shrug-l: Pondering NAD27 - WGS80 switch

Fri Jan 27 17:15:12 EST 2006

You can find out how much difference the change in horizontal datums from NAD27 to NAD83 has made on a control point marker located in your area of interest by using a few online tools offered by the USGS and the U.S. National Geodetic Survey.

The NGS (http://www.ngs.noaa.gov) maintains a database of the coordinate specifications of control points, including historical locations as well as more recent adjustments.  They also have an online conversion utility called NADCON.  NADCON is the Federal standard for NAD 27 to NAD 83 datum transformations.

The information about how to do this comes from a class lesson written by Dr. David DiBiase of Penn State University's GIS graduate program.

STEP 1.  Find the geographic coordinates of a populated place.  (If you already have lat/long coordinates, skip to STEP 2.) 
a.  Start at the USGS' Geographic Names Information System at http://geonames.usgs.gov .
b.  In the column on the left, beneath "Query GNIS", click on the link for  "U.S. and territories" place names. 
On the Query Form, enter the name of a town in the "Feature Name" field.  Then populate the State and County fields.  In the Feature Class field, choose "populated place." 
c.  Click the "Send Query" button.
The result should include latitude and longitude coordinates of a centroid that represents the place you searched for.  You'll need these coordinates for the next step, so write them down!

STEP 2.  Find the geographic coordinates of a nearby horizontal control point. 
a.  Visit the U.S. National Geodetic Survey home page at http://www.ngs.noaa.gov 
b.  Beneath the photos at the top of the screen, click on the link labeled "datasheets." 
c.  At the NGS Datasheet Page, under the Retrieval Links column, click on the "Datasheets" button.  (You may wish to begin with the "Info Link" labeled "Tell me more about datasheets.")
d.  At the NGS Datasheet Retrieval page, click on "Radial Search".
e.  At the NGS Datasheet Point Radius form, do the following: 
  - Enter the latitude and longitude coordinates you looked up in step #1. Pay attention to the input format. Notice that in their example the N and W come before the numbers.
  - Specify a Search Radius.
  - Select "Any Horizontal Control" from the Data Type Desired field.
  - Select "Any Stability" from the Stability Desired field.
  - Click the "Submit" button.
f.  The result is a Station List Results form.  The first column, "Dist", is the distance that the control point is from the coordinates you input.  The third column, "H V", stands for Horizontal and Vertical.  In this column, look for control points with a "1" in the column; 1 means it's a first-order control point, 2 means it's a second-order, and 3 is third-order. 
g.  Click on the station that is both the shortest distance away from the coordinates you entered AND the highest-order horizontal control point  (e.g.,  Dist   is   .03  and   H  is  1). 
h.  Click "Get Datasheets" button.  The station datasheet will list the geographic coordinates of the control point relative to the NAD 83 horizontal datum.
i.  Write down the NAD83 geographic coordinates of the control point.  (Do NOT write down the NAVD88 vertical datum coordinates.)

STEP 3.  Calculate the datum shift associated with a conversion from one horizontal datum to another.  This step will provide info on how much the control point "moved" as a result of the adjustment of those coordinates from the earlier NAD 27 datum.
a.  Return to the U.S. National Geodetic Survey home page at http://www.ngs.noaa.gov .
b.  Beneath the photos at the top of the page, click on the link labeled "geodetic tool kit". 
c.  At the NGS Geodetic Tool Kit page, click on the link labeled "NADCON."
d.  Click on "NADCON" again.  
e.  You'll see summary info about the NAD27 to NAD83 conversion at the top of the page.  Near the bottom of the screen, click on the link labeled "Interactively compute a datum shift between NAD27 and NAD83."
f.  At the NADCON computations form, under the heading "compute a datum shift for a specific location": 
  - Select NAD83 to NAD27 as the direction of conversion  (yes, you read that correctly, NAD83 to NAD27 ... not the other way around).
  - Enter the NAD83 latitude and longitude coordinates of the control station.  Pay attention to format as you input the coordinates. 
  - Click the "Compute Datum Shift for a Single Location" button.
g.  The result will be a NADCON Output Report.  For example, for a control point near Penn State, the geographic coordinate system grid shifted nearly 7 meters South and more than 23 meters East.  That grid shift is reflected in the adjustment of the coordinates that specify the control point's location.  Note that the point didn't move, rather, the grid shifted.

Hope this helps you find out what the datum conversion will mean for your area.

Valerie Johnson

*******************************************
Valerie Johnson
GIS Technician II
Tallahassee-Leon County GIS
Leon County Courthouse, Level P-3
301 S. Monroe St.
Tallahassee, FL 32301
PH (850) 606-5504
FAX (850) 606-5501
johnsonva at leoncountyfl.gov
www.tlcgis.org

>>> "Bill VanSickle" <bvansickle at sjrwmd.com> 01/27/06 12:27 PM >>>
Along a similar line of thought...

Some folks in our organization are preparing to shift their data sets from NAD27 to WGS80 (more of a vertical shift; we're staying in UTM meters), and I was wondering if any of you have had experience with that sort of thing. If so, how'd it go? What did you expect and what happened? What happened that you didn't expect, or didn't happen that you did? What did you do that helped? 

This is not an official request, it's just a in informal one so that I can get a sense of what we're facing.

   - beel

W. E. VanSickle
GIS Analyst III / ES Liaison
Environmental Sciences Division
SJRWMD 
phone: 386-329-4580
fax:     386-329-4585 

-----Original Message-----
From: shrug-l-bounces at lists.dep.state.fl.us [mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Dietrich, Mark
Sent: Thursday, January 26, 2006 12:33 PM
To: Thomas, Jim; Tan, Yurong; tdarsey at hsa.cc; Shrug (E-mail)
Subject: RE: shrug-l: .dgn files

In the old ArcInfo world they were pretty much the same, you just had to specify the FIPZONE in the parameters portion of the projection file. One may have been the state designation and one the Feds. 

Table 1: SPCS Zone Names, ZONE, and FIPSZONE numbers.
State & Zone Name Abbrev. Datum ZONE FIPSZONE

Florida, North FL_N  3576 903
Florida, East FL_E  3601 901
Florida, West FL_W  3626 902

  _____  

From: shrug-l-bounces at lists.dep.state.fl.us [mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Thomas, Jim
Sent: Thursday, January 26, 2006 12:15 PM
To: Tan, Yurong; Dietrich, Mark; tdarsey at hsa.cc; Shrug (E-mail)
Subject: RE: shrug-l: .dgn files

That's interesting.  One is a FIPSZONE (0901) and the other an ADSZONE (3601).  Does anyone know the difference?  ArcGIS lists only the FIPSZONE from what I can tell.

  _____  

From: shrug-l-bounces at lists.dep.state.fl.us [mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Tan, Yurong
Sent: Thursday, January 26, 2006 12:04 PM
To: Dietrich, Mark; tdarsey at hsa.cc; Shrug (E-mail)
Subject: RE: shrug-l: .dgn files

Also, you might need to check into the specified "ZONE" in the (Florida) State Plane coordinate system.  For instance, if I can recall correctly that Florida StatePlane East can have zone 901 or zone 3601 and the coordinates of the two zoning systems are completely different. 

Good luck!  

*****************************************************************

Yurong Tan, Ph.D., Program Analyst/GIS                                  Email: Yurong.tan at med.va.gov 

PSSG, field unit for the Office of the ADUSH (Policy & Planning)        Phone: 352-374-6080 (x5382)

Department of Veterans Affairs                                                                                Fax: 352-374-6119

300 E. University Avenue, Suite 430                                                                  Web: www.med.va.gov 

Gainesville, Florida 32601  USA     

________________________________________________________________

They all gave some and some gave all!

  _____  

From: shrug-l-bounces at lists.dep.state.fl.us [mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Dietrich, Mark
Sent: Thursday, January 26, 2006 11:30 AM
To: tdarsey at hsa.cc; Shrug (E-mail)
Subject: RE: shrug-l: .dgn files

If you are trying to view it in a projection other than the state plane try to first create a .prj file with the correct projection information. You can use a template from another shape file and simply change the prefix to the same file name as the .dgn file. 

If you have already brought it into ArcMap it may have created a .xml file which can contain incorrect information. You will have to delete that file first to make it work. Don't add it to ArcMap until you have the renamed .prj file in the same directory as the CAD file. 

Otherwise change your data view's projection to the state plane 83 north and everything should align with the CAD file.

Mark Dietrich 
NHD Coordinator 
GIS Subsection 
Bureau of Watershed Management 

  _____  

From: shrug-l-bounces at lists.dep.state.fl.us [mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Tony Darsey
Sent: Thursday, January 26, 2006 11:09 AM
To: Shrug (E-mail)
Subject: shrug-l: .dgn files

Can anyone tell me why I am having this problem. I am trying to add a Microstation .dgn file that is in state plane 83 north and when I bring it into ArcMap it is WAY OFF. Any suggestions?

Tony Darsey 
HSA Consulting Group, Inc. 
1311 Executive Center Drive, Suite 255 
Tallahassee, FL 32301 
Phone (850) 309-7510 
Fax     (850) 309-7510 
email   tdarsey at hsa.cc 