RE: [OKGIS] NAD27 to WGS84?


Chronological Thread 
  • From: wtackett@ODOT.ORG
  • To: Oklahoma GIS Community <okgis@gis.gis.ou.edu>
  • Subject: RE: [OKGIS] NAD27 to WGS84?
  • Date: Wed, 9 Jul 2008 07:58:09 -0500
  • List-archive: <http://gis.ou.edu/pipermail/okgis>
  • List-id: Oklahoma GIS Community <okgis.gis.ou.edu>


Please be warned that while the conversion from NAD27 to WGS84 (or any other datum) is a straightforward mathematical process, applying this particular datum conversion "globally" is a bad idea.  Nationwide, the NAD27 Adjustment, which derived coordinates for USC&GS (now NGS) Monuments, and other sources, contained huge errors.  Those errors were not due to any blunder or procedural mistakes, but were a consequence of the equipment and methods in use at the time.  

The errors were not known to exist, or at least, were not quantifiable, until GPS came into use.  The NGS HPGN (formerly HARN) Adjustment of the NAD83 Datum in 1993, made the errors in the NAD27 coordinates glaringly obvious.  When converting from NAD27 to WGS84 or other datums, if high accuracy is required, it is important to compare the differences between the datums on three (or more) control points that have positions known in both datums.  The easiest way to understand the magnitude and direction of the errors in a given area, is to convert to State Plane Coordinates in both datums, and compare the differences.  Note that the Easting Origin between NAD27 and NAD83 is different. (Subtract 31,500.000 US Survey Feet from the NAD27 Easting, for direct comparison to NAD83 Easting).  In small areas (less than one mile) there should be little if any difference.  However, if you plan to convert coordinates over project areas of three or four miles, or more, you may see regional differences of twenty feet or more.

The National Geodetic Survey maintains an excellent website, with various tools for converting between datums, and other resources.

http://www.ngs.noaa.gov/

Bottom line:  Be careful when doing this sort of conversion.

Regards,





"Alicia Nicely -ACOG" <asweethometx@aim.com>
Sent by: okgis-bounces@gis.gis.ou.edu

07/08/2008 04:41 PM

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RE: [OKGIS] NAD27 to WGS84?





If you have ArcEditor:

First,
You should be able to use "Add X Y data" from the Tools menu, and get those
points to display and export as a new shapefile. (remember to specify that
the coordinate system is NAD27 in the "Add X Y data" dialogue.)
Second,
Using the ArcToolbox tool: "Data Management Tools" > "Projection and
Transformation" > "Project"
to project this new shapefile from NAD27 to WGS84.

Hope this helps,

Alicia Nicely

Questions? Email: anicely@ACOGOK.org

_____________________________________________
From: okgis-bounces@gis.gis.ou.edu [mailto:okgis-bounces@gis.gis.ou.edu] On
Behalf Of Jenni Rush
Sent: Tuesday, July 08, 2008 3:42 PM
To: Oklahoma GIS Community
Subject: [OKGIS] NAD27 to WGS84?

Is there an easy way (maybe online) for converting NAD27 to WGS84?  I have
some lat/lon coordinates that I need to convert.

Thanks,

Jenni Rush
TILFORD PINSON EXPLORATION, LLC
23 W Main
Edmond, OK 73003
405.348.7201 Main
jenni@tilfordpinson.com






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