[Talk-us] NAD 27 data to OSM's native WGS 84 data

Paul Norman penorman at mac.com
Thu Jan 1 00:32:45 UTC 2015

On 12/31/2014 2:14 PM, stevea wrote:
> OSM uses WGS-84 as its default, and I erred in not realizing these 
> data are in NAD 27, so a conversion does seem to be necessary.
Not just by default - OSM is defined as only being in WGS84.

> When you open them as-is with JOSM (+ Shapefile plug-in), you get an 
> alert:
> "Unable to detect Coordinate Reference System.  Would you like to 
> fallback to ESPG:4326 (WGS 84)?"
> This implies that a "fix string" (perhaps via a .prj file, see below) 
> is required to get JOSM to "convert" (or "re-project") from the native 
> NAD 27 to WGS 84.  This is truly required, I believe.
To properly convert from NAD27 to WGS84 a grid shift file is required, 
but as you point out
> reasonable estimate of the geographic accuracy of active lines in 
> CONUS would be 25 m RMS
any differences from a crude conversion and a proper one are going to be 
swamped by the RMS error.

This also means that the differences between NAD83 and WGS84 are 
immaterial within the CONUS for this dataset, and that decently 
rectified imagery is much more accurate than it.

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