Example of a 2022 US Topo map. Quad is Morrison, Colorado. US Topo logo, bottom marginalia, and map content detail are shown. Please visit US Topo: Maps for America for full details.
Why are the NAD 83 position values so far from the NAD 27 values? Were the old coordinates wrong?
The old coordinates were not wrong, just different.
Positions obtained using the North American Datums of 1927 (NAD 27) and 1983 (NAD 83) are based on different earth shapes--or ellipsoids--and used the best technology available at the time. Mathematically, NAD 83 is a stronger datum because all previously existing horizontal stations and newer GPS surveyed stations were adjusted simultaneously. The positions within NAD 27 were adjusted in arcs, as the networks progressed across the country. Errors between stations adjusted in different arcs could have been substantial.
This issue is of declining importance and is seldom relevant to anything other than historical USGS maps (generally meaning maps published before 1990). All modern maps and GIS data are cast on NAD 83 or WGS 84, which are equivalent datums at map scales of 1:5,000 and smaller.
All federal agencies will replace NAD 83 and NAVD 88 (a vertical datum) with a new datum in the near future.
Learn more: Datum Shifts and Digital Map Coordinate Displays
Related Content
How large is the North American Datum of 1927 (NAD 27) to NAD 83 shift?
Within the conterminous 48 states, the North American Datum of 1927 (NAD 27) to the North American Datum of 1983 (NAD 83) shift of the latitude/longitude graticule (lines showing parallels of latitude and meridians of longitude for the earth) is in the range of 10-100 ground meters. Changes to UTM values are generally larger, around 200 meters, and changes for other coordinate systems are...
How are different map projections used?
The method used to portray a part of the spherical Earth on a flat surface, whether a paper map or a computer screen, is called a map projection. No flat map can rival a globe in truly representing the surface of the entire Earth, so every flat map misrepresents the surface of the Earth in some way. A flat map can show one or more--but never all--of the following: True directions True distances...
Why are USGS historical topographic maps referenced to outdated datums?
In the United States only three horizontal datums are commonly used: The North American Datum of 1927 ( NAD27 ) uses a starting point at a base station in Meades Ranch, Kansas and the Clarke Ellipsoid to calculate the shape of the Earth. The North American Datum of 1983 ( NAD83 ) was developed when satellites enabled a better model. NAD83 coordinates can be hundreds of meters away from coordinates...
Example of a 2022 US Topo map. Quad is Morrison, Colorado. US Topo logo, bottom marginalia, and map content detail are shown. Please visit US Topo: Maps for America for full details.
Examples of a variety of historical USGS topographic maps released between 1884 and 2006.
Examples of a variety of historical USGS topographic maps released between 1884 and 2006.
Examples of 7.5-minute, 1:24,000-scale historical USGS topographic map series released between 1884 and 2006.
Examples of 7.5-minute, 1:24,000-scale historical USGS topographic map series released between 1884 and 2006.
Monitoring the Earth's Landscape with Low-Cost High-Tech
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The Revolution in Mapping at the U.S. Geological Survey
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North American Datum of 1983, map data conversion tables; United States west of 96 degrees West longitude (including Hawaii)
Map projections used by the U.S. Geological Survey
Related Content
How large is the North American Datum of 1927 (NAD 27) to NAD 83 shift?
Within the conterminous 48 states, the North American Datum of 1927 (NAD 27) to the North American Datum of 1983 (NAD 83) shift of the latitude/longitude graticule (lines showing parallels of latitude and meridians of longitude for the earth) is in the range of 10-100 ground meters. Changes to UTM values are generally larger, around 200 meters, and changes for other coordinate systems are...
How are different map projections used?
The method used to portray a part of the spherical Earth on a flat surface, whether a paper map or a computer screen, is called a map projection. No flat map can rival a globe in truly representing the surface of the entire Earth, so every flat map misrepresents the surface of the Earth in some way. A flat map can show one or more--but never all--of the following: True directions True distances...
Why are USGS historical topographic maps referenced to outdated datums?
In the United States only three horizontal datums are commonly used: The North American Datum of 1927 ( NAD27 ) uses a starting point at a base station in Meades Ranch, Kansas and the Clarke Ellipsoid to calculate the shape of the Earth. The North American Datum of 1983 ( NAD83 ) was developed when satellites enabled a better model. NAD83 coordinates can be hundreds of meters away from coordinates...
Example of a 2022 US Topo map. Quad is Morrison, Colorado. US Topo logo, bottom marginalia, and map content detail are shown. Please visit US Topo: Maps for America for full details.
Example of a 2022 US Topo map. Quad is Morrison, Colorado. US Topo logo, bottom marginalia, and map content detail are shown. Please visit US Topo: Maps for America for full details.
Examples of a variety of historical USGS topographic maps released between 1884 and 2006.
Examples of a variety of historical USGS topographic maps released between 1884 and 2006.
Examples of 7.5-minute, 1:24,000-scale historical USGS topographic map series released between 1884 and 2006.
Examples of 7.5-minute, 1:24,000-scale historical USGS topographic map series released between 1884 and 2006.
Monitoring the Earth's Landscape with Low-Cost High-Tech
by Rian Bogle, Remote Sensing Specialist
Monitoring the Earth's Landscape with Low-Cost High-Tech
by Rian Bogle, Remote Sensing Specialist
The Revolution in Mapping at the U.S. Geological Survey
by Susan P. Benjamin, Research Geographer
The Revolution in Mapping at the U.S. Geological Survey
by Susan P. Benjamin, Research Geographer