[Proj] NAD27 and WGS84 woes

[Frank Warmerdam]

Michal Migurski wrote:
> Hello,
> 
> I'm having some difficulties with conversions between these two 
> projections:
>     +proj=utm +zone=10 +ellps=clrk66 +datum=NAD27 +units=m +no_defs
>     +proj=latlong +ellps=WGS84 +datum=WGS84 +no_defs
> 
> I'm seeing what looks like datum shift: when I convert between the two 
> using cs2cs and do a visual comparison to google maps (via 
> http://getlatlon.com), I get east/west shifts of approx. one city block. 
> The reference point I'm using is in Oakland CA, (37.804732, 
> -122.258477). I'm expecting to see it projected to (565345, 4184205) but 
> instead I'm getting (565278, 4184204), about 70m off.

Mike,

The result you are getting is congruent with no datum shift being applied
though I get (565276.44, 4184408.92) when I convert the above lat/long value
assuming WGS84 to WGS84 instead of WGS84 to NAD27.

If I do it properly using the normal NAD27 handling via the continental US
grid shift files:

   cs2cs -I +proj=utm +zone=10 +datum=NAD27 +to +proj=latlong +datum=WGS84

I get (565373.10, 4184212.50).  This is still pretty far off what you
were expecting, so I wonder if your expectations are wrong.  Or perhaps 30
meters the best you can accomplish with the data precision you have?

I presume what is happening is that you don't have the conus grid shift data
files in place.  I refer you to the datum shifting to topics in the FAQ:

   http://trac.osgeo.org/proj/wiki/FAQ


> I'm trying to use this elevation data from USGS, I need to have it match 
> the mercator projection used by OpenStreetMap:
>     http://bard.wr.usgs.gov/htmldir/dem_html/

Hmm, that may add some additional issues.

Best regards,
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