Converting
GPS Data to NAD83 from NAD27
September 26, 2005
Reminder: How to change the datum of GPS data collected using GPS in
a projection OTHER THAN WGS84.
*NOTE: This document only refers to GPS data, not other GIS
data.
Andrew Lister (alister@fs.fed.us), Research Forester, NE Forest Inventory
and Analysis
Background:
For several years, GPS data have been collected by the Forest
Service. In the early years of GPS, data were frequently
collected in NAD27 so that they would agree with NAD27
base maps (e.g., topographic
and other maps that were produced in NAD27) without any
additional effort. In 1992, there was a national mandate
to switch geographic datums to the NAD83 datum. For various reasons,
this has
not been done
consistently across the agency. Recently, the Geospatial
Advisory Committee has drafted a memo reaffirming this
mandate, and has created a timeline
for the final mandatory conversion. Conversion of legacy
GIS vector data stored in NAD27 or other datums to the
NAD83 datum will entail
using standard GIS transformation methods.
Problem identification:
GPS receivers are designed to receive data from satellites
in the WGS84 datum. If the GPS operator requests (via
the GPS setup) that the data be displayed in NAD27, the GPS unit performs
a transformation
of the WGS84 coordinate to the requested coordinate system,
e.g., NAD27, and outputs it to the display. This transformation
procedure
is often
a mathematical equation or set of equations that is applied
to the WGS84 coordinate to create the new coordinate. The
problem occurs when
that transformation method is not appropriate for the
area where the GPS is being used (see the below graphics).
Correct method of converting NAD27 GPS data to NAD83 coordinates:
- In order to correctly change a coordinate collected in
a non WGS84 datum (e.g., NAD27) back to WGS84 (and then
subsequently to NAD83), the operator needs to identify
the transformation method
that the GPS unit applied. For example, the Rockwell
PLGR GPS unit uses the following “transformation parameters” (which
are pieces of the equations used to perform the datum transformation)
to
transform the native WGS84 coordinate to NAD27 for display:
| Local Geodetic Systems |
Code |
Name |
 a(m) |
f
x104 |
X(m) |
Y(m) |
Z(m) |
| North American 1927 |
Mean Value (CONUS) |
NAS-C |
Clarke 1866 |
-69.4 |
-0.37264639 |
-8 |
160 |
176 |
The
GPS unit’s documentation or technical support staff should
be able to readily supply you with this information,
which is a critical component of the functionality
of the GPS.
- Next,
the user needs to identify which of the ESRI datum transformation
methods corresponds to this set of parameters. One way to
do this is to consult the ESRI document found at http://support.esri.com/index.cfm?fa=knowledgebase.whitepapers.viewPaper&PID=43&MetaID=302 ,
which lists the transformation method name and the parameters
used by ArcGIS to perform the transformation. Upon inspection
of the document,
the user will quickly see which parameters agree with those
supplied
by the GPS company. Then, in ArcToolbox or ArcMap, the
user will select this datum transformation method in the
projection dialogue when performing
the reprojection.
In the example of the PLGR with the above
parameters, I should use ArcMap’s
NAD_1927_To_WGS_1984_4, because its parameters are the same
as those used by the PLGR to transform from WGS84 to NAD27.
- To then convert the WGS84 coordinate generated by ArcToolbox in
step2 to NAD83, I should then consult the above document
again, and choose
the NAD83 to WGS84 conversion appropriate to your area – in the
case of much of the United States, “NAD_1983_To_WGS_1984_5”.
(Note – WGS84 and NAD83 are generally within ~1 meter of each other – many
people assume that WGS84 and NAD83 are equivalent for all
practical purposes. I advocate taking the extra step to make
the coordinates as correct as
possible, however).
Once this correct wgs84 to nad83 method
has been identified (typically NAD_1983_To_WGS_1984_1), this method should
be selected in the projection
dialogue of ArcToolbox to obtain the final NAD83 coordinate.
Summary
In summary, the main point is this: Convert NAD27 coordinates collected
in GPS to WGS84 using the same method that is used by the GPS (be it
NADCON or another transformation method). You can find the transformation
method used by examining the GPS support information, or by calling the
company with the GPS model number and software version. Cross reference
this information with the information in the above ESRI document, and
choose the appropriate ESRI transformation method (the one that corresponds
to your GPS transformation parameters) to convert to WGS84. Finally,
based on your location, choose once again from the ESRI document the
correct ESRI transformation method to convert from WGS84 to NAD83.
Important:
Conventional wisdom in the GPS community is that it is always
appropriate to use NADCON to transform any NAD27 GPS
coordinate to NAD83. This is generally correct, unless your
GPS does not use NADCON
to convert from WGS84 to NAD27 (which often is the case).
If you were
to convert a PLGR GPS-based NAD27 coordinate to NAD83
using NADCON, and then did the same conversion correctly
using the ArcMap #4
method
(described above), and compared distances between the
two sets of points, you would obtain the below distribution
of separation distances. It
is apparent that in the NE and NC US, and parts of the
PNW US, there are significant (up to 12 meter) shifts introduced
between
the correct
coordinate (correctly transformed using the Nad27_to_WGS84_#4
method in ArcMap), and the coordinate produced using NADCON
(in this case,
incorrectly). For Alaska, the effect might be more severe.
The same procedure described above applies in Alaska. The
following two figures
illustrate this effect graphically.
Example of How the PLGR's NAD27 Coordinates Were Converted
from WGS84 Incorrectly.
(Coord was taken at center of pitcher mound;
this
is typical of
findings
when repeating experiment at surveyed
benchmarks).
Conclusion:
By taking some extra effort to ascertain what transformation
method your GPS used to collect your data, you can
avoid the significant shifts portrayed above.
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