Why do the edges of old and new maps not line up?
This question has many variations. The following comments, from actual customer mail, are all about the same issue:
- Older topo maps seem to have a neatline that doesn’t quite align geographically with the quad boundaries of topos post-2000.
- Our [independent] coordinates, plotted on the USGS Topographic Map using the geospatial tool, have a discrepancy of about 600 feet.
- We are starting to use the new [US Topo] USGS topo maps...The boundaries of these do not line up with the “standard” quad boundaries any longer.
- [On a] 2014 map, the UTM values are not correct. I realized this by comparing it to the same map but the 1967 edition.
- The UTM grid on [the map] and the UTM grid that Arc generated do not match up.
The observations are valid, but do not indicate errors. These are effects of different spatial reference systems, which in turn are effects of improving models of the earth's size and shape.
Most USGS maps published between approximately 1940 and 1995 are cast on the North American Datum of 1927 (NAD27). Later maps are cast on the North American Datum of 1983 (NAD83). The "datum shift" changes the coordinate values of fixed ground points. Another way to say this is that a datum change moves lines of latitude and longitude relative to the physical surface of the earth, and therefore moves the edges of USGS map cells. A standard USGS map cell had a slightly different ground domain in the mid-20th century than it does today.
The shift is different for different coordinate systems. UTM, State Plane, and geographic (lat/lng) values change by different amounts. A change in UTM values of roughly 200 meters in the N-S direction (the exact value varies with location) indicates a NAD27-NAD83 datum shift. Changes in State Plane Coordinate System (SPCS) values can be much larger, and vary between states.
For more detail about the effects of datum changes on USGS quadrangle maps, see this technical paper.
For tutorial information on North American Datums, this Wikipedia article is a good starting point.
Spatial reference systems continue to be improved. The National Geodetic Survey is currently defining new reference frames for North America. This is important for many reasons, but the effects on medium-scale maps will be small. Lat/lng shifts from NAD27 to NAD83 are in the range of 10-100 ground meters, but changes from later refinements are almost always smaller than 10 meters, and therefore insignificant at 1:24,000 scale.