| During mid 1999 the Maine Office of GIS (OGIS) will be converting all of
its GIS data from the current NAD27 (North American Datum) to NAD83 to ensure Maine GIS
data will be as current as possible. Federal agencies, such as USGS (United States
Geological Survey), have already begun this conversion. WHAT IS A DATUM?
"A datum is a set of parameters defining a coordinate system, and a set of
control points whose geometric relationships are known, either through measurement or
calculation." (Dewhurst, 1990) The horizontal datum controls the position of mapped
features in reference to latitude and longitude lines.
WHY NAD83?
The North American Datum of 1927 uses the Clarke spheroid of 1866 to represent
the shape of the earth. The origin of this datum is a point on the earth referred to as
Meades Ranch in Kansas. The data and calculations used to estimate the NAD27 locations
were done over many years (much of the data was collected in the 1800’s) and were
done manually. As a result, the errors in the calculations vary considerably between
collection stations.
With the advent of new technologies, like GPS and Doppler systems, it was discovered
that the North American Datum of 1927 had accuracy discrepancies. Data collected using the
new technologies was noticeably different from the NAD27 data.
The North American Datum of 1983 was created to resolve the inaccuracies of NAD27.
NAD83 is based upon both earth and satellite observations, using the GRS80 spheroid. The
origin for this datum is the earth’s center of mass. This affects the surface
location of all latitude-longitude values enough to cause locations of previous control
points in North America to shift, sometimes over 500 feet. A ten-year multinational effort
linked a network of control points for the United States, Canada, Mexico, Greenland,
Central America, and the Caribbean.
TRANSFORMING BETWEEN NAD27 AND NAD83
As a result of the variations in the NAD27 collection processes and the
differences in the methodologies for defining each datum, converting data from one datum
to another cannot be accomplished by using one mathematical formula for each location. The
currently accepted method for transforming is to use an error-averaging, rubber sheeting
approach. A regular grid of control points whose datum shifts is known is used to estimate
the shift at other locations.
The Maine OGIS will be implementing the Arc/Info command project, which uses the datum
transformation program created by the U.S. National Geodetic Survey (NGS), known as
NADCON. 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 methodology for mapping at scales of 1:200 and smaller.
The following image is an example of the spatial difference between NAD27 and NAD83.
The image compares NAD27 and NAD83 hydrographic information from Augusta, Maine.
HOW WILL THE NAD CONVERSION AFFECT YOU?
GIS data that is in NAD27 is not identical to GIS data that is in NAD83. There
is a shift in the position of the GIS data that varies from location to location. This
means that NAD27 GIS data cannot be overlaid with NAD83 GIS data with any accuracy.
Not everybody will have to use NAD83 GIS data. NAD27 GIS data can still be used if all
of the data being used is in NAD27, or if the software is capable of dealing with datum
conversions. All of the OGIS NAD83 data will be available from the OGIS webpage and will
be available on CDs. Once the datum change becomes official, all data created, distributed
or posted on the OGIS webpage will be in NAD83. OGIS will provide more information on the
datum conversion as conversion time nears.
David Kirouac is a GIS programmer/analyst at the Maine Office of GIS. If there are any
questions, David can be reached at 287-6375 or at isdkiro@state.me.us
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