial data users knew more about transformational methods.