Figure 1. - 'CONUS' grid departure in degrees longitude (NAD27) from NAD83.Click Here to use the free RockWare Online Converter
The NAD 27 coordinate system (North American Datum 1927) is a geodetic coordinate system based on the Clarke Ellipsoid of 1866. Unlike the NAD83 coordinate system which is referenced from Latitude 0.0 and Longitude 0.0, the NAD27 system originates at Meades Ranch in Kansas (39.224079, -98.541808). USGS Topo Map location Because all NAD27 points are based off of the distance to this reference point, using this coordinate system on today's maps will give you quite an error the farther away you get from Meade's Ranch. Longitude is the most sensitive to error when looking at the shift from NAD27 to NAD83. Observe in Figures 1 and 2 what the deviation in degrees from NAD83 in the Conus (Contiguous US) grid as plotted by RockWorks16. It is very important for you to know which coordinate system your latitude and longitude are in. For example, a Lat/Lon position in NAD27 in central Colorado can be off by as much as 45m.
State Plane coordinate system
State Plane coordinate system coordinates are mostly legacy coordinates that originated in the 1930's. These early points were surveyed using the NAD27 datum. Later coordinates were surveyed using the NAD83 datum. To complicate things further, some states use US feel, while others use International feet. The coordinates in the State Plane system are based on a cylindrical projection. Some states have cylinders running north-south and others east-west depending on the orientation of the state. Some states are broken up into multiple zones. Also, it's important to note that some states changed the zones when they switched between the NAD27 and NAD83 systems. When doing conversions from State Plane coordinates, it's important to get the zone right. For more information on this check out this link USGS Geospatial Data FAQsFigure 2. - 'CONUS' grid departure in degrees latitude (NAD27) from NAD83.
Important Note about Google Map Output
It is important to note that Google Maps, as wtih most modern mapping applications, shows the map in the NAD83/WGS84 datum. This means that when displaying a NAD27 coordinate on a map in this datum, it will show it out of place, mostly East or West.
Map coordinate Conversion Methods
State Plane Conversions
State Plane coordinate conversions are done in the State Plane NAD27 datum. For each state and zone, the appropriate algorithm is used (Lambert, Mercator, Special Alaska, Azemuthal equidistant). The code for these computations are based on the original FORTRAN code from NOAA.
NAD27 to NAD83
Conversions from Latitude/Longitude NAD27 to Latitude/Longitude NAD83 were done using a variation of the NOAA NADCON software that was translated from FORTRAN to ICON, then to PHP. Conversions from NAD27 to NAD83 are based on grid data which are known variations from NAD27 to NAD83. Any location within the gridded area can be interpolated to within less than 0.5m of the corresponding NAD83 point.
