[Proj] NAD27 and WGS84 woes

[Michal Migurski]

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.

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/

I've searched for clues on how to approach this, and found these two  
old threads:
	http://lists.maptools.org/pipermail/proj/2000-July/000088.html
	http://lists.maptools.org/pipermail/proj/2006-February/002122.html

The first seems to indicate that PROJ does this conversion seamlessly,  
while the second recommends adding a +towgs84 parameter to the NAD27  
projection. I've tried this, and I don't see any difference in the  
numbers.

Any hints?

-mike.

----------------------------------------------------------------
michal migurski- mike at stamen.com
                  415.558.1610