[Gdal-dev] NAD83 vs. WGS84

[Matt Wilkie]

Grabowski, Hank wrote:

> I would find it to be an aggregious error that the ESRI code, which is
> used in a signficant number of GIS installations, is not doing the datum
> shift.  I'm going to call their tech support department in the next
> hour, once they open, to determine if the software isn't performing the
> task that they say it is performing.  If I look at the layer's active
> coordinate conversions when I have a WGS84 based layer and I import the
> NAD27 UTM data I see one coversion called NAD_1927_To_WGS_1984_1.  Doing
> the same thing with a NAD83 layer produces a conversion called
> NAD_1927_To_NAD_1983_NADCON.  The NAD27->NAD83 method is NADCON.  The
> NAD27->WGS84 method is a Geocentric Translation method.  Having never
> coded up a conversion method myself, I don't know if that is the
> standard mechanism of conversion between those two datums.  

I wonder if ArcMap's on the fly projection is taking shortcuts. In 
ArcInfo Workstation one cannot project directly from NAD27 to WGS84, but 
rather does things in two steps using what they call a 3 parameter 
transformation. From ArcDoc "NAD to WGS":

"The most accurate way to convert between WGS84 or WGS72 and NAD27is the 
following:

1.) PROJECT the dataset from "DATUM WGS84" to "DATUM NAR_C" (uses three 
parameter method).

2.) Use PROJECTDEFINE to update the datum to "DATUM NAD83".

3.) PROJECT from "DATUM NAD83" to "DATUM NAD27" (uses NADCON method).

Simply reverse the steps to convert from NAD27 to WGS84. This two step 
method takes advantage of the fact that the most accurate way to convert 
between NAD83 and NAD27 is the NADCON method. It's possible to convert 
directly to and from WGS84 and NAD27 (using the equivalent datum NAS_C) 
but not as accurate."

NOTE: Step 2 is a hack and doesn't change any values, it just changes 
the value in a text file (prj.adf) from NAR_C to NAD83; else the next 
step won't run.


ArcDoc "Transforming between NAD27 and NAD83" says:

"The datum transformation program known as NADCON, created by the U.S. 
National Geodetic Survey (NGS), has been implemented within the PROJECT 
command. A minimum curvature-derived surface based upon the National 
Geodetic Reference System determines the transformation of points and 
has an approximate accuracy of 0.15 to 0.5 meters. The accepted national 
standard, NADCON is the fastest, simplest, and most accurate datum 
transformation for mapping at scales of 1:200 and smaller. NADCON, an 
acronym for North American Datum CONversion, is suitable for the United 
States, Puerto Rico, and the Virgin Islands, but only for conversion 
between the NAD27, the NAD83, the High Precision GPS Network (HPGN) 
readjustment of NAD83, and selected local island datums discussed below. 
..."

An online doc which contains links to yet more docs, in particular the 
'Projection Engine', which might shed light on the ESRI way is
http://tinyurl.com/z0ve
(goes to 
http://support.esri.com/index.cfm?fa=knowledgebase.techArticles.articleShow&d=23025)

cheers,

-- 
matt wilkie
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